Table of contents

We perform the first kinematic analysis of a coronal mass ejection (CME) observed by both imaging and in situ instruments on board the Solar Terrestrial Relations Observatory (STEREO). Launched on 2008 February 4, the CME is tracked continuously from initiation to 1 AU using the imagers on both STEREO spacecraft, and is then detected by the particle and field detectors on board STEREO-B on February 7. The CME is also detected in situ by the Advanced Composition Explorer and Solar and Heliospheric Observatory at Earth's L1 Lagrangian point. This event provides a good example of just how different the same event can look when viewed from different perspectives. We also demonstrate many ways in which the comprehensive and continuous coverage of this CME by STEREO improves confidence in our assessment of its kinematic behavior, with potential ramifications for space weather forecasting. The observations provide several lines of evidence in favor of the observable part of the CME being narrow in angular extent, a determination crucial for deciding how best to convert observed CME elongation angles from Sun-center to actual Sun-center distances.

We present the first Chandra/ACIS imaging study of the circumnuclear region of the nearby Seyfert galaxy NGC 1365. The X-ray emission is resolved into pointlike sources and complex, extended emission. The X-ray morphology of the extended emission shows a biconical soft X-ray-emission region extending ∼5 kpc in projection from the nucleus, coincident with the high-excitation outflow cones seen in optical emission lines particularly to the northwest. Harder X-ray emission is detected from a kpc-diameter circumnuclear ring, coincident with the star-forming ring prominent in the Spitzer mid-infrared (IR) images; this X-ray emission is partially obscured by the central dust lane of NGC 1365. Spectral fitting of spatially separated components indicates a thermal plasma origin for the soft extended X-ray emission (kT = 0.57 keV). Only a small amount of this emission can be due to photoionization by the nuclear source. Detailed comparison with [O iii]λ5007 observations shows that the hot interstellar medium (ISM) is spatially anticorrelated with the [O iii]-emitting clouds and has thermal pressures comparable to those of the [O iii] media, suggesting that the hot ISM acts as a confining medium for the cooler photoionized clouds. The abundance ratios of the hot ISM are fully consistent with the theoretical values for enrichment from Type II supernovae, suggesting that the hot ISM is a wind from the starburst circumnuclear ring. X-ray emission from a ∼450 pc long nuclear radio jet is also detected to the southeast.

With a column density log N(O vi) = 14.95 ± 0.05, the O vi absorber at z_abs ≃ 0.2028 observed toward the quasi-stellar object PKS 0312–77 (z_em = 0.223) is the strongest yet detected at z < 0.5. At nearly identical redshift (z_abs ≃ 0.2026), we also identify a Lyman limit system (LLS, log N(H i) = 18.22^+0.19_−0.25). Combining FUV and NUV spectra of PKS 0312–77 with optical observations of galaxies in the surrounding field (15' × 32'), we present an analysis of these absorbers and their connection to galaxies. The observed O i/H i ratio and photoionization modeling of other low ions indicate that the metallicity of the LLS is [Z/H]_LLS ≃ −0.6, and that the LLS is nearly 100% photoionized. In contrast, the O vi-bearing gas is collisionally ionized at T ∼ (3–10) × 10⁵ K as derived from the high-ion ratios and profile broadenings. Our galaxy survey reveals 13 (0.3 ≲ L/L_* ≲ 1.6) galaxies at ρ < 2h⁻¹₇₀ Mpc and |δv| ≲ 1100 km s⁻¹ from the LLS. A probable origin for the LLS is debris from a galaxy merger, which led to a 0.7 L_* galaxy ([Z/H]_gal ≃ +0.15) at ρ ≃ 38h⁻¹₇₀ kpc. Outflow from this galaxy may also be responsible for the supersolar ([Z/H]_abs ≃ +0.15), fully ionized absorber at z_abs ≃ 0.2018 (−190 km s⁻¹ from the LLS). The hot O vi absorber likely probes coronal gas about the 0.7 L_* galaxy and/or (∼0.1 keV) intragroup gas of a spiral-rich system. The association of other strong O vi absorbers with LLS suggests that they trace galactic and not intergalactic structures.

We present the results of Spitzer Infrared Spectrograph 5–35 μm low-resolution spectroscopic energy diagnostics of ultraluminous infrared galaxies (ULIRGs) at z> 0.15, classified optically as non-Seyferts. Based on the equivalent widths of polycyclic aromatic hydrocarbon emission and the optical depths of silicate dust absorption features, we searched for signatures of intrinsically luminous, but optically elusive, buried active galactic nuclei (AGNs) in these optically non-Seyfert ULIRGs. We then combined the results with those of non-Seyfert ULIRGs at z< 0.15 and non-Seyfert galaxies with infrared luminosities L_IR < 10¹² L_☉. We found that the energetic importance of buried AGNs clearly increases with galaxy infrared luminosity, becoming suddenly discernible in ULIRGs with L_IR > 10¹² L_☉. For ULIRGs with buried AGN signatures, a significant fraction of infrared luminosities can be accounted for by the detected buried AGN and modestly obscured (A_V < 20 mag) starburst activity. The implied masses of spheroidal stellar components in galaxies for which buried AGNs become important roughly correspond to the value separating red massive and blue less-massive galaxies in the local universe. Our results may support the widely proposed AGN-feedback scenario as the origin of galaxy downsizing phenomena, where galaxies with currently larger stellar masses previously had higher AGN energetic contributions and star formation originating infrared luminosities, and have finished their major star formation more quickly, due to stronger AGN feedback.

We have examined the physical conditions in the narrow-line region (NLR) of the Seyfert 2 galaxy Markarian 3, using long-slit spectra obtained with the Hubble Space Telescope/Space Telescope Imaging Spectrograph, and photoionization models. We find three components of photoionized gas in the NLR. Two of these components, characterized by emission lines such as [Ne v] λ3426 and [O iii] λ5007, lie within the envelope of the biconical region described in our previous kinematic study. A component of lower ionization gas, in which lines such as [O ii] λ3727 arise, is found to lie outside the bicone. Each of these components is irradiated by a power-law continuum which is attenuated by intervening gas, presumably closer to the central source. The radiation incident upon the low-ionization gas, external to the bicone, is much more heavily absorbed. These absorbers are similar to the intrinsic UV and X-ray absorbers detected in many Seyfert 1 galaxies, which suggests that the collimation of the ionizing radiation occurs in a circumnuclear wind, rather than a thick, molecular torus. We estimate the mass for the observed NLR emitting gas to be 2 × 10⁶  M_☉. It is likely that Markarian 3 acquired this gas through an ongoing interaction with the spiral galaxy UGC 3422.

Cluster galaxies moving through the intracluster medium (ICM) are expected to lose some of their interstellar medium (ISM) through ISM–ICM interactions. We perform high-resolution (40 pc) three-dimensional hydrodynamical simulations of a galaxy undergoing ram pressure stripping, including radiative cooling, in order to investigate stripping of a multiphase medium. The clumpy, multiphase ISM is self-consistently produced by the inclusion of radiative cooling, and spans six orders of magnitude in gas density. We find no large variations in the amount of gas lost whether or not cooling is involved, although the gas in the multiphase galaxy is stripped more quickly and to a smaller radius. We also see significant differences in the morphology of the stripped disks. This occurs because the multiphase medium naturally includes high-density clouds set inside regions of lower density. We find that the lower density gas is stripped quickly from any radius of the galaxy, and the higher density gas can then be ablated. If high-density clouds survive, through interaction with the ICM, they lose enough angular momentum to drift toward the center of the galaxy where they are no longer stripped. Finally, we find that low ram pressure values compress gas into high-density clouds that could lead to enhanced star formation, while high ram pressure leads to a smaller amount of high-density gas.

Spitzer Space Observatory IRAC and MIPS photometric observations are presented for 20 white dwarfs with T_eff ≲ 20, 000 K and metal-contaminated photospheres. A warm circumstellar disk is detected at GD 16 and likely at PG 1457−086, while the remaining targets fail to reveal mid-infrared excess typical of dust disks, including a number of heavily polluted stars. Extending previous studies, over 50% of all single white dwarfs with implied metal-accretion rates dM/dt≳ 3 × 10⁸ g s⁻¹ display a warm infrared excess from orbiting dust; the likely result of a tidally destroyed minor planet. This benchmark accretion rate lies between the dust production rates of 10⁶ g s⁻¹ in the solar system zodiacal cloud and 10¹⁰ g s⁻¹ often inferred for debris disks at main-sequence A-type stars. It is estimated that between 1% and 3% of all single white dwarfs with cooling ages less than around 0.5 Gyr possess circumstellar dust, signifying an underlying population of minor planets.

Some general properties of the advective–acoustic instability are described and understood using a toy model, which is simple enough to allow for analytical estimates of the eigenfrequencies. The essential ingredients of this model, in the unperturbed regime, are a stationary shock and a subsonic region of deceleration. For the sake of analytical simplicity, the two-dimensional unperturbed flow is parallel and the deceleration is produced adiabatically by an external potential. The instability mechanism is determined unambiguously as the consequence of a cycle between advected and acoustic perturbations. The purely acoustic cycle, considered alone, is proven to be stable in this flow. Its contribution to the instability can be either constructive or destructive. A frequency cutoff is associated with the advection time through the region of deceleration. This cutoff frequency explains why the instability favors eigenmodes with a low frequency and a large horizontal wavelength. The relation between the instability occurring in this highly simplified toy model and the properties of standing accretion shock instability observed in the numerical simulations of stellar core collapse is discussed. This simple setup is proposed as a benchmark test to evaluate the accuracy, in the linear regime, of numerical simulations involving this instability. We illustrate such benchmark simulations in a companion paper.

The physical processes involved in the advective–acoustic instability are investigated with two-dimensional numerical simulations. Simple toy models, developed in a companion paper, are used to describe the coupling between acoustic and entropy/vorticity waves, produced either by a stationary shock or by the deceleration of the flow. Using two Eulerian codes based on different second-order upwind schemes, we confirm the results of the perturbative analysis. The numerical convergence with respect to the computation mesh size is studied with one-dimensional simulations. We demonstrate that the numerical accuracy of the quantities that depend on the physics of the shock is limited to a linear convergence. We argue that this property is likely to be true for most current numerical schemes dealing with standing accretion shock instability in the core-collapse problem, and could be solved by the use of advanced techniques for the numerical treatment of the shock. We propose a strategy to choose the mesh size for an accurate treatment of the advective–acoustic coupling in future numerical simulations.

Observations of the intergalactic medium (IGM) suggest that quasars reionize He ii in the IGM at z ≈ 3. We have run a set of 190 and 430 comoving Mpc simulations of He ii being reionized by quasars to develop an understanding of the nature of He ii reionization and its potential impact on observables. We find that He ii reionization heats regions in the IGM by as much as 25, 000 K above the temperature that is expected otherwise, with the volume-averaged temperature increasing by ∼12, 000 K and with large temperature fluctuations on ∼50 Mpc scales. Much of this heating occurs far from quasars by photons with long mean free path. We find a temperature–density equation of state of γ − 1 ≈ 0.3 during He ii reionization, but with a wide dispersion in this relation having σ_T ∼ 10⁴ K. He ii reionization by the observed population of quasars cannot produce an inverted relation (γ − 1 < 0). Our simulations are consistent with the observed evolution in the mean transmission of the He ii Lyα forest. We argue that the heat input from He ii reionization is unable to cause the observed depression at z ≈ 3.2 in the H i Lyα forest opacity as has been suggested. We investigate how uncertainties in the properties of QSOs and of He ii Lyman limit systems influence our predictions.

We utilize the local velocity dispersion function (VDF) of spheroids, together with their inferred age distributions, to predict the VDF at higher redshifts (0 < z ≲ 6), under the assumption that (1) most of the stars in each nearby spheroid formed in a single episode and, (2) the velocity dispersion σ remained nearly constant afterward. We assume further that a supermassive BH forms concurrently with the stars, and within ±1 Gyr of the formation of the potential well of the spheroid, and that the relation between the mass of the BH and host velocity dispersion maintains the form M_BH ∝ σ^β with β ≈ 4, but with the normalization allowed to evolve with redshift as ∝(1 + z)^α. We compute the BH mass function associated with the VDF at each redshift, and compare the accumulated total BH mass density with that inferred from the integrated quasar luminosity function (LF; the so-called Sołtan argument). This comparison is insensitive to the assumed duty cycle or Eddington ratio of quasar activity, and we find that the match between the two BH mass densities favors a relatively mild redshift evolution, with α ∼ 0.33, with a positive evolution as strong as α ≳ 1.3 excluded at more than 99% confidence level. A direct match between the characteristic BH mass in the VDF-based and quasar LF-based BH mass functions also yields a mean Eddington ratio of λ ∼ 0.5–1 that is roughly constant within 0 ≲ z ≲ 3. A strong positive evolution in the M_BH–σ relation is still allowed by the data if galaxies increase, on average, their velocity dispersions since the moment of formation due to dissipative processes. If we assume that the mean velocity dispersion of the host galaxies evolves as σ(z) = σ(0) × (1 + z)^−γ, we find a lower limit of γ ≳ 0.23 for α ≳ 1.5. The latter estimate represents an interesting constraint for galaxy evolution models and can be tested through hydro simulations. This dissipative model, however, also implies a decreasing λ at higher z, at variance with several independent studies.

