Table of contents

Galaxy cluster merger statistics are an important component in understanding the formation of large-scale structure. Cluster mergers are also potential sources of systematic error in the mass calibration of upcoming cluster surveys. Unfortunately, it is difficult to study merger properties and evolution directly because the identification of cluster mergers in observations is problematic. We use large N-body simulations to study the statistical properties of massive halo mergers, specifically investigating the utility of close halo pairs as proxies for mergers. We examine the relationship between pairs and mergers for a wide range of merger timescales, halo masses, and redshifts (0 < z < 1). We also quantify the utility of pairs in measuring merger bias. While pairs at very small separations will reliably merge, these constitute a small fraction of the total merger population. Thus, pairs do not provide a reliable direct proxy to the total merger population. We do find an intriguing universality in the relation between close pairs and mergers, which in principle could allow for an estimate of the statistical merger rate from the pair fraction within a scaled separation, but including the effects of redshift space distortions strongly degrades this relation. We find similar behavior for galaxy-mass halos, making our results applicable to field galaxy mergers at high redshift. We investigate how the halo merger rate can be statistically described by the halo mass function via the merger kernel (coagulation), finding an interesting environmental dependence of merging: halos within the mass resolution of our simulations merge less efficiently in overdense environments. Specifically, halo pairs with separations less than a few h⁻¹ Mpc are more likely to merge in underdense environments; at larger separations, pairs are more likely to merge in overdense environments.

We apply machine learning in the form of a nearest neighbor instance-based algorithm (NN) to generate full photometric redshift probability density functions (PDFs) for objects in the Fifth Data Release of the Sloan Digital Sky Survey (SDSS DR5). We use a conceptually simple but novel application of NN to generate the PDFs, perturbing the object colors by their measurement error and using the resulting instances of nearest neighbor distributions to generate numerous individual redshifts. When the redshifts are compared to existing SDSS spectroscopic data, we find that the mean value of each PDF has a dispersion between the photometric and spectroscopic redshift consistent with other machine learning techniques, being σ = 0.0207 ± 0.0001 for main sample galaxies to r < 17.77 mag, σ = 0.0243 ± 0.0002 for luminous red galaxies to r≲ 19.2 mag, and σ = 0.343 ± 0.005 for quasars to i < 20.3 mag. The PDFs allow the selection of subsets with improved statistics. For quasars, the improvement is dramatic: for those with a single peak in their probability distribution, the dispersion is reduced from 0.343 to σ = 0.117 ± 0.010, and the photometric redshift is within 0.3 of the spectroscopic redshift for 99.3% ± 0.1% of the objects. Thus, for this optical quasar sample, we can virtually eliminate "catastrophic" photometric redshift estimates. In addition to the SDSS sample, we incorporate ultraviolet photometry from the Third Data Release of the Galaxy Evolution Explorer All-Sky Imaging Survey (GALEX AIS GR3) to create PDFs for objects seen in both surveys. For quasars, the increased coverage of the observed-frame UV of the SED results in significant improvement over the full SDSS sample, with σ = 0.234 ± 0.010. We demonstrate that this improvement is genuine and not an artifact of the SDSS-GALEX matching process.

We have conducted a systematic survey for intervening O VI absorbers in available echelle spectra of 16 QSOs at z_QSO = 0.17–0.57. These spectra were obtained using HST STIS with the E140M grating. Our search uncovered a total of 27 foreground O VI absorbers with rest-frame absorption equivalent width W_r(1031) ≳ 25 mÅ. Ten of these QSOs exhibit strong O VI absorbers in their vicinity. Our O VI survey does not require the known presence of Ly α , and the echelle resolution allows us to identify the O VI absorption doublet based on their common line centroid and known flux ratio. We estimate the total redshift survey path, Δ z, using a series of Monte Carlo simulations, and find that Δ z = 1.66,2.18, and 2.42 for absorbers of strength W_r = 30,50, and 80 mÅ, respectively, leading to a number density of d(W ⩾ 50 mÅ)/dz = 6.7 ± 1.7 and d(W ⩾ 30 mÅ)/dz = 10.4 ± 2.2. In contrast, we also measure d/dz = 27 ± 9 for O VI absorbers of W_r > 50 mÅ at | Δ v| < 5000 km s⁻¹ from the background QSOs. Using the random sample of O VI absorbers with well-characterized survey completeness, we estimate a mean cosmological mass density of the O VI gas Ω (O^{5 +}) h = (1.7 ± 0.3) × 10⁻⁷. In addition, we show that <5% of O VI absorbers originate in underdense regions that do not show a significant trace of H I. Furthermore, we show that the neutral gas column N(H I) associated with these O VI absorbers spans nearly 5 orders of magnitude, and shows moderate correlation with N(O VI). Finally, while the number density of O VI absorbers varies substantially from one sight line to another, it also appears to be inversely correlated with the number density of H I absorbers along individual lines of sight.