The first stars in the universe formed out of pristine primordial gas clouds that were radiatively cooled to a few hundreds of degrees kelvin either via molecular or atomic (Lyman-α) hydrogen lines. This primordial mode of star formation was eventually quenched once radiative and/or chemical (metal enrichment) feedbacks marked the transition to Population II stars. In this paper, we present a model for the formation rate of Population III stars based on Press–Schechter modeling coupled with analytical recipes for gas cooling and radiative feedback. Our model also includes a novel treatment for metal pollution based on self-enrichment due to a previous episode of Population III star formation in progenitor halos. With this model, we derive the star formation history of Population III stars, their contribution to the reionization of the universe and the time of the transition from Population III star formation in minihalos (M ≈ 10⁶ M_☉, cooled via molecular hydrogen) to that in more massive halos (M ≳ 2 × 10⁷ M_☉, where atomic hydrogen cooling is also possible). We consider a grid of models highlighting the impact of varying the values for the free parameters used, such as star formation and feedback efficiency. The most critical factor is the assumption that only one Population III star is formed in a halo. In this scenario, metal-free stars contribute only to a minor fraction of the total number of photons required to reionize the universe. In addition, metal-free star formation is primarily located in minihalos, and chemically enriched halos become the dominant locus of star formation very early in the life of the universe—at redshift z ≈ 25—even assuming a modest fraction (0.5%) of enriched gas converted in stars. If instead multiple metal-free stars are allowed to form out of a single halo, then there is an overall boost of Population III star formation, with a consequent significant contribution to the reionizing radiation budget. In addition, the bulk of metal-free stars are produced in halos with M ≳ 2 × 10⁷ M_☉.

Cosmological simulations of dark matter (DM) structures have shown that the equilibrated DM structures have a fairly small angular momentum. It appears from these N-body simulations that the radial profile of the angular momentum has an almost universal behavior, even if the different DM structures have experienced very different formation and merger histories. We suggest a perturbed Jeans equation, which includes a rotational term. This is done under a reasonable assumed form of the change in the distribution function. By conjecturing that the (new) subdominant rotation term must be proportional to the (old) dominant mass term, we find a clear connection, which is in rather good agreement with the results of recent high-resolution simulations. We also present a new connection between the radial profiles of the angular momentum and the velocity anisotropy, which is also in fair agreement with numerical findings. Finally, we show how the spin parameter λ increases as a function of radius.

The first paper in this series explored the effects of altering the chemical mixture of the stellar population on an element-by-element basis on stellar evolutionary tracks and isochrones to the end of the red giant branch. This paper extends the discussion by incorporating the fully consistent synthetic stellar spectra with those isochrone models in predicting integrated colors, Lick indices, and synthetic spectra. Older populations display element ratio effects in their spectra at higher amplitude than younger populations. In addition, spectral effects in the photospheres of stars tend to dominate over effects from isochrone temperatures and lifetimes, but, further, the isochrone-based effects that are present tend to fall along the age–metallicity degeneracy vector, while the direct stellar spectral effects usually show considerable orthogonality.

We expect direct lens modeling to be the key to successful and meaningful automated strong galaxy-scale gravitational lens detection. We have implemented a lens-modeling "robot" that treats every bright red galaxy (BRG) in a large imaging survey as a potential gravitational lens system. Having optimized a simple model for "typical" galaxy-scale gravitational lenses, we generate four assessments of model quality that are then used in an automated classification. The robot infers from these four data the lens classification parameter H that a human would have assigned; the inference is performed using a probability distribution generated from a human-classified training set of candidates, including realistic simulated lenses and known false positives drawn from the Hubble Space Telescope (HST) Extended Groth Strip (EGS) survey. We compute the expected purity, completeness, and rejection rate, and find that these statistics can be optimized for a particular application by changing the prior probability distribution for H; this is equivalent to defining the robot's "character." Adopting a realistic prior based on expectations for the abundance of lenses, we find that a lens sample may be generated that is ∼100% pure, but only ∼20% complete. This shortfall is due primarily to the oversimplicity of the model of both the lens light and mass. With a more optimistic robot, ∼90% completeness can be achieved while rejecting ∼90% of the candidate objects. The remaining candidates must be classified by human inspectors. Displaying the images used and produced by the robot on a custom "one-click" web interface, we are able to inspect and classify lens candidates at a rate of a few seconds per system, suggesting that a future 1000 deg² imaging survey containing 10⁷ BRGs, and some 10⁴ lenses, could be successfully, and reproducibly, searched in a modest amount of time. We have verified our projected survey statistics, albeit at low significance, using the HST EGS data, discovering four new lens candidates in the process.

We present emission maps of the Sgr A molecular cloud complex at the Galactic center (GC) in the J = 2 → 1  line of SiO observed with the IRAM 30 m telescope at Pico Veleta. Comparing our SiO(2–1) data cube with that of CS(1–0) emission with similar angular and velocity resolution, we find a correlation between the SiO/CS line intensity ratio and the equivalent width of the Fe Kα fluorescence line at 6.4 keV. We discuss the SiO abundance enhancement in terms of the two most plausible scenarios for the origin of the 6.4 keV Fe line: X-ray reflection nebula (XRN) and low-energy cosmic rays (LECRs). Both scenarios could explain the enhancement in the SiO/CS intensity ratio with the intensity of the 6.4 keV Fe line, but both present difficulties. The XRN scenario requires a population of very small grains to produce the SiO abundance enhancement, together with a past episode of bright X-ray emission from some source in the GC, possibly the central supermassive black hole, SgrA*, ∼300 yr ago. The LECR scenario needs higher gas column densities to produce the observed 6.4 keV Fe line intensities than those derived from our observations. It is possible to explain the SiO abundance enhancement if the LECRs originate in supernovae and their associated shocks produce the SiO abundance enhancement. However, the LECR scenario cannot account for the time variability of the 6.4 keV Fe line, which can be naturally explained by the XRN scenario.

Stationary solutions to the equations of nonlinear diffusive shock acceleration play a fundamental role in the theory of cosmic-ray acceleration. Their existence usually requires that a fraction of the accelerated particles be allowed to escape from the system. Because the scattering mean free path is thought to be an increasing function of energy, this condition is conventionally implemented as an upper cutoff in energy space—particles are then permitted to escape from any part of the system, once their energy exceeds this limit. However, because accelerated particles are responsible for the substantial amplification of the ambient magnetic field in a region upstream of the shock front, we examine an alternative approach in which particles escape over a spatial boundary. We use a simple iterative scheme that constructs stationary numerical solutions to the coupled kinetic and hydrodynamic equations. For parameters appropriate for supernova remnants, we find stationary solutions with efficient acceleration when the escape boundary is placed at the point where growth and advection of strongly driven nonresonant waves are in balance. We also present the energy dependence of the distribution function close to the energy where it cuts off—a diagnostic that is in principle accessible to observation.

Two-body relaxation times of nuclear star clusters are short enough that gravitational encounters should substantially affect their structure in 10 Gyr or less. In nuclear star clusters without massive black holes, dynamical evolution is a competition between core collapse, which causes densities to increase, and heat input from the surrounding galaxy, which causes densities to decrease. The maximum extent of a nucleus that can resist expansion is derived numerically for a wide range of initial conditions; observed nuclei are shown to be compact enough to resist expansion, although there may have been an earlier generation of low-density nuclei that were dissolved. An evolutionary model for NGC 205 is presented which suggests that the nucleus of this galaxy has already undergone core collapse. Adding a massive black hole to a nucleus inhibits core collapse, and nuclear star clusters with black holes always expand, due primarily to heat input from the galaxy and secondarily to heating from stellar disruptions. The expansion rate is smaller for larger black holes due to the smaller temperature difference between galaxy and nucleus when the black hole is large. The rate of stellar tidal disruptions and its variation with time are computed for a variety of initial models. The disruption rate generally decreases with time due to the evolving nuclear density, particularly in the faintest galaxies, assuming that scaling relations derived for luminous galaxies can be extended to low luminosities.

A re-analysis of the fluorine abundance in three Galactic asymptotic giant branch (AGB) carbon stars (TX Psc, AQ Sgr, and R Scl) has been performed from the molecular HF (1-0) R9 line at 2.3358 μm. High resolution (R ∼   50,000) and high signal-to-noise spectra obtained with the CRIRES spectrograph and the VLT telescope or from the NOAO archive (for TX Psc) have been used. Our abundance analysis uses the latest generation of MARCS model atmospheres for cool carbon-rich stars. Using spectral synthesis in local thermodynamic equilibrium, we derive for these stars fluorine abundances that are systematically lower by ∼0.8 dex in average with respect to the sole previous estimates by Jorissen et al. The possible reasons of this discrepancy are explored. We conclude that the difference may rely on the blending with C-bearing molecules (CN and C₂) that were not properly taken into account in the former study. The new F abundances are in better agreement with the prediction of full network stellar models of low-mass AGB stars. These models also reproduce the s-process elements distribution in the sampled stars. This result, if confirmed in a larger sample of AGB stars, might alleviate the current difficulty to explain the largest [F/O] ratios found by Jorissen et al. In particular, it may not be necessary to search for alternative nuclear chains affecting the production of F in AGB stars.

A statistical equilibrium treatment of a simplified three-level hydrogen molecule has been used by several authors to predict the volume densities of hydrogen nuclei produced by photodissociation in cold low-density clouds. This procedure does not require knowledge of the excitation conditions in the molecular gas, but the derived densities depend linearly on an integral function, $\mathcal {G}(\sc N_{2})$, which approaches a constant value as the molecular hydrogen column (N₂) becomes infinite. Previous studies presumed this function could be replaced by that constant without determining the function's actual value in locations of interest. Numerical integrations of $\mathcal {G}(\sc N_{2})$ have been performed to assess the impact of that assumption using four different published forms of the H₂ self-shielding function embedded in its integrand. In each case, these calculations showed that N₂ must exceed ≈6 × 10²⁰ cm⁻² for $\mathcal {G}$ to be constant when solar neighborhood values are assumed for the dust-to-gas mass ratio (δ₀), but that constant's actual value, $(\mathcal {G}_\infty)_0$, depends on the function chosen to describe self-shielding. When the latter is represented by N₂^−1/2 as often assumed in the literature for an isolated H₂ absorption line, $\mathcal {G}_\infty$ scales as ($\mathcal {G}_\infty$)₀(δ/δ₀)^1/2. However, use of a more exact self-shielding function derived previously for an ensemble of overlapping lines shows this quantity actually scales as (δ/δ₀)^0.7. The volume density of photodissociated hydrogen nuclei found in this case, n_E, is related to the value computed by assuming self-shielding arises solely from a single line, n_S, by n_E = n_S(δ/δ₀)^0.2.

To explore the high-frequency radio spectra of galaxies in clusters, we used NRAO's Very Large Array at four frequencies, 4.9–43 GHz, to observe 139 galaxies in low redshift (z < 0.25), X-ray detected, clusters. The clusters were selected from the survey conducted by Ledlow and Owen, who provided redshifts and 1.4 GHz flux densities for all the radio sources. We find that more than half of the observed sources have steep microwave spectra as generally expected (α < −0.5, in the convention S ∝ ν^α). However, 60%–70% of the unresolved or barely resolved sources have flat or inverted spectra. Most of these show an upward turn in flux at ν>22 GHz, implying a higher flux than would be expected from an extrapolation of the lower-frequency flux measurements. Our results quantify the need for careful source subtraction in increasingly sensitive measurements of the Sunyaev–Zel'dovich effect in clusters of galaxies (as currently being conducted by, for instance, the Atacama Cosmology Telescope and South Pole Telescope groups).

We numerically evolve turbulence driven by the magnetorotational instability (MRI) in a three-dimensional, unstratified shearing box and study its structure using two-point correlation functions. We confirm Fromang and Papaloizou's result that shearing box models with zero net magnetic flux are not converged; the dimensionless shear stress α is proportional to the grid scale. We find that the two-point correlation of $\mbox{\boldmath $B$}$ shows that it is composed of narrow filaments that are swept back by differential rotation into a trailing spiral. The correlation lengths along each of the correlation function principal axes decrease monotonically with the grid scale. For mean azimuthal field models, which we argue are more relevant to astrophysical disks than the zero net field models, we find that: α increases weakly with increasing resolution at fixed box size; α increases slightly as the box size is increased; α increases linearly with net field strength, confirming earlier results; the two-point correlation function of the magnetic field is resolved and converged, and is composed of narrow filaments swept back by the shear; the major axis of the two-point increases slightly as the box size is increased; these results are code independent, based on a comparison of ATHENA and ZEUS runs. The velocity, density, and magnetic fields decorrelate over scales larger than ∼H, as do the dynamical terms in the magnetic energy evolution equations. We conclude that MHD turbulence in disks is localized, subject to the limitations imposed by the absence of vertical stratification, the use of an isothermal equation of state, finite box size, finite run time, and finite resolution.

The thermal evolution of neutron stars is coupled to their spin down and the resulting changes in structure and chemical composition. This coupling correlates stellar surface temperatures with rotational state as well as time. We report an extensive investigation of the coupling between spin down and cooling for hybrid stars which undergo a phase transition to deconfined quark matter at the high densities present in stars at low rotation frequencies. The thermal balance of neutron stars is re-analyzed to incorporate phase transitions and the related latent heat self-consistently, and numerical calculations are undertaken to simultaneously evolve the stellar structure and temperature distribution. We find that the changes in stellar structure and chemical composition with the introduction of a pure quark matter phase in the core delay the cooling and produce a period of increasing surface temperature for strongly superfluid stars of strong and intermediate magnetic field strength. The latent heat of deconfinement is found to reinforce this signature if quark matter is superfluid and it can dominate the thermal balance during the formation of a pure quark matter core. At other times, it is less important and does not significantly change the thermal evolution.