We study the evolution of the number of close companions of similar luminosities per galaxy (N_c) by choosing a volume-limited subset of the photometric redshift catalog from the Red-Sequence Cluster Survey (RCS-1). The sample contains over 157,000 objects with a moderate redshift range of 0.25 ⩽ z⩽ 0.8 and M_Rc ⩽ − 20. This is the largest sample used for pair evolution analysis, providing data over nine redshift bins with about 17,500 galaxies in each. After applying incompleteness and projection corrections, N_c shows a clear evolution with redshift. The N_c value for the whole sample grows with redshift as (1 + z)^m, where m = 2.83 ± 0.33 in good agreement with N-body simulations in a ΛCDM cosmology. We also separate the sample into two different absolute magnitude bins: -25 ⩽ M_Rc ⩽ − 21 and -21 < M_Rc ⩽ − 20, and find that the brighter the absolute magnitude, the smaller the m-value. Furthermore, we study the evolution of the pair fraction for different projected separation bins and different luminosities. We find that the m-value becomes smaller for larger separation, and the pair fraction for the fainter luminosity bin has stronger evolution. We derive the major merger remnant fraction f_rem = 0.06, which implies that about 6% of galaxies with -25 ⩽ M_Rc ⩽ − 20 have undergone major mergers since z = 0.8.

We present the number counts of K_s-band-selected high-redshift galaxy populations such as extremely red objects (EROs), B-, z-, and K-band-selected galaxies (BzKs) and distant red galaxies (DRGs) in the AKARI NEP field. These high-redshift galaxy samples are extracted from a multicolor catalog combining optical data from Suprime-Cam on the 8.2 m Subaru Telescope with near-infrared data from the Florida Multiobject Imaging Near-IR Grism Observational Spectrometer on the Kitt Peak National Observatory 2.1 m telescope over 540 arcmin² in the NEP region field. The final catalog contains 308 EROs (K_s < 19.0; 54% are dusty star-forming EROs, and the rest are passive old EROs), 137 star-forming BzKs, and 38 passive old BzKs (K_s < 19.0) and 64 DRGs (K_s < 18.6). We also produce individual component source counts for both the dusty star-forming and passive populations. We compare the observed number counts of the high redshift passively evolving galaxy population with a backward pure luminosity evolution (PLE) model allowing different degrees of number density evolution. We find that the PLE model without density evolution fails to explain the observed counts at faint magnitudes, while the model incorporating negative density evolution is consistent with the observed counts of the passively evolving population. We also compare our observed counts of dusty star-forming EROs with a phenomenological evolutionary model postulating that the near-infrared EROs can be explained by the source densities of the far-infrared-submillimeter populations. Our model predicts that the dusty ERO source counts can be explained assuming a 25% contribution of submillimeter star-forming galaxies with the majority of brighter K_s-band-detected dusty EROs having luminous (rather than HR 10 type ultraluminous) submillimeter counterparts. We propose that the fainter K_s > 19.5 population is dominated by the submillijansky submillimeter population. We also predict a turnover in dusty ERO counts around 19 < K_s < 20.

We establish a simple model for the distribution of cool (~10⁴ K) gas around L^⋆ galaxies using the properties of strong Mg II absorption line systems as observational constraints. Our analysis suggests that the halos of L^⋆ galaxies are filled with cool gas clouds having sizes of order 1 kpc and densities of ~10⁻² cm⁻³. We then investigate the physical conditions of a similar ensemble of clouds situated around a central quasar. We show that the flux from the quasar gives rise to (1) extended narrow line emission on ~100 kpc scales and (2) an anisotropy in the properties of the absorbing gas arising from the geometry of the quasar radiation field. Provided that quasars reside in gaseous halos more massive than those of L^⋆ galaxies, our predictions agree with the results from detections of both narrow emission line nebulae and ~100 kpc Mg II-absorbing halos around quasars, suggesting a common origin for these phenomena. We discuss the implications of our results for understanding quasar absorption line systems, quasar environments at high redshifts, and the quasar unification scheme.