We investigate the utility of a new, self-similar pressure profile for fitting Sunyaev–Zel'dovich (SZ) effect observations of galaxy clusters. Current SZ imaging instruments–such as the Sunyaev–Zel'dovich Array (SZA)–are capable of probing clusters over a large range in a physical scale. A model is therefore required that can accurately describe a cluster's pressure profile over a broad range of radii from the core of the cluster out to a significant fraction of the virial radius. In the analysis presented here, we fit a radial pressure profile derived from simulations and detailed X-ray analysis of relaxed clusters to SZA observations of three clusters with exceptionally high-quality X-ray data: A1835, A1914, and CL J1226.9+3332. From the joint analysis of the SZ and X-ray data, we derive physical properties such as gas mass, total mass, gas fraction and the intrinsic, integrated Compton y-parameter. We find that parameters derived from the joint fit to the SZ and X-ray data agree well with a detailed, independent X-ray-only analysis of the same clusters. In particular, we find that, when combined with X-ray imaging data, this new pressure profile yields an independent electron radial temperature profile that is in good agreement with spectroscopic X-ray measurements.

Observations indicate that mass accretion rates onto low-mass protostars are generally lower than the rates of infall to their disks; this suggests that much of the protostellar mass must be accreted during rare, short outbursts of rapid accretion. We explore when protostellar disk accretion is likely to be highly variable. While constant α disks can in principle adjust their accretion rates to match infall rates, protostellar disks are unlikely to have constant α. In particular, we show that neither models with angular momentum transport due solely to the magnetorotational instability (MRI) nor gravitational instability (GI) are likely to transport disk mass at protostellar infall rates over the large range of radii needed to move infalling envelope material down to the central protostar. We show that the MRI and GI are likely to combine to produce outbursts of rapid accretion starting at a few AU. Our analysis is consistent with the time-dependent models of Armitage et al. and agrees with our observational study of the outbursting object FU Ori.

The molecular cloud, DR21 Main, is an example of a large-scale gravitational collapse about an axis near the plane of the sky where the collapse is free of major disturbances due to rotation or other effects. Using flux maps, polarimetric maps, and measurements of the field inclination by comparing the line widths of ion and neutral species, we estimate the temperature, mass, magnetic field, and the turbulent kinetic, mean magnetic, and gravitational potential energies, and present a three-dimensional model of the cloud and magnetic field.

In the epoch of precise and accurate cosmology, cross-confirmation using a variety of cosmographic methods is paramount to circumvent systematic uncertainties. Owing to progenitor histories and explosion physics differing from those of Type Ia supernovae (SNe Ia), Type II-plateau supernovae (SNe II-P) are unlikely to be affected by evolution in the same way. Based on a new analysis of 17 SNe II-P, and on an improved methodology, we find that SNe II-P are good standardizable candles, almost comparable to SNe Ia. We derive a tight Hubble diagram with a dispersion of 10% in distance, using the simple correlation between luminosity and photospheric velocity introduced by Hamuy and Pinto. We show that the descendent method of Nugent et al. can be further simplified and that the correction for dust extinction has low statistical impact. We find that our SN sample favors, on average, a very steep dust law with total to selective extinction R_V < 2. Such an extinction law has been recently inferred for many SNe Ia. Our results indicate that a distance measurement can be obtained with a single spectrum of a SN II-P during the plateau phase combined with sparse photometric measurements.

A time series of full-Stokes spectropolarimetric observations of the sunspot NOAA 10944, acquired with HINODE/SOT in 2007 February, is analyzed. The data were inverted using the code SIR into a series of 34 maps covering 3 hr of umbra and penumbra evolution. The retrieved maps of plasma parameters show the spatial distribution of temperature, line-of-sight velocity, magnetic field strength, and inclination in two different ranges of optical depths corresponding to the low and high photosphere. In these maps, the evolution of central and peripheral umbral dots (CUDs and PUDs) and penumbral grains (PGs) was traced. While CUDs do not show any excess of line-of-sight velocity and magnetic field inclination with respect to the surrounding umbra, upflows of 400 m s⁻¹ and a more horizontal magnetic field are detected in the low photospheric layers of PUDs. PGs have even stronger upflows and magnetic field inclination in the low photosphere than PUDs. The absolute values of these parameters decrease when PGs evolve into PUDs. It seems that PGs and PUDs are of a similar physical nature. Both classes of features appear in regions with a weaker and more horizontal magnetic field and their formation height reaches the low photosphere. On the other hand, CUDs appear in regions with a stronger and more vertical magnetic field and they are formed too deep to detect upflows and changes in magnetic field inclination.

We present interferometric angular sizes for 12 stars with known planetary companions, for comparison with 28 additional main-sequence stars not known to host planets. For all objects we estimate bolometric fluxes and reddenings through spectral-energy distribution (SED) fits, and in conjunction with the angular sizes, measurements of effective temperature. The angular sizes of these stars are sufficiently small that the fundamental resolution limits of our primary instrument, the Palomar Testbed Interferometer, are investigated at the sub-milliarcsecond level and empirically established based upon known performance limits. We demonstrate that the effective temperature scale as a function of dereddened (V − K)₀ color is statistically identical for stars with and without planets. A useful byproduct of this investigation is a direct calibration of the T_EFF scale for solarlike stars, as a function of both spectral type and (V − K)₀ color, with an precision of $\overline{\Delta T}_{\it {(V-K)}_0} = 138\,{\rm K}$ over the range (V − K)₀ = 0.0–4.0 and $\overline{\Delta T}_{\rm {SpType}} = 105\,{\rm K}$ for the range F6V–G5V. Additionally, in an Appendix we provide SED fits for the 166 stars with known planets which have sufficient photometry available in the literature for such fits; this derived "XO-Rad" database includes homogeneous estimates of bolometric flux, reddening, and angular size.

In order to try to understand the internal evolution of galaxies and relate this to the global evolution of the galaxy population, we present a comparative study of the dependence of star formation rates on the average surface mass densities (Σ_M) of galaxies at 0.5 < z < 0.9 and 0.04 < z < 0.08, using the zCOSMOS and Sloan Digital Sky Survey (SDSS) surveys, respectively. We derive star formation rates, stellar masses, and structural parameters in a consistent way for both samples, and apply them to samples that are complete down to the same stellar mass at both redshifts. We first show that the characteristic step-function dependence of median specific star formation rate (SSFR) on Σ_M in SDSS, seen by Brinchmann et al., is due to the change over from predominantly disk galaxies to predominantly spheroidal galaxies at the surface mass density logΣ_Mchar∼8.5 at which the SSFR is seen to drop. Turning to zCOSMOS, we find a similar shape for the median SSFR–Σ_M relation, but with median SSFR values that are about 5–6 times higher than for SDSS, across the whole range of Σ_M, and in galaxies with both high and low Sersic indices. This emphasizes that galaxies of all types are contributing, proportionally, to the global increase in star formation rate density in the Universe back to these redshifts. The Σ_Mchar "step" shifts to slightly higher values of Σ_M in zCOSMOS relative to SDSS, but this can be explained by a modest differential evolution in the size–mass relations of disk and spheroid galaxies. For low Sersic index galaxies, there is little change in the size–mass relation, as seen by Barden et al., although we suggest that this does not necessarily imply inside-out growth of disks, at least not in this redshift range. On the other hand, there is a modest evolution in the stellar mass–size relation for high Sersic index galaxies, with galaxies smaller by ∼25% at z ∼ 0.7. Taken together these produce a modest increase in Σ_Mchar. Low Sersic index galaxies have a SSFR that is almost independent of Σ_M, and the same is probably also true of high Sersic index galaxies once obvious disk systems are excluded.

We report on optical imaging of the X-ray binary SAX J1808.4 − 3658 with the 8 m Gemini South Telescope. The binary, containing an accretion-powered millisecond pulsar, appears to have a large periodic modulation in its quiescent optical emission. In order to clarify the origin of this modulation, we obtained three time-resolved r'-band light curves (LCs) of the source over five days. The LCs can be described by a sinusoid, and the long time-span between them allows us to determine optical period P = 7251.9 s and phase 0.671 at MJD 54599.0 (TDB; phase 0.0 corresponds to the ascending node of the pulsar orbit), with uncertainties of 2.8 s and 0.008 (90% confidence), respectively. This periodicity is highly consistent with the X-ray orbital ephemeris. By considering this consistency and the sinusoidal shape of the LCs, we rule out the possibility of the modulation arising from the accretion disk. Our study supports the previous suggestion that the X-ray pulsar becomes rotationally powered in quiescence, with its energy output irradiating the companion star, causing the optical modulation. While it has also been suggested that the accretion disk would be evaporated by the pulsar, we argue that the disk exists and gives rise to the persistent optical emission. The existence of the disk can be verified by long-term, multiwavelength optical monitoring of the source in quiescence, as an increasing flux and spectral changes from the source would be expected based on the standard disk-instability model.

We study the Wouthuysen–Field (W–F) coupling at early universe with numerical solutions of the integrodifferential equation describing the kinetics of photons undergoing resonant scattering. The numerical solver is developed based on the weighted essentially nonoscillatory (WENO) scheme for the Boltzmann-like integrodifferential equation. This method has perfectly passed the tests of the analytic solution and conservation property of the resonant scattering equation. We focus on the time evolution of the Wouthuysen–Field (W–F) coupling in relation to the 21 cm emission and absorption at the epoch of reionization. We especially pay attention to the formation of the local Boltzmann distribution, $e^{-(\nu -\nu _0)/kT}$, of photon frequency spectrum around resonant frequency ν₀ within width ν_l, i.e., |ν − ν₀| ⩽ ν_l. We show that a local Boltzmann distribution will be formed if photons with frequency ∼ν₀ have undergone a 10,000 or more times of scattering, which corresponds to the order of 10³ yr for neutral hydrogen density of the concordance ΛCDM model. The time evolution of the shape and width of the local Boltzmann distribution actually do not depend on the details of atomic recoil, photon sources, or initial conditions very much. However, the intensity of photon flux at the local Boltzmann distribution is substantially time dependent. The timescale of approaching the saturated intensity can be as long as 10⁵–10⁶ yr for typical parameters of the ΛCDM model. The intensity of the local Boltzmann distribution at time less than 10⁵ yr is significantly lower than that of the saturation state. Therefore, it may not be always reasonable to assume that the deviation of the spin temperature of 21 cm energy states from cosmic background temperature is mainly due to the W–F coupling if first stars or their emission/absorption regions evolved with a timescale equal to or less than Myr.

At redshifts around 0.1 the Canada–France–Hawaii Telescope Legacy Survey Deep fields contain some 6 × 10⁴ galaxies spanning the mass range from 10⁵ to 10¹² M_☉. We measure the stellar mass dependence of the two-point correlation using angular measurements to largely bypass the errors, approximately 0.02 in the median, of the photometric redshifts. Inverting the power-law fits with Limber's equation we find that the autocorrelation length increases from a very low 0.4 h⁻¹ Mpc at 10^5.5 M_☉ to the conventional 4.5 h⁻¹ Mpc at 10^10.5 M_☉. The power-law fit to the correlation function has a slope which increases from γ ≃ 1.6 at high mass to γ ≃ 2.3 at low mass. The spatial cross-correlation of dwarf galaxies with more massive galaxies shows fairly similar trends, with a steeper radial dependence at low mass than predicted in numerical simulations of subhalos within galaxy halos. To examine the issue of "missing satellites" we combine the cross-correlation measurements with our estimates of the low-mass galaxy number density. We find on the average there are 60 ± 20 dwarfs in subhalos with M(total)>10⁷ M_☉ for a typical Local Group M(total)/M(stars) = 30, corresponding to M/L_V ≃ 100 for a galaxy with no recent star formation. The number of dwarfs per galaxy is about a factor of 2 larger than currently found for the Milky Way. Nevertheless, the average dwarf counts are about a factor of 30 below lambda cold dark matter (LCDM) simulation results. The divergence from LCDM predictions is one of the slope of the relation, approximately dN/dln M ≃ −0.5 rather than the predicted −0.9, not sudden onset at some characteristic scale. The dwarf galaxy star formation rates span the range from passive to bursting, which suggests that there are few completely dark halos.

We explore the ability of high-energy observations to constrain orbital parameters of long-period massive binary systems by means of an inverse Compton (IC) model acting in colliding wind environments. This is particularly relevant for (very) long-period binaries where orbital parameters are often poorly known from conventional methods, as is the case, e.g. for the Wolf-Rayet (W-R) star binary system WR 147, where INTEGRAL and MAGIC upper limits on the high-energy emission have recently been presented. We conduct a parameter study of the set of free quantities describing the yet vaguely constrained geometry and respective effects on the nonthermal high-energy radiation from WR 147. The results are confronted with the recently obtained high-energy observations and with sensitivities of contemporaneous high-energy instruments like Fermi-LAT. For binaries with sufficient long periods, like WR 147, γ-ray attenuation is unlikely to cause any distinctive features in the high-energy spectrum. This leaves the anisotropic IC scattering as the only process that reacts sensitively on the line-of-sight angle with respect to the orbital plane, and therefore allows the deduction of system parameters even from observations not covering a substantial part of the orbit. Provided that particle acceleration acts sufficiently effectively to allow the production of GeV photons through IC scattering, our analysis indicates a preference for WR 147 to possess a large inclination angle. Otherwise, for low inclination angles, electron acceleration is constrained to be less efficient as anticipated here.

We have carried out mid-infrared spectroscopy of seven Galactic protoplanetary nebulae (PPNs) using the Spitzer Space Telescope. They were observed from 10 to 36 μm at relatively high spectral resolution, R ≈ 600. The sample was chosen because they all gave some evidence in the visible of a carbon-rich chemistry. All seven of the sources show the broad, unidentified 21 μm emission feature; three of them are new detections (IRAS 06530 − 0213, 07430+1115, and 19477+2401) and the others are observed at higher signal-to-noise ratio than in previous spectra. These have the same shape and central wavelength (20.1 μm) as found in the Infrared Space Observatory (ISO) spectra of the brighter PPNs. The 30 μm feature was seen in all seven objects. However, it is not resolved into two separate features (26 and 33 μm) as was claimed on the basis of ISO spectra, which presumably suffered from the noisy detector bands in this region. All showed the aromatic infrared bands at 11.3, 12.4, and 13.3 μm. Five of these also appear to have the C₂H₂ molecular band at 13.7 μm, one in absorption and four in emission. This is extremely rare, with only one other evolved star, IRC+10216, in which C₂H₂ emission has been observed. Four also possessed a broad, unidentified emission feature at 15.8 μm that may possibly be related to the 21 μm feature. Model fits were made to the spectral energy distributions for these PPNs to determine properties of the detached circumstellar envelopes. The 21 μm feature has been seen in all Galactic carbon-rich PPNs observed, and thus its carrier appears to be a common component of the outflow around these objects.

The formation of the first stars out of metal-free gas appears to result in stars at least an order of magnitude more massive than in the present-day case. We here consider what controls the transition from a primordial to a modern initial mass function. It has been proposed that this occurs when effective metal line cooling occurs at a metallicity threshold of Z/Z_☉ > 10^−3.5. We study the influence of low levels of metal enrichment on the cooling and collapse of initially ionized gas in small protogalactic halos using three-dimensional, smoothed particle hydrodynamics simulations with particle splitting. Our initial conditions represent protogalaxies forming within a previously ionized H ii region that has not yet had time to cool and recombine. These differ considerably from those used in simulations predicting a metallicity threshold, where the gas was initially cold and only partially ionized. In the centrally condensed potential that we study here, a wide variety of initial conditions for the gas yields a monolithic central collapse. Our models show no fragmentation during collapse to number densities as high as 10⁵ cm⁻³, for metallicities reaching as high as 10⁻¹ Z_☉, far above the threshold suggested by previous work. Rotation allows for the formation of gravitationally stable gas disks over large fractions of the local Hubble time. Turbulence slows the growth of the central density slightly, but both spherically symmetric and turbulent initial conditions collapse and form a single sink particle. We therefore argue that fragmentation at moderate density depends on the initial conditions for star formation more than on the metal abundances present. The actual initial conditions to be considered still need to be determined in detail by observation and modeling of galaxy formation. Metal abundance may still drive fragmentation at very high densities due to dust cooling, perhaps giving an alternative metallicity threshold.

We present the color–magnitude and color–stellar mass diagrams for galaxies with z_phot ≲ 2, based on a K^(AB) < 22 catalog of the $\frac{1}{2} \times \frac{1}{2} \square ^{\circ }$ Extended Chandra Deep Field South from the MUltiwavelength Survey by Yale–Chile. Our main sample of 7840 galaxies contains 1297 M_*>10¹¹M_☉ galaxies in the range 0.2 < z_phot < 1.8. We show empirically that this catalog is approximately complete for M_*>10¹¹ M_☉ galaxies for z_phot < 1.8. For this mass-limited sample, we show that the locus of the red sequence color–stellar mass relation evolves as Δ(u − r) ∝ (−0.44 ± 0.02)z_phot for z_phot ≲ 1.2. For z_phot ≳ 1.3, however, we are no longer able to reliably distinguish red and blue subpopulations from the observed color distribution; we show that this would require much deeper near-infrared data. At 1.5 < z_phot < 1.8, the comoving number density of M_*>10¹¹ M_☉ galaxies is ≈50% of the local value, with a red fraction of ≈33%. Making a parametric fit to the observed evolution, we find n_tot(z) ∝ (1 + z_phot)^{−0.52±0.12(±0.20)}. We find stronger evolution in the red fraction: f_red(z) ∝ (1 + z_phot)^{−1.17±0.18(±0.21)}. Through a series of sensitivity analyses, we show that the most important sources of systematic error are (1) systematic differences in the analysis of the z ≈ 0 and z ≫ 0 samples; (2) systematic effects associated with details of the photometric redshift calculation; and (3) uncertainties in the photometric calibration. With this in mind, we show that our results based on photometric redshifts are consistent with a completely independent analysis which does not require redshift information for individual galaxies. Our results suggest that, at most, 1/5 of local red sequence galaxies with M_*>10¹¹M_☉ were already in place at z ∼ 2.

In this paper, we present results from the complete set of cosmic microwave background (CMB) radiation temperature anisotropy observations made with the Arcminute Cosmology Bolometer Array Receiver (ACBAR) operating at 150 GHz. We include new data from the final 2005 observing season, expanding the number of detector hours by 210% and the sky coverage by 490% over that used for the previous ACBAR release. As a result, the band-power uncertainties have been reduced by more than a factor of two on angular scales encompassing the third to fifth acoustic peaks as well as the damping tail of the CMB power spectrum. The calibration uncertainty has been reduced from 6% to 2.1% in temperature through a direct comparison of the CMB anisotropy measured by ACBAR with that of the dipole-calibrated WMAP5 experiment. The measured power spectrum is consistent with a spatially flat, ΛCDM cosmological model. We include the effects of weak lensing in the power spectrum model computations and find that this significantly improves the fits of the models to the combined ACBAR+WMAP5 power spectrum. The preferred strength of the lensing is consistent with theoretical expectations. On fine angular scales, there is weak evidence (1.1σ) for excess power above the level expected from primary anisotropies. We expect any excess power to be dominated by the combination of emission from dusty protogalaxies and the Sunyaev–Zel'dovich effect (SZE). However, the excess observed by ACBAR is significantly smaller than the excess power at ℓ>2000 reported by the CBI experiment operating at 30 GHz. Therefore, while it is unlikely that the CBI excess has a primordial origin; the combined ACBAR and CBI results are consistent with the source of the CBI excess being either the SZE or radio source contamination.

Modeling the structure formation in the universe, we extend the spherical-collapse model in the context of modified Newtonian dynamics (MOND) starting with the linear Newtonian structure formation followed by the MONDian evolution. In MOND, the formation of structures speeds up without a need for dark matter. Starting with the top-hat overdense distribution of the matter, the structures virialize with a power-law profile of the distribution of matter. We show that the virialization process takes place gradually from the center of the structure to the outer layers. In this scenario, the smaller structures enter the MONDian regime earlier and evolve faster, hence they are older than larger structures. We also show that the virialization of the structures occurs in the MONDian regime, in which the smaller structures have stronger gravitational acceleration than the larger ones. This feature of the dynamical behavior of the structures is in agreement with the fact that the smaller structures, as the globular clusters or galactic bulges, have been formed earlier and need less dark matter in cold dark matter scenario.

We have obtained the first successful interferometric measurements of asteroid sizes and shapes by means of the Very Large Telescope Interferometer-Mid-Infrared Interferometric Instrument (VLTI-MIDI). The VLTI can spatially resolve asteroids in a range of sizes and heliocentric distances that are not accessible to other techniques such as adaptive optics and radar. We have observed, as a typical bench mark, the asteroid (951) Gaspra, visited in the past by the Galileo space probe, and we derive a size in good agreement with the ground truth coming from the in situ measurements by the Galileo mission. Moreover, we have also observed the asteroid (234) Barbara, known to exhibit unusual polarimetric properties, and we found evidence of a potential binary nature. In particular, our data are best fit by a system of two bodies of 37 and 21 km in diameter, separated by a center-to-center distance of ∼24  km (projected along the direction of the baseline at the epoch of our observations).

We report VRI CCD observations of nine Cepheids in the South Polar (Sculptor) Group spiral galaxy NGC 0247. Periods of these Cepheids range from 20 to 70 days. Over the past 20 years, the brightest Cepheid in our sample, NGC 0247:[MF09] C1, has decreased its period by 6%, faded by 0.8 mag in the V band, and become bluer by 0.23 mag in (V−I). A multiwavelength analysis of the Cepheid data yields a true distance modulus of μ_o = 27.81 ± 0.10 mag (3.65 ± 0.17 Mpc) with a total line-of-sight reddening of E(V − I) = 0.07 ± 0.04 mag, after adopting an LMC true distance modulus of 18.5 mag and reddening of E(B − V) = 0.10 mag. These results are in excellent agreement with other very recently published (Cepheid and tip of the red giant branch) distances to NGC 0247. Combining both Cepheid data sets gives μ_o = 27.85 ± 0.09 mag (3.72 ± 0.15 Mpc) with E(V − I) = 0.11 ± 0.03 mag.

The density profile of simulated dark matter structures is fairly well-established, and several explanations for its characteristics have been put forward. In contrast, the radial variation of the velocity anisotropy has still not been explained. We suggest a very simple origin, based on the shapes of the velocity distribution functions, which are shown to differ between the radial and tangential directions. This allows us to derive a radial variation of the anisotropy profile which is in good agreement with both simulations and observations. One of the consequences of this suggestion is that the velocity anisotropy is entirely determined once the density profile is known. We demonstrate how this explains the origin of the γ–β relation, which is the connection between the slope of the density profile and the velocity anisotropy. These findings provide us with a powerful tool, which allows us to close the Jeans equations.

We have carried out a study of active region loops using observations from the Extreme-ultraviolet Imaging Spectrometer (EIS) on board Hinode using 1'' raster data for an active region observed on 2007 May 19. We find that active region structures which are clearly discernible in cooler lines (≈1 MK) become "fuzzy" at higher temperatures (≈2 MK). The active region was comprised of redshifted emissions (downflows) in the core and blueshifted emissions (upflows) at the boundary. The flow velocities estimated in the two regions located near the footpoints of coronal loop showed redshifted emission at transition region temperature and blueshifted emission at coronal temperature. The upflow speed in these regions increased with temperature. For more detailed study we selected one particular well-defined loop. Downward flows are detected along the coronal loop, being stronger in lower-temperature lines (rising up to 60 km s⁻¹ near the footpoint). The downflow was localized toward the footpoint in transition region lines (Si vii) and toward the loop top in high-temperature line (Fe xv). By carefully accounting for the background emission we found that the loop structure was close to isothermal for each position along the loop, with the temperature rising from around 0.8 MK to 1.5 MK from the close to the base to higher up toward the apex (≈75 Mm). We derived electron density using well-established line ratio diagnostic techniques. Electron densities along the active region loop were found to vary from 10¹⁰ cm⁻³ close to the footpoint to 10^8.5 cm⁻³ higher up. A lower electron density, varying from 10⁹ cm⁻³ close to the footpoint to 10^8.5 cm⁻³ higher up, was found for the lower temperature density diagnostic. Using these densities we derived filling factors in along the coronal loop which can be as low as 0.02 near the base of the loop. The filling factor increased with projected height of the loop. These results provide important constraints on coronal loop modeling.

With Spitzer IRS, we have obtained sensitive low-resolution spectroscopy from 5 to 35 μm for six supernova remnants (SNRs) that show evidence of shocked molecular gas: Kes 69, 3C 396, Kes 17, G346.6−0.2, G348.5−0.0, and G349.7+0.2. Bright pure rotational lines of molecular hydrogen are detected at the shock front in all remnants, indicative of radiative cooling from shocks interacting with dense clouds. We find the excitation of H₂ S(0)–S(7) lines in these SNRs requires two nondissociative shock components: a slow 10 km s⁻¹ C-shock through clumps of density 10⁶ cm⁻³, and a faster 40–70 km s⁻¹ C-shock through a medium of density 10⁴ cm⁻³. The ortho-to-para ratio for H₂ in the warm shocked gas is typically found to be much less than the LTE value, suggesting that these SNRs are propagating into cold quiescent clouds. Additionally, a total of 13 atomic fine-structure transitions of Ar⁺, Ar⁺⁺, Fe⁺, Ne⁺, Ne⁺⁺, S⁺⁺, and Si⁺ are detected. The ionic emitting regions are spatially segregated from the molecular emitting regions within the IRS slits. The presence of ionic lines with high appearance potential requires the presence of much faster, dissociative shocks through a lower density medium. The IRS slits are sufficiently wide to include regions outside the SNR which permits emission from diffuse gas around the remnants to be separated from the shocked emission. We find the diffuse H₂ gas projected outside the SNR is excited to a temperature of 100–300 K with a warm gas fraction of at least 0.5%–15% along the line of sight.

We present a detailed study of the Galaxy Evolution Explorer's (GALEX) photometric catalogs with special focus on the statistical properties of the All-sky and Medium Imaging Surveys. We introduce the concept of primaries to resolve the issue of multiple detections and follow a geometric approach to define clean catalogs with well understood selection functions. We cross-identify the GALEX sources (GR2+3) with Sloan Digital Sky Survey (SDSS; DR6) observations, which indirectly provides an invaluable insight into the astrometric model of the UV sources and allows us to revise the band merging strategy. We derive the formal description of the GALEX footprints as well as their intersections with the SDSS coverage along with analytic calculations of their areal coverage. The crossmatch catalogs are made available for the public. We conclude by illustrating the implementation of typical selection criteria in SQL for catalog subsets geared toward statistical analyses, e.g., correlation and luminosity function studies.

On 2006 May 23, we used the ACIS-S instrument on Chandra to study the X-ray emission from the B fragment of comet 73P/2006 (Schwassmann-Wachmann 3) (73P/B). We obtained a total of 20 ks of Chandra observation time of Fragment B, and also investigated contemporaneous Advanced Composition Explorer and Solar and Heliospheric Observatory solar wind physical data. The Chandra data allow us to spatially resolve the detailed structure of the interaction zone between the solar wind and the fragment's coma at a resolution of ∼1000 km, and to observe the X-ray emission due to multiple comet-like bodies. We detect a change in the spectral signature with the ratio of the C V/O VII line increasing with increasing collisional opacity as predicted by Bodewits et al. The line fluxes arise from a combination of solar wind speed, the species that populate the wind, and the gas density of the comet. We are able to understand some of the observed X-ray morphology in terms of nongravitational forces that act upon an actively outgassing comet's debris field. We have used the results of the Chandra observations on the highly fragmented 73P/B debris field to reanalyze and interpret the mysterious emission seen from comet C/1999 S4 (LINEAR) on 2000 August 1, after the comet had completely disrupted. We find the physical situations to be similar in both cases, with extended X-ray emission due to multiple, small outgassing bodies in the field of view. Nevertheless, the two comets interacted with completely different solar winds, resulting in distinctly different spectra.