We have mapped the central region of the Seyfert 1 galaxy NGC 1097 in ¹²CO J = 2–1 with the Submillieter Array (SMA). The ¹²CO J = 2–1 map shows a central concentration and a surrounding ring coinciding, respectively, with the Seyfert nucleus and a starburst ring. The line intensity peaks at the nucleus, whereas in a previously published ¹²CO J = 1–0 map the intensity peaks at the starburst ring. The azimuthally averaged ¹²CO J = 1–0 intensity ratio R₂₁ of the ring is about unity, which is similar to those in nearby active star-forming galaxies, suggesting that most of the molecular gas in the ring is involved in fueling the starburst. The ratio of molecular gas to dynamical mass in the starburst ring shows a somewhat lower value than that found in nearby star-forming galaxies, suggesting that the high R₂₁ of unity may be caused by additional effects, such as shocks induced by gas infall along the bar. The molecular gas can last for about 1.2 × 10⁸ yr without further replenishment, assuming a constant star formation rate. The central gas is rotating with the molecular ring in the same direction, while its velocity gradient is steeper than that of the ring, and similar to what usually observed in Seyfert 2 galaxies. To view the Seyfert nucleus without obscuration, the central gas can be a low-inclined disk or torus but not too low to be less massive than the mass of the host galaxy, or be a highly inclined thin disk or clumpy and thick torus, inner part of the galactic disk is also possible. The R₂₁ of ~1.9 of the central gas is significantly higher than that of the ring, indicates that the activity of the Seyfert nucleus may significant influence the central gas.

We present distance estimates for 11 peculiar Virgo Cluster spiral galaxies based on measurements of the stellar kinematics of their central 2 kpc. Stellar circular velocities were obtained using two-integral dynamical models. Distances were obtained by comparing, at each radius, the stellar circular velocities with synthetic Hα rotation curves derived from NIR Tully-Fisher relations. The results show that most of our galaxies are located within 4 Mpc of the core of the cluster. Three of these galaxies, previously classified as "low rotator galaxies" or with "truncated/compact" Hα radial distributions, have stellar kinematics-based distances that are discrepant with H I-based distances by at least 60% and are likely to be located within the virial radius of the cluster. These discrepancies appear due to very truncated gas distributions plus noncircular gas motions or gas motions not in the plane of the stellar disk, perhaps as the result of gravitational interactions. Our results show that environmental effects can significantly reduce the measured H I line widths for some disturbed cluster galaxies, thus affecting the accurate determination of distances based on gas kinematics methods.

We investigate the physical properties of tidal structures in a disk galaxy created by gravitational interactions with a companion using numerical N-body simulations. We consider a simple galaxy model consisting of a rigid halo/bulge and an infinitesimally thin stellar disk with Toomre parameter Q ≈ 2. A perturbing companion is treated as a point mass moving on a prograde parabolic orbit, with varying mass and pericenter distance. Tidal interactions produce well-defined spiral arms and extended tidal features, such as bridge and tail, that are all transient, but distinct in nature. In the extended disks, a strong tidal force is able to lock the perturbed epicycle phases of the near-side particles to the perturber, shaping them into a tidal bridge that corotates with the perturber. A tidal tail develops on the opposite side as strongly perturbed, near-side particles overtake mildly perturbed, far-side particles. The tail is essentially a narrow material arm with a roughly logarithmic shape, dissolving with time because of large velocity dispersions. Inside the disks where the tidal force is relatively weak, on the other hand, a two-armed logarithmic spiral pattern emerges due to the kinematic alignment of perturbed particle orbits. While self-gravity makes the spiral arms a bit stronger, the arms never become fully self-gravitating, wind up progressively with time, and decay almost exponentially after the peak in a timescale of ~1 Gyr. The arm pattern speed varying with both radius and time converges to Ω − κ/2 at late time, suggesting that the pattern speed of tidally driven arms may depend on radius in real galaxies. Here Ω and κ denote the angular and epicycle frequencies, respectively. We present the parametric dependences of various properties of tidal features on the tidal strength and discuss our findings as applied to tidal spiral arms in grand-design spiral galaxies.

Extragalactic sources from the IRAS Faint Source Catalog (FSC) that have the optically faintest magnitudes (E≳ 18) were selected by spatial coincidence with a source in the FIRST radio survey, and 28 of these sources have been observed with the Infrared Spectrograph on Spitzer (IRS). While an infrared source is always detected with the IRS at the FIRST position, only ~50% of the infrared sources are real FSC detections, as estimated from the number of sources for which the f_ν(25 μm) determined with the IRS is fainter than the sensitivity limit for the FSC. Sources have 0.12 < z < 1.0 and luminosities (ergs s⁻¹) 43.3 < log [ ν L_ν(5.5 μ m) ] < 46.7, encompassing the range from local ULIRGs to the most luminous sources discovered by Spitzer at z ∼ 2. Detectable PAH features are found in 15 of the sources (54%), and measurable silicate absorption is found in 19 sources (68%); both PAH emission and silicate absorption are present in 11 sources. PAH luminosities are used to determine the starburst fraction of bolometric luminosity, and model predictions for a dusty torus are used to determine the AGN fraction of luminosity in all sources based on νL_ν(5.5 μm). Approximately half of the sources have luminosity dominated by an AGN and approximately half by a starburst. The ratio of infrared to radio flux, defined as q = log [ f_ν(25 μ m)/f_ν(1.4 GHz) ] , does not distinguish between AGNs and starbursts for these sources.

We propose a model for allocating galaxies in cosmological N-body simulations. We identify each subhalo with a galaxy and assign luminosity and morphological type, assuming that the galaxy luminosity is a monotonic function of the host subhalo mass. Morphology is assigned using two simple relations between the subhalo mass and galaxy luminosity for different galaxy types. The first uses a constant luminosity ratio between early-type (E/SO) and late-type (S/Irr) galaxies at a fixed subhalo mass. The other assumes that galaxies of different morphological types but equal luminosity have a constant ratio of subhalo mass. We made a series of comparisons of the properties of these mock galaxies with those of SDSS galaxies. The resulting mock galaxy sample is found to successfully reproduce the observed local number density distribution except in high-density regions. We study the luminosity function as a function of local density, and find that the observed luminosity functions in different local density environments are overall well reproduced by the mock galaxies. A discrepancy is found at the bright end of the luminosity function of early types in the underdense regions and at the faint end of both morphological types in very high density regions. A significant fraction of the observed early-type galaxies in voids seem to have undergone relatively recent star formation and become brighter. The lack of faint mock galaxies in dense regions may be due to the strong tidal force of the central halo, which destroys less massive satellite subhalos around the simulation. The mass-to-light ratio is found to depend on the local density in a way similar to that observed in the SDSS sample. We have found an impressive agreement between our mock galaxies and the SDSS galaxies in the dependence of central velocity dispersion on the local density and luminosity.

Using the Tuorla-Heidelberg model for the mass distribution of the Milky Way, I determine the rotation curve predicted by MOND (modified Newtonian dynamics). The result is in good agreement with the observed terminal velocities interior to the solar radius and with estimates of the Galaxy's rotation curve exterior thereto. There are no fit parameters: given the mass distribution, MOND provides a good match to the rotation curve. The Tuorla-Heidelberg model does allow for a variety of exponential scale lengths; MOND prefers short scale lengths in the range 2.0 kpc ≲ R_d≲ 2.5 kpc. The favored value of R_d depends somewhat on the choice of interpolation function. There is some preference for the "simple" interpolation function as found by Famaey & Binney. I introduce an interpolation function that shares the advantages of the simple function on galaxy scales while having a much smaller impact in the solar system. I also solve the inverse problem, inferring the surface mass density distribution of the Milky Way from the terminal velocities. The result is a Galaxy with "bumps and wiggles" in both its luminosity profile and rotation curve that are reminiscent of those frequently observed in external galaxies.

The median observed velocity width v₉₀ of low-ionization species in damped Lyα systems is close to 90 km s⁻¹, with ~10% of all systems showing v₉₀≳ 210 km s⁻¹ at z = 3. We show that a relative shortage of such high-velocity neutral gas absorbers in state-of-the-art galaxy formation models is a fundamental problem, present in both grid-based and particle-based numerical simulations. Using a series of numerical simulations of varying resolution and box size to cover a wide range of halo masses, we demonstrate that energy from gravitational infall alone is insufficient to produce the velocity dispersion observed in damped Lyα systems, nor does this dispersion arise from an implementation of star formation and feedback in our highest resolution (~45 pc) models, if we do not put any galactic winds into our models by hand. We argue that these numerical experiments highlight the need to separate the dynamics of different components of the multiphase interstellar medium at z = 3.

We present a Chandra study of the hot interstellar medium (ISM) in the giant elliptical galaxy NGC 4649. In common with other group-centered ellipticals, its temperature profile rises with radius in the outer parts of the galaxy, from ~0.7 keV at 2 kpc to ~0.9 keV by 20 kpc. However, within the central ~2 kpc the trend reverses, and the temperature peaks at ~1.1 keV within the innermost 200 pc. Under the assumption of hydrostatic equilibrium, we demonstrate that the central temperature spike arises due to the gravitational influence of a quiescent central supermassive black hole. We constrain the black hole mass (M_BH) to (3.35^{+ 0.67}_−0.95) × 10⁹M_☉ (90% confidence), in good agreement with stellar kinematics measurements. This is the first direct measurement of M_BH based on studies of hydrostatic X-ray-emitting gas, which are sensitive to the most massive black holes, and is a crucial validation of both mass-determination techniques. This agreement clearly demonstrates that the gas must be close to hydrostatic, even in the very center of the galaxy, which is consistent with the lack of morphological disturbances in the X-ray image. NGC 4649 is now one of only a handful of galaxies for which M_BH has been measured by more than one method. At larger radii, we were able to decompose the gravitating mass profile into stellar and dark matter (DM) components. Unless one accounts for the DM, a standard virial analysis of the stars dramatically overestimates the stellar mass of the galaxy. We find that the measured J-band stellar mass-to-light ratio, 1.37 ± 0.10 M_☉L⁻¹_☉, is in good agreement with simple stellar population model calculations for this object.