We present the surface density of luminous active galactic nuclei (AGNs) associated with a uniformly selected galaxy cluster sample identified in the 8.5 deg² Boötes field of the NOAO Deep Wide-Field Survey. The clusters are distributed over a large range of redshift (0 < z < 1.5), and we identify AGN using three different selection criteria: mid-IR color, radio luminosity, and X-ray luminosity. Relative to the field, we note a clear overdensity of the number of AGNs within 0.5 Mpc of the cluster centers at z > 0.5. The amplitude of this AGN overdensity increases with redshift. Although there are significant differences between the AGN populations probed by each selection technique, the rise in cluster AGN surface density generally increases more steeply than that of field quasars. In particular, X-ray-selected AGNs are at least 3 times more prevalent in clusters at 1 < z < 1.5 compared to clusters at 0.5 < z < 1. This effect is stronger than can be explained by the evolving median richness of our cluster sample. We thus confirm the existence of a Butcher–Oemler-type effect for AGN in galaxy clusters, with the number of AGNs in clusters increasing with redshift.

We report results of three-dimensional magnetohydrodynamic simulations of the dynamics of buoyant bubbles in magnetized galaxy cluster media. The simulations are three-dimensional extensions of two-dimensional calculations reported by Jones and De Young. Initially, spherical bubbles and briefly inflated spherical bubbles all with radii a few times smaller than the intracluster medium (ICM) scale height were followed as they rose through several ICM scale heights. Such bubbles quickly evolve into a toroidal form that, in the absence of magnetic influences, is stable against fragmentation in our simulations. This ring formation results from (commonly used) initial conditions that cause ICM material below the bubbles to drive upwards through the bubble, creating a vortex ring; that is, hydrostatic bubbles develop into "smoke rings," if they are initially not very much smaller or very much larger than the ICM scale height. Even modest ICM magnetic fields with β = P_gas/P_mag ≲ 10³ can influence the dynamics of the bubbles, provided the fields are not tangled on scales comparable to or smaller than the size of the bubbles. Quasi-uniform, horizontal fields with initial β ∼ 10² bifurcated our bubbles before they rose more than about a scale height of the ICM, and substantially weaker fields produced clear distortions. These behaviors resulted from stretching and amplification of ICM fields trapped in irregularities along the top surface of the young bubbles. On the other hand, tangled magnetic fields with similar, modest strengths are generally less easily amplified by the bubble motions and are thus less influential in bubble evolution. Inclusion of a comparably strong, tangled magnetic field inside the initial bubbles had little effect on our bubble evolution, since those fields were quickly diminished through expansion of the bubble and reconnection of the initial field.

Distances of galaxies in the Hubble Space Telescope (HST) Key Project are based on the Cepheid period–luminosity relation. An alternative basis is the tip of the red giant branch (TRGB). Using archival HST data, we calibrate surface brightness fluctuations using 16 galaxies with TRGB branch measurements. The TRGB and Cepheid distance scales are shown to be consistent to approximately ±6% in the Hubble constant.

We constrain the iron abundance in a sample of 33 low-ionization Galactic planetary nebulae (PNe) using [Fe iii] lines and correcting for the contribution of higher ionization states with ionization correction factors that take into account uncertainties in the atomic data. We find very low iron abundances in all the objects, suggesting that more than 90% of their iron atoms are condensed onto dust grains. This number is based on the solar iron abundance and implies a lower limit on the dust-to-gas mass ratio, solely due to iron, of M_dust/M_gas ⩾ 1.3 × 10⁻³ for our sample. The depletion factors of different PNe cover about two orders of magnitude, probably reflecting differences in the formation, growth, or destruction of their dust grains. However, we do not find any systematic difference between the gaseous iron abundances calculated for C-rich and O-rich PNe, suggesting similar iron depletion efficiencies in both environments. The iron abundances of our sample PNe are similar to those derived following the same procedure for a group of 10 Galactic H ii regions. These high depletion factors argue for high depletion efficiencies of refractory elements onto dust grains both in molecular clouds and asymptotic giant branch stars, and low dust destruction efficiencies both in interstellar and circumstellar ionized gas.

We present the first results from the largest spectroscopic survey to date of an intermediate redshift galaxy cluster, the z = 0.834 cluster RX J0152.7−1357. We use the colors of galaxies, assembled from a D ∼ 12 Mpc region centered on the cluster, to investigate the properties of the red sequence as a function of density and clustercentric radius. Our wide-field multislit survey with a low-dispersion prism in the Inamori Magellan Areal Camera and Spectrograph at the 6.5 m Baade telescope allowed us to identify 475 new members of the cluster and its surrounding large-scale structure with a redshift accuracy of σ_z/(1 + z) ≈ 1% and a contamination rate of ∼2% for galaxies with i' <23.75 mag. We combine these new members with the 279 previously known spectroscopic members to give a total of 754 galaxies from which we obtain a mass-limited sample of 300 galaxies with stellar masses M>4 × 10¹⁰M_☉ (log M/M_☉>10.6). We find that the red galaxy fraction is 93 ± 3% in the two merging cores of the cluster and declines to a level of 64 ± 3% at projected clustercentric radii R ≳ 3 Mpc. At these large projected distances, the correlation between clustercentric radius and local density is nonexistent. This allows an assessment of the influence of the local environment on galaxy evolution, as opposed to mechanisms that operate on cluster scales (e.g., harassment, ram-pressure stripping). Even beyond R>3 Mpc we find an increasing fraction of red galaxies with increasing local density. The red galaxy fraction at the highest local densities in two large groups at R>3 Mpc matches the red galaxy fraction found in the two cores. Strikingly, galaxies at intermediate densities at R>3 Mpc, that are not obvious members of groups, also show signs of an enhanced red galaxy fraction. Our results point to such intermediate-density regions and the groups in the outskirts of the cluster, as sites where the local environment influences the transition of galaxies onto the red sequence.

Polarized light provides the most reliable source of information at our disposal for diagnosing the physical properties of astrophysical plasmas, including the three-dimensional (3D) structure of the solar atmosphere. Here we formulate and solve the 3D radiative transfer problem of the linear polarization of the solar continuous radiation, which is principally produced by Rayleigh and Thomson scattering. Our approach takes into account not only the anisotropy of the solar continuum radiation but also the symmetry-breaking effects caused by the horizontal atmospheric inhomogeneities produced by the solar surface convection. We show that such symmetry-breaking effects do produce observable signatures in Q/I and U/I, even at the very center of the solar disk where we observe the forward scattering case, but their detection would require obtaining very high resolution linear polarization images of the solar surface. Without spatial and/or temporal resolution U/I ≈ 0 and the only observable quantity is Q/I, whose wavelength variation at a solar disk position close to the limb has been recently determined semi-empirically. Interestingly, our 3D radiative transfer modeling of the polarization of the Sun's continuous spectrum in a well-known 3D hydrodynamical model of the solar photosphere shows remarkable agreement with the semi-empirical determination, significantly better than that obtained via the use of one-dimensional (1D) atmospheric models. Although this result confirms that the above-mentioned 3D model was indeed a suitable choice for our Hanle-effect estimation of the substantial amount of "hidden" magnetic energy that is stored in the quiet solar photosphere, we have found however some small discrepancies whose origin may be due to uncertainties in the semi-empirical data and/or in the thermal and density structure of the 3D model. For this reason, we have paid some attention also to other (more familiar) observables, like the center-limb variation of the continuum intensity, which we have calculated taking into account the scattering contribution to the continuum source function. The overall agreement with the observed center-limb variation turns out to be impressive, but we find a hint that the model's temperature gradients in the continuum-forming layers could be slightly too steep, perhaps because all current simulations of solar surface convection and magnetoconvection compute the radiative flux divergence ignoring the fact that the effective polarizability is not completely negligible, especially in the downward-moving intergranular lane plasma.

Near-infrared (near-IR) spectroscopic data for the five Seyfert galaxies with jet–gas interaction Mrk 348, Mrk 573, Mrk 1066, NGC 7212, and NGC 7465, taken with the Long-slit Intermediate Resolution Infrared Spectrograph near-IR camera/spectrometer at the William Herschel Telescope are reported. The long-slit spectra reveal the characteristic strong emission lines of this type of objects. Many forbidden transitions and hydrogen recombination lines are employed here to study the excitation and ionization mechanisms that are dominating the narrow-line region emission of these objects, that is affected by the radio–jet interaction. Several absorption features are also detected in the H and K bands of these galaxies, allowing us to identify the spectral types that are producing them. We find that the continuum can be reproduced by a combination of late-type stellar templates plus a blackbody component associated with hot dust, mainly contributing to the K-band emission. The detection of the permitted O i and Fe ii lines and broad components of the hydrogen recombination lines in the spectra of Mrk 573 and NGC 7465 allows the reclassification of these two galaxies that are not canonical Type-2 Seyferts: Mrk 573 is confirmed to be an obscured narrow-line Seyfert 1, and NGC 7465 is revealed for the first time as a Type-1 LINER through its near-IR spectrum.

We have mapped the central region of the HH 111 protostellar system in 1.33 mm continuum, C¹⁸O (J = 2 − 1), ¹³CO (J = 2 − 1), and SO (N_J = 5₆ − 4₅) emissions at ∼3'' resolution with the Submillimeter Array. There are two sources, VLA 1 (=IRAS 05491+0247) and VLA 2, with the VLA 1 source driving the HH 111 jet. Thermal emission is seen in 1.33 mm continuum tracing the dust in the envelope and the putative disks around the sources. A flattened, toruslike envelope is seen in C¹⁸O and ¹³CO around the VLA 1 source surrounding the dust lane perpendicular to the jet axis, with an inner radius of ∼400 AU (1''), an outer radius of ∼3200 AU (8''), and a thickness of ∼1000 AU (25). It seems to be infalling toward the center with the conservation of specific angular momentum rather than with a Keplerian rotation as assumed by Yang et  al. An inner envelope is seen in SO, with a radius of ∼500 AU (13). The inner part of this inner envelope, which is spatially coincident with the dust lane, seems to have a differential rotation and thus may have formed a rotationally supported disk. The outer part of this inner envelope, however, may have a rotation velocity decreasing toward the center and thus represent a region where an infalling envelope is in transition to a rotationally supported disk. A brief comparison with a collapsing model suggests that the flattened, toruslike envelope seen in C¹⁸O and ¹³CO could result from a collapse of a magnetized rotating toroid.

Utilizing the CLASS statistical sample, we investigate the constraint of the splitting angle statistic of strong gravitational lenses (SGL) on the equation-of-state parameter w = p/ρ of the dark energy in the flat cold dark matter (CDM) cosmology. Through the comoving number density of dark halos described by the Press–Schechter theory, dark energy affects the efficiency with which dark-matter concentrations produce strong lensing signals. The constraints on both constant w and time-varying w(z) = w₀ + w_az/(1 + z) from the SGL splitting angle statistic are consistently obtained by adopting a two-model combined mechanism of a dark halo density profile matched at the mass scale M_c. Our main observations are that (1) the resulting model parameter M_c is found to be M_c ∼ 1.4 for both constant w and time-varying w(z), which is larger than M_c ∼ 1 obtained in literatures; (2) the fitting results for the constant w are found to be w = −0.89^+0.49_−0.26 and w = −0.94^+0.57_−0.16 for the source redshift distributions of the Gaussian models g(z_s) and g^c(z_s), respectively, which are consistent with the ΛCDM at 95% C.L.; (3) the time-varying w(z) is found to be σ₈ = 0.74: (M_c; w₀, w_a) = (1.36; − 0.92, − 1.31) and (M_c; w₀, w_a) = (1.38; − 0.89, − 1.21) for g(z_s) and g^c(z_s), respectively; the influence of σ₈ is investigated and found to be sizable for σ₈ = 0.74–0.90. After marginalizing the likelihood functions over the cosmological parameters (Ω_M, h, σ₈) and the model parameter M_c, we find that the data of SGL splitting angle statistic lead to the best-fit results (w₀, w_a) = (−0.88^+0.65_−1.03, − 1.55^+1.77_−1.88) and (w₀, w_a) = (−0.91^+0.60_−1.46, − 1.60^+1.60_−2.57) for g(z_s) and g^c(z_s), respectively.

A four-dimensional statistical description of electromagnetic radiation is developed and applied to the analysis of radio pulsar polarization. The new formalism provides an elementary statistical explanation of the modal-broadening phenomenon in single-pulse observations. It is also used to argue that the degree of polarization of giant pulses has been poorly defined in past studies. Single- and giant-pulse polarimetry typically involves sources with large flux-densities and observations with high time-resolution, factors that necessitate consideration of source-intrinsic noise and small-number statistics. Self-noise is shown to fully explain the excess polarization dispersion previously noted in single-pulse observations of bright pulsars, obviating the need for additional randomly polarized radiation. Rather, these observations are more simply interpreted as an incoherent sum of covariant, orthogonal, partially polarized modes. Based on this premise, the four-dimensional covariance matrix of the Stokes parameters may be used to derive mode-separated pulse profiles without any assumptions about the intrinsic degrees of mode polarization. Finally, utilizing the small-number statistics of the Stokes parameters, it is established that the degree of polarization of an unresolved pulse is fundamentally undefined; therefore, previous claims of highly polarized giant pulses are unsubstantiated.