It has been found that globular clusters (GCs) in dwarf galaxies and those in the Milky Way (MW) outer halo mostly have the same parent distributions, while GCs in the MW disk and inner halo have a different origin from those in dwarf galaxies. Thus, these dwarf galaxies did not play a crucial role in the formation of the Galactic disk or inner halo. In order to investigate this phenomenon in a more objective manner, a statistical comparison of the GCs of our Galaxy and those of neighboring dwarf galaxies has been carried out by a multivariate nonparametric method. For the various parameters of GCs in the MW and in dwarf galaxies, the multivariate Gaussian assumption fails, so a nonparametric method of comparison (instead of multivariate analysis of variance [MANOVA]) has been chosen. The test is performed on GCs of the MW disk, inner halo, and outer halo separately, with GCs from neighboring dwarf galaxies Canis Major, Fornax, and Sculptor, and the LMC dwarf irregular galaxy. The test is also performed for GCs from dwarf spheroidal galaxies in the neighborhood of M31: M33, NGC 147, NGC 185, and NGC 205.

We report the discovery of active star formation in Digel's Cloud 2, which is one of the most distant giant molecular clouds known in the extreme outer Galaxy (EOG). At the probable Galactic radius of ~20 kpc, Cloud 2 has a quite different environment from that in the solar neighborhood, including lower metallicity, much lower gas density, and small or no perturbation from the spiral arms. With new wide-field near-infrared (NIR) imaging that covers the entire Cloud 2, we discovered two young embedded star clusters located in the two dense cores of the cloud. Using our NIR and ¹²CO data, as well as H I, radio continuum, and IRAS data in the archives, we discuss the detailed star formation processes in this unique environment. We show clear evidence of sequential star formation triggered by the nearby huge supernova remnant GSH 138-01-94. The two embedded clusters show a distinct morphology difference: the one in the northern molecular cloud core is a loose association with isolated-mode star formation, while the one in the southern molecular cloud core is a dense cluster with cluster-mode star formation. We propose that high compression due to the combination of the supernova remnant shell and an adjacent shell caused the dense cluster formation in the southern core. In view of the special environment, in particular the low-metallicity range, we suggest that the EOG could be an excellent laboratory for the study of star formation processes, such as those triggered by supernovae, that occurred during an early epoch of the Galaxy's formation. In particular, the study of the EOG may shed light on the origin and role of the thick disk, whose metallicity range well matches that of the EOG.

We present the results of GBT observations of all four ground-state hydroxyl (OH) transitions toward 15 supernova remnants (SNRs) which show OH (1720 MHz) maser emission. This species of maser is well established as an excellent tracer of an ongoing interaction between the SNR and dense molecular material. For the majority of these objects we detect significantly higher flux densities with a single dish than have been reported with interferometric observations. We infer that spatially extended, low-level maser emission is a common phenomenon that traces the large-scale interaction in maser-emitting SNRs. In addition, we use a collisional pumping model to fit the physical conditions under which OH is excited behind the SNR shock front. We find the observed OH gas associated with the SNR interaction having columns N_OH ⩽ 1.5 × 10¹⁷ cm⁻², temperatures of 20-125 K, and densities ~10⁵ cm⁻³.

We present a comprehensive survey of C II* absorption detections toward stars within 100 pc in order to measure the distribution of electron densities present in the local interstellar medium (LISM). Using high spectral resolution observations obtained by GHRS and STIS on board HST, we searched for all detections of LISM C II* absorption. We identify 13 sight lines with 23 individual C II* absorption components, which provide electron density measurements. We employ several strategies to determine more accurate C II column densities from the saturated C II resonance line, including constraints of the line width from the optically thin C II* line, constraints from independent temperature measurements of the LISM gas based on line widths of other ions, and third, using measured S II column densities as a proxy for C II column densities. The distribution of electron densities based on using S II as a proxy for C II is similar to the distribution based on carbon alone, while significantly tighter, and proves to be a promising technique to avoid grossly overestimating the C II column density based on the saturated line profile. The sample of electron densities appears consistent with a lognormal distribution and an unweighted mean value of n_e(C II_{S II}) = 0.11^+0.10_−0.05 cm⁻³. Seven individual sight lines probe the Local Interstellar Cloud (LIC), and all present a similar value for the electron density, with a weighted mean of n_e(LIC) = 0.12 ± 0.04 cm⁻³. Given some simple assumptions, the range of observed electron densities translates into a range of thermal pressures, P/k = 3300^{+ 5500}₋₁₉₀₀ K cm ⁻³. This work greatly expands the number of electron density measurements and provides important constraints on the ionization, abundance, and evolutionary models of the LISM.

We study the collapse of protoclusters within a giant molecular cloud (GMC) to determine the conditions under which collapse is significantly disrupted. Motivated by observations of star-forming regions which exhibit flattened cloud structures, this study considers collapsing protoclusters with disk geometries. The collapse of a 10³M_☉ protocluster initially a distance of 2-10 pc from a 10³-10⁶M_☉ point mass is numerically calculated. Simulations with zero initial relative velocity between the two are completed, as well as simulations with relative velocities consistent with those observed in GMCs. The results allow us to define the conditions under which it is safe to assume that protocluster collapse proceeds as if in isolation. For instance, we find that the collapse of a 10³M_☉ protocluster will be significantly disrupted if it is within 2-4 pc of a 10⁴M_☉ point mass. Thus, the collapse of a 10³M_☉ protocluster can be considered to proceed as if in isolation if it is more than ~4 pc away from a 10⁴M_☉ compact object. In addition, in no portion of the sampled parameter space does the gravitational interaction between the protocluster disk and the massive particle significantly disperse the disk into the background GMC. We discuss the distribution of clusters of young stellar objects within the Perseus and Mon R2 star-forming regions, which are consistent with the results of our simulations and the limitations of our results in gas dominated regions such as the Orion cloud.

We describe a model for the thermal and dynamical equilibrium of starless cores that includes the radiative transfer of gas and dust and simple CO chemistry. The model shows that the structure and behavior of the cores is significantly different depending on whether the central density is either above or below about 10⁵ cm⁻³. This density is significant as the critical density for gas cooling by gas-dust collisions and as the critical density for dynamical stability, given the typical properties of the starless cores. Starless cores thus divide into two classes that we refer to as thermally supercritical and thermally subcritical. This two-class distinction allows an improved interpretation of the different observational data of starless cores within a single model.

We present the measurement and analysis of hydrogen energetic neutral atoms (ENAs) recorded with the NPD sensor of the ASPERA-3 instrument on board Mars Express during the cruise phase and the Mars orbit phase. We conclude that the origin of these ENAs is the inner heliosheath. The ENA energy spectra are all very similar and can be fitted well by a two-component power law. The ENA intensities, integrated from 0.3 to 10 keV, vary in the range of 5 × 10³ to 3 × 10⁴ cm⁻² sr⁻¹ s⁻¹. This report is an update of our earlier paper using the final NPD calibration data and improved sensor knowledge from two years of NPD operation. The present ENA measurements fit together well with earlier ENA data that were obtained from other spacecraft at higher energies, and which also have their likely origin in the inner heliosheath. Comparison of the measured ENA energy spectra with results from several heliospheric models shows that some of these models predict significantly lower ENA intensities at Earth orbit.

We have observed the Orion Molecular Cloud-2 FIR 3/4 region in the H¹³CO⁺ (J = 1–0),¹²CO (J = 1–0), SiO (v = 0, J = 2–1), and CS (J = 2–1) lines and the 3.3 mm continuum emission with the Nobeyama Millimeter Array (NMA) and in the CO (J = 3–2) and CH₃OH (J_K = 7_K–6_K) lines with Atacama Submillimeter Telescope Experiment (ASTE). Our NMA observations in the H¹³CO⁺ emission have revealed 0.07 pc scale dense gas associated with FIR 4 (FIR 4 clump). The ¹²CO (J = 3–2,1-0) emission shows high-velocity blue- and redshifted components to both the northeast and southwest of FIR 3, suggesting an outflow from FIR 3 along the plane of the sky. The SiO and CH₃OH emission, known as shock tracers, are detected around the interface between the outflow and FIR 4 clump. Furthermore, the ¹²CO (J = 1–0) emission shows an L-shaped structure in the PV diagram. These results suggest the presence of an interaction between the outflow and FIR 4 clump. Moreover, our interferometric 3.3 mm continuum observations have first found that FIR 4 consists of 11 dusty cores at a scale of ~2000 AU. The separation among these cores (~5 × 10³ AU) is on the same order of the Jeans length (~13 × 10³ AU), and the estimated time scale of the fragmentation (~3.8 × 10⁴ yr) is similar to the time scale of the outflow interaction (~1.4 × 10⁴ yr). We suggest that the interaction triggered the fragmentation into these dusty cores, and hence the next generation of the cluster formation in FIR 4.