A census of classical T Tauri stars and Herbig Ae/Be stars has been performed around the Orion-Eridanus Superbubble that is ionized and created by the Ori OB1 association. This sample is used to study the spatial distribution of newborn stars, hence the recent star formation sequence, in the region that includes two giant molecular clouds (Orions A and B) and additional smaller clouds (NGC 2149, GN 05.51.4, VdB 64, the Crossbones, the Northern Filament, LDN 1551, LDN 1558, and LDN 1563). Most of the molecular clouds are located on the border of the Superbubble, and associated with Hα filaments and star formation activity, except the Northern Filament which is probably located outside the Superbubble. This suggests that while star formation progresses from the oldest Ori OB1a subgroup to 1b, 1c, and 1d, the Superbubble compresses and initiates starbirth in clouds such as NGC 2149, GN 05.51.4, VdB 64, and the Crossbones, which are located more than 100 pc away from the center of the Superbubble, and even in clouds some 200 pc away, i.e., in LDN 1551, LDN 1558, and LDN 1563. A superbubble appears to have potentially a long-range influence in triggering next-generation star formation in an OB association.

We present a study on the effects of the intracluster medium (ICM) on the interstellar medium (ISM) of 10 Virgo Cluster galaxies using Spitzer far-infrared (FIR) and Very Large Array radio continuum imaging. Relying on the FIR–radio correlation within normal galaxies, we use our infrared data to create model radio maps, which we compare to the observed radio images. For six of our sample galaxies, we find regions along their outer edges that are highly deficient in the radio compared with our models. We also detect FIR emission slightly beyond the observed radio disk along these outer edges. We believe these observations are the signatures of ICM ram pressure. For NGC 4522, we find the radio-deficit region to lie just exterior to a region of high radio polarization and flat radio spectral index, although the total 20 cm radio continuum in this region does not appear strongly enhanced. These characteristics seem consistent for other galaxies with radio polarization data in the literature. The strength of the radio deficit is inversely correlated with the time since peak pressure as inferred from stellar population studies and gas-stripping simulations, consistent with the strength of the radio deficit being a good indicator of the strength of the current ram pressure. We also find that galaxies having local radio deficits appear to have enhanced global radio fluxes. Our preferred physical picture is that the observed radio-deficit regions arise from the ICM wind sweeping away cosmic-ray (CR) electrons and the associated magnetic field, thereby creating synchrotron tails as observed for some of our galaxies. We propose that CR particles are also reaccelerated by ICM-driven shocklets behind the observed radio-deficit regions which, in turn, enhances the remaining radio disk brightness. The high radio polarization and lack of precisely coincident enhancement in the total synchrotron power for these regions suggest shearing, and possibly mild compression of the magnetic field, as the ICM wind drags and stretches the leading edge of the ISM.

We confirm the reality of a reversal of the sign of the Faraday rotation measure in the Galactic plane in Cygnus (Lazio et al. 1990), possibly associated with the Cygnus OB1 association. The rotation measure changes by several hundred rad m⁻² over an angular scale of 2°–5°. We show that a simple model of an expanding plasma shell with an enhanced density and magnetic field can account for the magnitude and angular scale of this feature. This model is consistent with observations of Hα emission as well as other observations in this part of sky. We suggest that this structure is physically associated with a superbubble produced by the Cygnus OB1 association.

Steady magnetic reconnection in the framework of incompressible Hall magnetohydrodynamics is considered. The principal role of the Hall effect in the formation of the structure of the reconnecting current sheet is emphasized. Analytical expressions for the velocity and the magnetic field in the sheet are derived, based on the approximation of a weak two dimensionality of the planar components of the solution. The analytical solution illustrates key features of Hall magnetic reconnection, including the reconnection rate enhancement and the sheet thinning due to the Hall effect, the presence of a quadrupolar axial (out-of-the-plane) magnetic field that controls the geometry of the reconnecting planar magnetic field, and the dynamical coupling of the axial and planar components of the solution, with the coupling strength that is proportional to the ion skin depth. Scalings for the sheet thickness, width, and the reconnection inflow and outflow speeds in terms of the electric resistivity and the axial magnetic field are determined. Implications of the results for fast magnetic reconnection in a weakly collisional plasma of the solar corona are discussed.

We have used Sun–Earth Connection Coronal and Heliospheric Investigation observations obtained from the STEREO A and B spacecraft to study complementary face-on and edge-on views of coronal streamers. The face-on views are analogous to what one might see looking down on a flat equatorial streamer belt at sunspot minimum, and show streamer blobs as diffuse arches gradually expanding outward from the Sun. With the passage of time, the legs of the arches fade, and the ejections appear as a series of azimuthal structures like ripples on a pond. The arched topology is similar to that obtained in face-on views of streamer disconnection events (including in/out pairs and streamer blowout mass ejections), and suggests that streamer blobs have the helical structure of magnetic flux ropes.

We report the serendipitous detection by the XMM-Newton X-ray Observatory of an X-ray source at the position of the Type I (He- and N-rich) bipolar planetary nebula (PN) Hb 5. The Hb 5 X-ray source appears marginally resolved. While the small number of total counts (∼170) and significant off-axis angle of the X-ray source (∼78) precludes a definitive spatial analysis, the morphology of the X-ray emission appears to trace the brightest features seen in optical images of Hb 5. The X-ray spectrum is indicative of a thermal plasma at a temperature between 2.4 and 3.7 MK and appears to display strong Neon emission. The inferred X-ray luminosity is L_X = 1.5 × 10³²ergs⁻¹. These results suggest that the detected X-ray emission is dominated by shock-heated gas in the bipolar nebula, although we cannot rule out the presence of a pointlike component at the position of the central star. The implications for and correspondence with current models of shock-heated gas in PNe is discussed.

We present the results of a parsec-scale polarization study of three FRI radio galaxies—3C66B, 3C78, and 3C264—obtained with Very Long Baseline Interferometry at 5, 8, and 15 GHz. Parsec-scale polarization has been detected in a large number of beamed radio-loud active galactic nuclei, but in only a handful of the relatively unbeamed radio galaxies. We report here the detection of parsec-scale polarization at one or more frequencies in all three FRI galaxies studied. We detect Faraday rotation measures (RMs) of the order of a few hundred rad m⁻² in the nuclear jet regions of 3C78 and 3C264. In 3C66B, polarization was detected at 8 GHz only. A transverse RM gradient is observed across the jet of 3C78. The inner-jet magnetic field, corrected for Faraday rotation, is found to be aligned along the jet in both 3C78 and 3C264, although the field becomes orthogonal further from the core in 3C78. The RM values in 3C78 and 3C264 are similar to those previously observed in nearby radio galaxies. The transverse RM gradient in 3C78, the increase in the degree of polarization at the jet edge, the large rotation in the polarization angles due to Faraday rotation, and the low depolarization between frequencies suggest that a layer surrounding the jet with a sufficient number of thermal electrons and threaded by a toroidal or helical magnetic field is a good candidate for the Faraday rotating medium. This suggestion is tentatively supported by Hubble Space Telescope optical polarimetry but needs to be examined in a greater number of sources.

The ACS Survey of Galactic globular clusters is a Hubble Space Telescope Treasury program designed to provide a new large, deep, and homogeneous photometric database. Based on observations from this program, we have measured precise relative ages for a sample of 64 Galactic globular clusters by comparing the relative position of the clusters' main-sequence (MS) turnoffs, using MS fitting to cross-compare clusters within the sample. This method provides relative ages to a formal precision of 2%–7%. We demonstrate that the calculated relative ages are independent of the choice of theoretical model. We find that the Galactic globular cluster sample can be divided into two groups—a population of old clusters with an age dispersion of ∼5% and no age–metallicity relation, and a group of younger clusters with an age–metallicity relation similar to that of the globular clusters associated with the Sagittarius dwarf galaxy. These results are consistent with the Milky Way halo having formed in two phases or processes. The first one would be compatible with a rapid (<0.8 Gyr) assembling process of the halo, in which the clusters in the old group were formed. Also these clusters could have been formed before re-ionization in dwarf galaxies that would later merge to build the Milky Way halo as predicted by ΛCDM cosmology. However, the galactocentric metallicity gradient shown by these clusters seems difficult to reconcile with the latter. As for the younger clusters, it is very tempting to argue that their origin is related to their formation within Milky Way satellite galaxies that were later accreted, but the origin of the age–metallicity relation remains unclear.

We present the serendipitous discovery of molecular gas CO emission lines with the IRAM Plateau de Bure interferometer coincident with two luminous submillimeter galaxies (SMGs) in the Great Observatories Origins Deep Survey North (GOODS-N) field. The identification of the millimeter emission lines as CO[4–3] at z = 4.05 is based on the optical and near-IR photometric redshifts, radio-infrared photometric redshifts, and Keck+DEIMOS optical spectroscopy. These two galaxies include the brightest submillimeter source in the field (GN20; S_850 μm = 20.3 mJy, z_CO = 4.055 ± 0.001) and its companion (GN20.2; S_850 μm = 9.9 mJy, z_CO = 4.051 ± 0.003). These are among the most distant submillimeter-selected galaxies reliably identified through CO emission and also some of the most luminous known. GN20.2 has a possible additional counterpart and a luminous active galactic nucleus inside its primary counterpart revealed in the radio. Continuum emission of 0.3 mJy at 3.3 mm (0.65 mm in the rest frame) is detected at 5σ for GN20, the first dust continuum detection in an SMG at such long wavelength, unveiling a spectral energy distribution that is similar to local ultra luminous IR galaxies. In terms of CO to bolometric luminosities, stellar mass, and star formation rates (SFRs), these newly discovered z > 4 SMGs are similar to z ∼ 2–3 SMGs studied to date. These z ∼ 4 SMGs have much higher specific star formation rates than those of typical B-band dropout Lyman break galaxies at the same redshift. The stellar mass–SFR correlation for normal galaxies does not seem to evolve much further, between z ∼ 2 and z ∼ 4. A significant z = 4.05 spectroscopic redshift spike is observed in GOODS-N, and a strong spatial overdensity of B-band dropouts and IRAC selected z > 3.5 galaxies appears to be centered on the GN20 and GN20.2 galaxies. This suggests a protocluster structure with total mass ∼10¹⁴ M_☉. Using photometry at mid-IR (24 μm), submillimeter (850 μm), and radio (20 cm) wavelengths, we show that reliable photometric redshifts (Δz/(1 + z) ∼ 0.1) can be derived for SMGs over 1 ≲ z ≲ 4. This new photometric redshift technique has been used to provide a first estimate of the space density of 3.5 < z < 6 hyper-luminous starburst galaxies, and to show that they both contribute substantially to the SFR density at early epochs and that they can account for the presence of old galaxies at z ∼ 2–3. Many of these high-redshift starbursts will be within reach of Herschel. We find that the criterion S_1.4 GHz ≳ S_24 μm, coupled to optical, near-IR and mid-IR photometry, can be used to select z > 3.5 starbursts, regardless of their submillimeter/millimeter emission.

We examine the UV emission from luminous early-type galaxies as a function of redshift. We perform a stacking analysis using Galaxy Evolution Explorer images of galaxies in the NOAO Deep Wide Field Survey Boötes field and examine the evolution in the UV colors of the average galaxy. Our sample, selected to have minimal ongoing star formation based on the optical to mid-IR spectral energy distributions of the galaxies, includes 1843 galaxies spanning the redshift range 0.05 ⩽ z ⩽ 0.65. We find evidence that the strength of the UV excess decreases, on average, with redshift, and our measurements also show moderate disagreement with previous models of the UV excess. Our results show little evolution in the shape of the UV continuum with redshift, consistent either with the binary model for the formation of extreme horizontal branch (EHB) stars or with no evolution in EHB morphology with look-back time. However, the binary formation model predicts that the strength of the UV excess should also be relatively constant, in contradiction with our measured results. Finally, we see no significant influence of a galaxy's environment on the strength of its UV excess.

We report a discovery of extended counterrotating gaseous disks in early-type disk galaxies NGC 2551 and NGC 5631. To find them, we have undertaken complex spectral observations including integral-field spectroscopy for the central parts of the galaxies and long-slit deep spectroscopy to probe the external parts. The line-of-sight velocity fields have been constructed and compared to the photometric structure of the galaxies. As a result, we have revealed full-size counterrotating gaseous disks, the one coplanar to the stellar disk in NGC 2551 and the other inclined to the main stellar disk in NGC 5631. We suggest that we observe the early stages of minor-merger events which may be two different stages of the process of lenticular galaxy formation in rather sparse environments.

This paper presents multiband photometric follow-up observations of the Neptune-mass transiting planet GJ 436b, consisting of five new ground-based transit light curves obtained in 2007 May. Together with one already published light curve, we have at hand a total of six light curves, spanning 29 days. The analysis of the data yields an orbital period P = 2.64386 ± 0.00003 days, midtransit time T_c [HJD] = 2454235.8355 ± 0.0001, planet mass M_p = 23.1 ± 0.9 M_⊕ = 0.073 ± 0.003 M_Jup, planet radius R_p = 4.2 ± 0.2 R_⊕ = 0.37 ± 0.01 R_Jup, and stellar radius R_s = 0.45 ± 0.02 R_☉. Our typical precision for the midtransit timing for each transit is about 30 s. We searched the data for a possible signature of a second planet in the system through transit timing variations (TTV) and variation of the impact parameter. The analysis could not rule out a small, of the order of a minute, TTV and a long-term modulation of the impact parameter, of the order of +0.2 yr⁻¹.