The HH 24 MMS protostellar system was observed in the 6.9 mm continuum with a high angular resolution (0.5^''). HH 24 MMS was resolved into two sources. The separation between sources 1 and 2 is ~0.9^'' or 360 AU. The spectral energy distribution suggests that the 6.9 mm flux is almost entirely from dust. The 6.9 mm image and the spectrum suggest that HH 24 MMS may be a protostellar binary system. Total mass including the accretion disks and the inner protostellar envelope is ~1.4 M_☉. Disk masses of sources 1 and 2 are 0.6 and 0.3 M_☉, respectively. Both sources are highly elongated. The difference in the position angle of the two disks is ~45°, which means that HH 24 MMS is a highly misaligned protobinary system. The misalignment suggests that turbulent fragmentation may be the formation mechanism relevant to the binary systems with a separation of a few hundreds of AU, such as the HH 24 MMS system.

With its remarkable double-S shape, IC 4634 is an archetype of point-symmetric planetary nebulae (PNe). In this paper, we present a detailed study of this PN using archival HST WFPC2 and ground-based narrowband images to investigate its morphology, and long-slit spectroscopic observations to determine its kinematics and to derive its physical conditions and excitation. The data reveal new structural components, including a distant string of knots distributed along an arclike feature 40^''-60^'' from the center of the nebula, a skin of enhanced [O III]/Hα ratio enveloping the inner shell and the double-S feature, and a triple-shell structure. The spatio-kinematical study also finds an equatorial component of the main nebula that is kinematically independent of the bright inner S-shaped arc. We have investigated in detail the bow shock-like features in IC 4634 and found that their morphological, kinematical, and emission properties are consistent with the interaction of a collimated outflow with surrounding material. Indeed, the morphology and kinematics of some of these features can be interpreted using a three-dimensional numerical simulation of a collimated outflow precessing at a moderate, time-dependent velocity. Apparently, IC 4634 has experienced several episodes of point-symmetric ejections oriented in different directions, with the outer S-shaped feature being related to an earlier point-symmetric ejection, and the outermost arclike string of knots being the relic of a still much earlier point-symmetric ejection. There is tantalizing evidence that the action of these collimated outflows has also taken part in the shaping of the innermost shell and inner S-shaped arc of IC 4634.

We present self-consistent models of gas in optically thick dusty disks and calculate its thermal, density, and chemical structure. The models focus on an accurate treatment of the upper layers where line emission originates and at radii ≳0.7 AU. Although our models are applicable to stars of any mass, we present here only results around ~1 M_☉ stars where we have varied dust properties, X-ray luminosities, and UV luminosities. We separately treat gas and dust thermal balance and calculate line luminosities at infrared and submillimeter wavelengths from all transitions originating in the predominantly neutral gas that lies below the very tenuous and completely ionized surface of the disk. We find that the [Ar II] 7 μm, [Ne II] 12.8 μm, [Fe I] 24 μm, [S I] 25 μm, [Fe II] 26 μm, [Si II] 35 μm, [O I] 63 μm, and pure rotational lines of H₂ and CO can be quite strong and are good indicators of the presence and distribution of gas in disks. Water is an important coolant in the disk, and many water emission lines can be moderately strong. Current and future observational facilities such as the Spitzer Space Telescope, Herschel Space Observatory, and SOFIA are capable of detecting gas emission from young disks. We apply our models to the disk around the nearby young star, TW Hya, and find good agreement between our model calculations and observations. We also predict strong emission lines from the TW Hya disk that are likely to be detected by future facilities. We suggest that the gas disk around TW Hya may be truncated to ~120 AU, compared to its dust disk of 174 AU. We speculate that photoevaporation due to the strong stellar FUV field from TW Hya is responsible for the gas disk truncation.

We imaged a 2^' × 2^' region of the Orion Nebula Cluster (ONC) in 1.3 mm wavelength continuum emission with the recently commissioned Combined Array for Research in Millimeter Astronomy (CARMA) and with the Submillimeter Array (SMA). Our mosaics include >250 known near-IR cluster members, of which 36 are so-called ``proplyds'' that have been imaged previously with the Hubble Space Telescope. We detected 40 sources in 1 mm continuum emission (one of which is the BN Object), and several of them are spatially resolved with our observations. The emission from most objects arises predominantly from dust, and circumstellar masses inferred for detected sources range from 0.01 to 0.5 M_☉. The average circumstellar mass for undetected sources is estimated to be ~0.001 M_☉, approximately an order of magnitude smaller than the minimum-mass solar nebula. Most stars in the ONC thus do not appear to currently possess sufficient mass in small dust grains to form Jupiter-mass (or larger) planets. Comparison with previous results for younger and older regions indicates that massive disks evolve significantly on ~Myr timescales. We also show that the percentage of stars in Orion surrounded by disks more massive than ~0.01 M_☉ is substantially lower than in Taurus, indicating that environment has an impact on the disk-mass distribution. Disks in Orion may be truncated through photoevaporation caused by the intense radiation field of the Trapezium stars, and we see marginal evidence for such a scenario in the spatial distribution of massive disks within the cluster. Our data show no statistically significant correlation between disk and stellar masses, although we see hints of a higher percentage of massive disks around lower mass stars.