Planets have been observed in tight binary systems with separations less than 20 AU. A likely formation scenario for such systems involves a dynamical capture, after which high relative inclinations are likely and may lead to Kozai oscillations. We numerically investigate the fate of an initially coplanar double-planet system in a class of binaries with separation ranging between 12 and 20 AU. Dynamical integrations of representative four-body systems are performed, each including a hot Jupiter and a second planet on a wider orbit. We find that, although such systems can remain stable at low relative inclinations (≲40°), high relative inclinations are likely to lead to instabilities. This can be avoided if the planets are placed in a Kozai-stable zone within which mutual gravitational perturbations can suppress the Kozai mechanism. We investigate the possibility of inducing Kozai oscillations in the inner orbit by a weak coupling mechanism between the planets in which the coplanarity is broken due to a differential nodal precession. Propagating perturbations from the stellar companion through a planetary system in this manner can have dramatic effects on the dynamical evolution of planetary systems, especially in tight binaries and can offer a reasonable explanation for eccentricity trends among planets observed in binary systems. We find that inducing such oscillations into the orbit of a hot Jupiter is more likely in tight binaries and an upper limit can be set on the binary separation above which these oscillations are not observed.

We exploit the recent observations of extremely metal-poor (EMP) stars in the Galactic halo and investigate the constraints on the initial mass function (IMF) of the stellar population that left these low-mass survivors of [Fe/H] ≲ −2.5 and the chemical evolution that they took part in. In previous study, the high-mass nature of the IMF with typical mass ≃10 M_☉ for the stars of the EMP population and the overwhelming contribution of low-mass members of binaries to the EMP survivors are derived from the statistics of carbon-enriched EMP stars with and without the enhancement of s-process elements. We first examine the analysis to confirm their results for various assumptions on the mass-ratio distribution function of binary members. As compared with the uniform distribution they used, the increase or decrease function of the mass ratio gives a higher- or lower-mass IMF. For the independent distribution a lower-mass IMF results in both the members in the same IMF, but the derived ranges of typical mass differ less than by a factor of 2 and overlap for the extreme cases. Furthermore, we prove that the same constraints are placed on the IMF from the surface density of EMP stars estimated from the surveys and the chemical evolution consistent with the metal yields of theoretical supernova (SN) models. We then apply the derived high-mass IMF with the binary contribution to show that the observed metallicity distribution function (MDF) of EMP stars can be reproduced not only for the shape but also for the number of EMP stars. In particular, the scarcity of stars below [Fe/H] ≃ −4 is naturally explained in terms of hierarchical structure formation, and there is no indication of significant changes in the IMF for the EMP population. The present study indicates that three hyper-metal-poor/ultra-metal-poor stars of [Fe/H] < −4 are the primordial stars that were born as the low-mass members of binaries before the host clouds were polluted by their own SNe.

Synthetic heating of solar coronal loops combining Joule and wave-heating mechanisms on current sheets is proposed. The formation of singular current structures such as current sheets can be caused not only by magnetic reconnection and footpoint convection of coronal loops, but also Alfvén resonance induced by shear flows. On the other hand, with the two-fluid effect, the kinetic Alfvén wave can also be excited on such current structures. Therefore, a synthetic energy dispassion process of both collisional and wave-heating mechanisms on the current structures may lead to a better understanding of the coronal-heating problem.

Observations show that a p-mode may lose up to 70% of its energy flux when it interacts with a sunspot. Part of the absorbed energy is assumed to be converted into other types of waves, while part of it is re-emitted into modes with different radial orders n. In the present paper, we investigate absorption of p-modes with the azimuthal order m = 0 due to their interaction with magnetic flux tubes and attempt to determine the role of mode mixing in this phenomenon. We consider the linearized magnetohydrodynamic equations in two-dimensional, cylindrical geometry, with all the model parameters depending only on radius r and depth z. It is assumed that the wave field may be decomposed into incoming and outgoing components that separately satisfy the governing equations. These components are calculated numerically using a second-order Runge–Kutta finite difference scheme. The calculations reveal substantial scattering from higher-to-lower radial orders n, predominantly into the f-mode (n = 0). Only weak scattering occurs from lower-to-higher radial orders. At the same time, the amount of energy transferred from the p-modes to the f-mode can account for 25%–30% of the energy lost by an incoming p-mode.

The Yuan-Tseh Lee Array for microwave background anisotropy is the first interferometer dedicated to study the cosmic microwave background radiation at 3 mm wavelength. The choice of 3 mm is to minimize the contributions from foreground synchrotron radiation and Galactic dust emission. The initial configuration of seven 0.6 m telescopes mounted on a 6 m hexapod platform was dedicated in 2006 October on Mauna Loa, Hawaii. Scientific operations began with the detection of a number of clusters of galaxies via the thermal Sunyaev–Zel'dovich effect. We compare our data with Subaru weak-lensing data to study the structure of dark matter. We also compare our data with X-ray data to derive the Hubble constant.

We present observations, analysis, and results for the first-year operation of Array for Microwave Background Anisotropy (AMiBA), an interferometric experiment designed to study cosmology via the measurement of cosmic microwave background (CMB). AMiBA is the first CMB interferometer operating at 3 mm to have reported successful results, currently with seven close-packed antennas of 60 cm diameter giving a synthesized resolution of around 6'. During 2007, AMiBA detected the Sunyaev–Zel'dovich effects (SZEs) of six galaxy clusters at redshift 0.091 ⩽ z ⩽ 0.322. An observing strategy with on–off-source switching is used to minimize the effects from electronic offset and ground pickup. Planets were used to test the observational capability of AMiBA and to calibrate the conversion from correlator time-lag data to visibilities. The detailed formalism for data analysis is given. We summarize our early tests including observations of planets and quasars, and present images, visibility profiles, the estimated central coordinates, sizes, and SZE amplitudes of the galaxy clusters. Scientific implications are summarized. We also discuss possible systematic effects in the results.

The Y.T. Lee Array for Microwave Background Anisotropy started scientific operation in early 2007. This work describes the optimization of the system performance for the measurements of the Sunyaev–Zel'dovich effect for six massive galaxy clusters at redshifts 0.09–0.32. We achieved a point-source sensitivity of 63 ± 7 mJy with the seven 0.6 m dishes in 1 hr of on-source integration in two-patch differencing observations. We measured and compensated for the delays between the antennas of our platform-mounted interferometer. Beam switching was used to cancel instrumental instabilities and ground pick up. Total power and phase stability were good on timescales of hours, and the system was shown to integrate down on equivalent timescales of 300 hr per baseline/correlation, or about 10 hr for the entire array. While the broadband correlator leads to good sensitivity, the small number of lags in the correlator resulted in poorly measured bandpass response. We corrected for this by using external calibrators (Jupiter and Saturn). Using Jupiter as the flux standard, we measured the disk brightness temperature of Saturn to be 149⁺⁵₋₁₂ K.

We describe methods used to validate data from the Y.T. Lee Array for Microwave Background Anisotropy (AMiBA), an interferometric array designed to measure the Sunyaev–Zel'dovich effect and the anisotropy of the cosmic microwave background. We perform several statistical tests on data from pointed observations of galaxy clusters taken in 2007 and noise data from long-term blank-sky observations and measurements with the feeds covered by the absorbers. We apply power-spectrum analysis, cross-power-spectrum analysis among different outputs with different time lags in our analog correlator, and sample-variance law tests to noise data. We find that (1) there is no time variation of electronic offsets on the timescale of our two-patch observations (∼10 minutes); (2) noise is correlated by less than 10% between different lags; and (3) the variance of noise scales with the inverse of time. To test the Gaussianity of the data, we apply Kolmogorov–Smirnov tests to cluster data and find that a 5% significance level efficiently detects data sets with known hardware problems without rejecting an excess of acceptable data. We also calculate third- and fourth-order moments and cumulants for the noise residual visibilities and find that about 95% of our data are within the 99% confidence regions of Gaussianity.

We present a multiwavelength analysis of a sample of four hot (T_X > 8 keV) X-ray galaxy clusters (A1689, A2261, A2142, and A2390) using joint AMiBA Sunyaev–Zel'dovich effect (SZE) and Subaru weak-lensing observations, combined with published X-ray temperatures, to examine the distribution of mass and the intracluster medium (ICM) in massive cluster environments. Our observations show that A2261 is very similar to A1689 in terms of lensing properties. Many tangential arcs are visible around A2261, with an effective Einstein radius ∼40'' (at z ∼ 1.5), which when combined with our weak-lensing measurements implies a mass profile well fitted by a Navarro–Frenk–White model with a high concentration c_vir ∼ 10, similar to A1689 and to other massive clusters. The cluster A2142 shows complex mass substructure, and displays a shallower profile (c_vir ∼ 5), consistent with detailed X-ray observations which imply recent interaction. The AMiBA map of A2142 exhibits an SZE feature associated with mass substructure lying ahead of the sharp northwest edge of the X-ray core suggesting a pressure increase in the ICM. For A2390 we obtain highly elliptical mass and ICM distributions at all radii, consistent with other X-ray and strong-lensing work. Our cluster gas fraction measurements, free from the hydrostatic equilibrium assumption, are overall in good agreement with published X-ray and SZE observations, with the sample-averaged gas fraction of 〈f_gas(<r₂₀₀)〉 = 0.133 ± 0.027, for our sample with 〈M_vir〉 = (1.2 ± 0.1) × 10¹⁵ M_☉ h⁻¹. When compared to the cosmic baryon fraction f_b = Ω_b/Ω_m constrained by the WMAP five-year data, this indicates 〈f_gas,200〉/f_b = 0.78 ± 0.16, i.e., (22 ± 16)% of the baryons are missing from the hot phase of clusters.

The Y. T. Lee Array for Microwave Background (AMiBA) has reported the first results on the detection of galaxy clusters via the Sunyaev–Zel'dovich effect. The objectives required small reflectors in order to sample large-scale structures (20'), while interferometry provided modest resolutions (2'). With these constraints, we designed for the best sensitivity by utilizing the maximum possible continuum bandwidth matched to the atmospheric window at 86–102 GHz, with dual polarizations. A novel wide-band analog correlator was designed that is easily expandable for more interferometer elements. Monolithic millimeter-wave integrated circuit technology was used throughout as much as possible in order to miniaturize the components and to enhance mass production. These designs will find application in other upcoming astronomy projects. AMiBA is now in operation since 2006, and we are in the process to expand the array from seven to 13 elements.

The Array for Microwave Background Anisotropy (AMiBA) is the largest hexapod astronomical telescope in current operation. We present a description of this novel hexapod mount with its main mechanical components—the support cone, universal joints, jack screws, and platform—and outline the control system with the pointing model and the operating modes that are supported. The AMiBA hexapod mount performance is verified based on optical pointing tests and platform photogrammetry measurements. The photogrammetry results show that the deformations in the inner part of the platform are less than 120 μm rms. This is negligible for optical pointing corrections, radio alignment, and radio phase errors for the currently operational seven-element compact configuration. The optical pointing error in azimuth and elevation is successively reduced by a series of corrections to about 0 4 rms which meets our goal for the seven-element target specifications.

Pulsar timing can detect the discreteness of the mass distribution in the Galaxy. The jerk imposed on millisecond pulsars by their nearest neighbors, and on the Sun by its nearest neighbor, may be detectable presently or within a decade. The method can detect planets in the outer solar system out to distances of several thousand AU, with the sensitivity rapidly increasing with time.

Most Galaxy-sized systems (M_host ≃ 10¹² M_☉) in the ΛCDM cosmology are expected to have interacted with at least one satellite with a total mass M_sat ≃ 10¹¹ M_☉ ≃ 3M_disk in the past 8 Gyr. Analytic and numerical investigations suggest that this is the most precarious type of accretion for the survival of thin galactic disks because more massive accretion events are relatively rare and less massive ones preserve thin disk components. We use high-resolution, dissipationless N-body simulations to study the response of an initially thin, fully formed Milky Way-type stellar disk to these cosmologically common satellite accretion events, and show that the thin disk does not survive. Regardless of orbital configuration, the impacts transform the disks into structures that are roughly three times as thick and more than twice as kinematically hot as the observed dominant thin disk component of the Milky Way. We conclude that if the Galactic thin disk is a representative case, then the presence of a stabilizing gas component is the only recourse for explaining the preponderance of disk galaxies in a ΛCDM universe; otherwise, the disk of the Milky Way must be uncommonly cold and thin for its luminosity, perhaps as a consequence of an unusually quiescent accretion history.

V2491 Cygni (Nova Cygni 2008 No. 2) was detected as a transient supersoft X-ray source with the Swift XRT as early as 40 days after the outburst, suggesting a very massive white dwarf (WD) close to the Chandrasekhar limit. We present a unified model of near infrared, optical, and X-ray light curves for V2491 Cyg, and have estimated, from our best-fit model, the WD mass to be 1.3 ± 0.02 M_☉ with an assumed chemical composition of the envelope, X = 0.20, Y = 0.48, X_CNO = 0.20, X_Ne = 0.10, and Z = 0.02 by mass weight. We strongly recommend detailed composition analysis of the ejecta because some enrichment of the WD matter suggests that the WD mass does not increase like in RS Oph, which is a candidate of Type Ia supernova progenitors. V2491 Cyg shows a peculiar secondary maximum in the optical light curve as well as V1493 Aql and V2362 Cyg. Introducing magnetic activity as an adding energy source to nuclear burning, we propose a physical mechanism of the secondary maxima.

In the frame of unification schemes for radio-loud active galactic nuclei (AGNs), FR I radio galaxies are believed to be BL Lacertae (BL Lac) objects with the relativistic jet misaligned to our line of sight, and FR II radio galaxies correspond to misaligned radio quasars. The Ledlow–Owen dividing line for the FR I/FR II dichotomy in the optical absolute magnitude of the host galaxy–radio luminosity (M_R–L_Rad) plane can be translated to the line in the black hole mass–jet power (M_bh–Q_jet) plane by using two empirical relations: Q_jet–L_Rad and M_bh–M_R. We use a sample of radio quasars and BL Lac objects with measured black hole masses to explore the relation of the jet power with black hole mass, in which the jet power is estimated from the extended radio emission. It is found that the BL Lac objects are clearly separated from radio quasars by the Ledlow–Owen FR I/II dividing line in the M_bh–Q_jet plane. This strongly supports the unification schemes for FR I/BL Lac object and FR II/radio quasar. We find that the Eddington ratios L_bol/L_Edd of BL Lac objects are systematically lower than those of radio quasars in the sample with a rough division at L_bol/L_Edd ∼ 0.01, and the distribution of Eddington ratios of BL Lac objects/quasars exhibits a bimodal nature, which imply that the accretion mode of BL Lac objects may be different from that of radio quasars.

We present the determination of the geometric R-band albedos of two main-belt comet (MBC) nuclei based on data from the Spitzer Space Telescope and a number of ground-based optical facilities. For 133P/Elst-Pizarro, we find an albedo of p_R = 0.05 ±  0.02 and an effective radius of r_e = 1.9 ±  0.3 km (estimated semiaxes of a ∼ 2.3 km and b ∼ 1.6 km). For 176P/LINEAR, we find an albedo of p_R = 0.06 ±  0.02 and an effective radius of r_e = 2.0 ±  0.2 km (estimated semiaxes of a ∼ 2.6 km and b ∼ 1.5 km). In terms of albedo, 133P and 176P are similar to each other and are typical of other Themis family asteroids, C-class asteroids, and other comet nuclei. We find no indication that 133P and 176P are compositionally unique among other dynamically similar (but inactive) members of the Themis family, in agreement with previous assertions that the two objects most likely formed in situ. We also note that low albedo (p_R < 0.075) remains a consistent feature of all cometary (i.e., icy) bodies, whether they originate in the inner solar system (the MBCs) or in the outer solar system (all other comets).

We report on a signature of chromospheric downflows in two emerging flux regions detected by time–distance helioseismology analysis. We use both chromospheric intensity oscillation data in the Ca ii H line and photospheric Dopplergrams in the Fe i 557.6 nm line obtained by Hinode/SOT for our analyses. By cross-correlating the Ca ii oscillation signals, we have detected a travel-time anomaly in the plage regions; outward travel times are shorter than inward travel times by 0.5–1 minute. However, such an anomaly is absent in the Fe i data. These results can be interpreted as evidence of downflows in the lower chromosphere. The downflow speed is estimated to be below 10 km s^-1. This result demonstrates a new possibility of studying chromospheric flows by time–distance analysis.

The data collected in the Shapley–Ames catalog of bright galaxies show that lenticular (S0) galaxies are typically about a magnitude fainter than both elliptical (E) and early spiral (Sa) galaxies. Hubble was therefore wrong to regard S0 galaxies as being intermediate between morphological types E and Sa. The observation that E5–E7 galaxies are significantly fainter than objects of subtypes E0–E5 suggests that many of the flattest "ellipticals" may actually be misclassified lenticular galaxies. In particular, it is tentatively suggested all E7 galaxies might actually be misclassified S0₁(7) galaxies. The present results are consistent with the view that galaxies belonging to the S0 class evolved in environments in which they typically lost more than half of their original luminous material.

In hierarchical structure formation, merging of galaxies is frequent and known to dramatically affect their properties. To comprehend these interactions high-resolution simulations are indispensable because of the nonlinear coupling between pc and Mpc scales. To this end, we present the first adaptive mesh refinement (AMR) simulation of two merging, low mass, initially gas-rich galaxies (1.8 × 10¹⁰ M_☉ each), including star formation and feedback. With galaxies resolved by ∼2 × 10⁷ total computational elements, we achieve unprecedented resolution of the multiphase interstellar medium, finding a widespread starburst in the merging galaxies via shock-induced star formation. The high dynamic range of AMR also allows us to follow the interplay between the galaxies and their embedding medium depicting how galactic outflows and a hot metal-rich halo form. These results demonstrate that AMR provides a powerful tool in understanding interacting galaxies.

The interpretation of imagery of the solar chromosphere in the widely used Ca ii 854.2 nm infrared line is hampered by its complex, three-dimensional, and non-LTE formation. Forward modeling is required to aid understanding. We use a three-dimensional non-LTE radiative transfer code to compute synthetic Ca ii 854.2 nm images from a radiation-MHD simulation of the solar atmosphere spanning from the convection zone to the corona. We compare the simulation with observations obtained with the CRISP filter at the Swedish 1 m Solar Telescope. We find that the simulation reproduces dark patches in the blue line wing caused by Doppler shifts, brightenings in the line core caused by upward-propagating shocks, and thin dark elongated structures in the line core that form the interface between upward and downward gas motion in the chromosphere. The synthetic line core is narrower than the observed one, indicating that the Sun exhibits both more vigorous large-scale dynamics as well as small scale motions that are not resolved within the simulation, presumably owing to a lack of spatial resolution.

It is widely believed that the low-frequency quasi-periodic X-ray oscillations observed in microquasars are correlated to, but do not originate at, the physical radius of the inner edge of the accretion disk. Models relating the quasi-periodic oscillation (QPO) frequency and color radius are hindered by observations showing contradicting trend correlations between the microquasars GRO 1655−40, XTE J1550−564, and GRS 1915+105. The first shows a negative correlation and the latter two a positive one. By taking into account relativistic rotation in the accretion disk, the accretion–ejection instability (AEI) model predicts a turnover in the frequency–radius relationship, and has been successfully compared with observations of GRO J1655−40 and GRS 1915+105. We present further evidence supporting the AEI model prediction by using observations of the microquasar GRS 1915+105. By combining a data set including θ-, β-, and α-class X-ray light curves, we observe positive, negative, and null correlations in the frequency–radius relationship. This is the first time a single source has shown a possible inversion in the QPO frequency–color radius curve predicted by the AEI model.

We present the optical and X-ray properties of four clusters recently discovered by the South Pole Telescope (SPT) using the Sunyaev–Zel'dovich effect (SZE). The four clusters are located in one of the common survey areas of the southern sky that is also being targeted by the Atacama Cosmology Telescope (ACT) and imaged by the CTIO Blanco 4 m telescope. Based on publicly available griz optical images and XMM-Newton and ROSAT X-ray observations, we analyze the physical properties of these clusters and obtain photometric redshifts, luminosities, richness, and mass estimates. Each cluster contains a central elliptical whose luminosity is consistent with SDSS cluster studies. Our mass estimates are well above the nominal detection limit of the SPT and ACT; the new SZE clusters are very likely massive systems with M ≳ 5 × 10¹⁴ M_☉.

We report the discovery of a high proper motion L subdwarf (μ = 0617 yr⁻¹) in the Sloan Digital Sky Survey (SDSS) spectral database. The optical spectrum from the star SDSS J125637–022452 has mixed spectral features of both late-M spectral subtype (strong TiO and CaH at 7000 Å) and mid-L spectral subtype (strong wings of KI at 7700 Å, CrH, and FeH), which is interpreted as the signature of a very low-mass, metal-poor star (ultracool subdwarf) of spectral-type sdL. The near-infrared (NIR) (J − K_s) colors from 2MASS show the object to be significantly bluer compared to normal L dwarfs, which is probably due to a strong collision-induced absorption (CIA) from the H₂ molecule. This is consistent with the idea that CIA from H₂ is more pronounced at low metallicities. Proper motion and radial velocity measurements also indicate that the star is kinematically "hot" and probably associated with the Galactic halo population.

We present a spectroscopic study of Leo V, a recently discovered satellite of the Milky Way (MW). From stellar spectra obtained with the MMT/Hectochelle spectrograph we identify seven likely members of Leo V. Five cluster near the Leo V center (R < 3') and have a velocity dispersion of 2.4^+2.4_−1.4 km s⁻¹. The other two likely members lie near each other but far from the center (R ∼ 13' ∼ 700 pc) and inflate the global velocity dispersion to 3.7^+2.3_−1.4 km s⁻¹. Assuming the five central members are bound, we obtain a dynamical mass of M = 3.3^+9.1_−2.5 × 10⁵M_☉ (M/L_V = 75⁺²³⁰₋₅₈[M/L_V]_☉). From the stacked spectrum of the five central members we estimate a mean metallicity of [Fe/H]=−2.0 ± 0.2 dex. Thus, with respect to dwarf spheroidals of similar luminosity, Leo V is slightly less massive and slightly more metal rich. Since we resolve the central velocity dispersion only marginally, we do not rule out the possibility that Leo V is a diffuse star cluster devoid of dark matter. The wide separation of its two outer members implies Leo V is losing mass; however, its large distance (D ∼ 180 kpc) is difficult to reconcile with MW tidal stripping unless the orbit is very radial.

Three planets have been directly imaged around the young star HR 8799. The planets are 5–13 M_Jup and orbit the star at projected separations of 24–68 AU. While the initial detection occurred in 2007, two of the planets were recovered in a reanalysis of data obtained in 2004. Here we present a detection of the furthest planet of that system, HR 8799 b, in archival Hubble Space Telescope (HST)/Near Infrared Camera and Multi-Object Spectrometer (NICMOS) data from 1998. The detection was made using the locally optimized combination of images algorithm to construct, from a large set of HST/NICMOS images of different stars taken from the archive, an optimized reference point-spread function image used to subtract the light of the primary star from the images of HR 8799. This new approach improves the sensitivity to planets at small separations by a factor of ∼10 compared to traditional roll deconvolution. The new detection provides an astrometry point 10 years before the most recent observations, and is consistent with a Keplerian circular orbit with a∼ 70 AU and low orbital inclination. The new photometry point, in the F160W filter, is in good agreement with an atmosphere model with intermediate clouds and vertical stratification, and thus suggests the presence of significant water absorption in the planet's atmosphere. The success of the new approach used here highlights a path for the search and characterization of exoplanets with future space telescopes, such as the James Webb Space Telescope or a Terrestrial Planet Finder.

We analyze the variability in accretion-related emission lines for 40 Classical T Tauri stars to probe the extent of accretion variations in young stellar objects. Our analysis is based on multi-epoch high-resolution spectra for young stars in Taurus-Auriga and Chamaeleon I. For all stars, we typically obtain four spectra, covering timescales from hours to months. As proxies for the accretion rate, we use the Hα 10% width and the Ca ii-λ8662 line flux. We find that while the two quantities are correlated, their variability amplitude is not. Converted to accretion rates, the Ca ii fluxes indicate typical accretion rate changes of 0.35 dex, with 32% exceeding 0.5 dex, while Hα 10% width suggests changes of 0.65 dex, with 66% exceeding 0.5 dex. We conclude that Ca ii fluxes are a more robust quantitative indicator of accretion than Hα 10% width, and that intrinsic accretion rate changes typically do not exceed 0.5 dex on timescales of days to months. The maximum extent of the variability is reached after a few days, suggesting that rotation is the dominant cause of variability. We see a decline of the inferred accretion rates toward later spectral types, reflecting the $\dot{M}$ versus M relationship. There is a gap between accretors and nonaccretors, pointing to a rapid shutdown of accretion. We conclude that the ∼two orders of magnitude scatter in the $\dot{M}$ versus M relationship is dominated by object-to-object scatter instead of intrinsic source variability.

Galaxies above redshift 1 can be very clumpy, with irregular morphologies dominated by star complexes as large as 2 kpc and as massive as a few ×10⁸ or 10⁹M_☉. Their co-moving densities and rapid evolution suggest that most present-day spirals could have formed through a clumpy phase. The clumps may form by gravitational instabilities in gas-rich turbulent disks; they do not appear to be separate galaxies merging together. We show here that the formation of the observed clumps requires initial disks of gas and stars with almost no stabilizing bulge or stellar halo. This cannot be achieved in models where disk galaxies grow by mergers. Mergers tend to make stellar spheroids even when the gas fraction is high, and then the disk is too stable to make giant clumps. The morphology of high-redshift galaxies thus suggests that inner disks assemble mostly by smooth gas accretion, either from cosmological flows or from the outer disk during a grazing interaction.

In this Letter, we studied the time evolution of the energy-dependent spectral indices for the 2004 November 3 solar hard X-ray flare observed by RHESSI. The common soft–hard–soft (SHS) pattern spectra were found at the lower energies, while a new feature, hard–soft–hard (HSH), was found at higher energies for each subpeak. As the energy increases, the SHS pattern is gradually converted into the HSH pattern. Some possible explanations for the spectral evolution and its energy dependence are discussed, such as the return current.

We investigate the relationship between black hole mass and bulge luminosity for active galactic nuclei (AGNs) with reverberation-based black hole mass measurements and bulge luminosities from two-dimensional decompositions of Hubble Space Telescope host galaxy images. We find that the slope of the relationship for AGNs is 0.76–0.85 with an uncertainty of ∼0.1, somewhat shallower than the M_BH ∝ L^1.0±0.1 relationship that has been fit to nearby quiescent galaxies with dynamical black hole mass measurements. This difference is somewhat perplexing, as the AGN black hole masses include an overall scaling factor that brings the AGN M_BH–σ_⋆ relationship into agreement with that of quiescent galaxies. We discuss biases that may be inherent to the AGN and quiescent galaxy samples and could cause the apparent inconsistency in the forms of their M_BH–L_bulge relationships. Recent work by Graham, however, presents a similar slope of ∼0.8 for the quiescent galaxies and may bring the relationship for AGNs and quiescent galaxies into agreement.

Based on a suite of Monte Carlo simulations, I show that a stellar-mass-dependent lifetime of the gas disks from which planets form can explain the lack of hot Jupiters/close-in giant planets around high-mass stars and other key features of the observed semimajor axis distribution of radial velocity-detected giant planets. Using reasonable parameters for the Type II migration rate, regions of planet formation, and timescales for gas giant core formation, I construct synthetic distributions of Jovian planets. A planet formation/migration model assuming a stellar-mass-dependent gas disk lifetime reproduces key features in the observed distribution by preferentially stranding planets around high-mass stars at large semimajor axes.