We test the hypothesis that the host galaxies of long-duration gamma-ray bursts (GRBs) as well as quasar-selected damped Lyα (DLA) systems are drawn from the population of UV-selected star-forming, high-z galaxies (generally referred to as Lyman break galaxies). Specifically, we compare the metallicity distributions of the GRB and DLA populations against simple models where these galaxies are drawn randomly from the distribution of star-forming galaxies according to their star formation rate and H I cross section, respectively. We find that it is possible to match both observational distributions assuming very simple and constrained relations between luminosity, metallicity, and H I sizes. The simple model can be tested by observing the luminosity distribution of GRB host galaxies and by measuring the luminosity and impact parameters of DLA-selected galaxies as a function of metallicity. Our results support the expectation that GRB and DLA samples, in contrast with magnitude-limited surveys, provide an almost complete census of z ≈ 3 star-forming galaxies that are not heavily obscured.

It is usually proposed that hyperaccretion disks surrounding stellar-mass black holes, with an accretion rate of a fraction of 1 M_☉ s⁻¹, produced during the mergers of double compact stars or the collapses of massive stars, are central engines of gamma-ray bursts (GRBs). In some origin/afterglow models, however, newborn compact objects are introduced as neutron stars rather than black holes. Thus, hyperaccretion disks around neutron stars may exist in some GRBs. Such disks may also occur in Type II supernovae. In this paper we study the structure of a hyperaccretion disk around a neutron star. We consider a steady-state hyperaccretion disk, and as a reasonable approximation, divide it into two regions, the inner and outer disks. The outer disk is similar to that of a black hole, and the inner disk has a self-similar structure. In order to study the physical properties of the entire disk clearly, we first adopt a simple model, in which some microphysical processes in the disk are simplified, and analytically and numerically investigate the size of the inner disk, the efficiency of neutrino cooling, and the radial distributions of the disk density, temperature, and pressure. We see that the neutron star disk can cool more efficiently and produce a much higher neutrino luminosity than a black hole disk. Finally, we consider an elaborate model with more physical considerations of the thermodynamics and microphysics in the neutron star disk, and compare this elaborate model with our simple model. We find that most of the results of these two models are basically consistent with each other.

We present theoretical models for the formation and evolution of populations of low-mass X-ray binaries (LMXBs) in the two elliptical galaxies NGC 3379 and NGC 4278. The models are calculated with the recently updated StarTrack code, assuming only a primordial galactic field LMXB population. StarTrack is an advanced population synthesis code that has been tested and calibrated using detailed binary star calculations and incorporates all the important physical processes of binary evolution. The simulations are targeted to modeling and understanding the origin of the X-ray luminosity functions (XLFs) of point sources in these galaxies. For the first time we explore the population XLF in luminosities below 10³⁷ ergs s⁻¹, as probed by the most recent observational results. We consider models for the formation and evolution of LMXBs in galactic fields with different CE efficiencies, stellar wind prescriptions, magnetic braking laws, and IMFs. We identify models that produce XLFs consistent with the observations both in shape and absolute normalization, suggesting that a primordial galactic field LMXB population can make a significant contribution to the total population of an elliptical galaxy. We also find that the treatment of the outburst luminosity of transient systems remains a crucial factor for the determination of the XLF, since the modeled populations are dominated by transient X-ray systems.

We show three-dimensional magnetohydrodynamical simulations of a core-collapse supernova in which the progenitor has magnetic fields inclined to the rotation axis. The simulations employed a simple empirical equation of state in which the pressure of degenerate gas is approximated by piecewise polytropes for simplicity. Energy loss due to neutrinos is not taken into account for simplicity as well. The simulations start from the stage of dynamical collapse of an iron core. The dynamical collapse halts at t = 189 ms by the pressure of high-density gas, and a proto-neutron star (PNS) forms. The evolution of the PNS was followed for about 40 ms in typical models. When the initial rotation is mildly fast and the initial magnetic fields are mildly strong, bipolar jets are launched from the upper atmosphere (r ∼ 60 km ) of the PNS. The jets are accelerated to ~3 × 10⁴ km s⁻¹, which is comparable to the escape velocity at the footpoint. The jets are parallel to the initial rotation axis. Before the launch of the jets, magnetic fields are twisted by rotation of the PNS. The twisted magnetic fields form torus-shaped multilayers in which the azimuthal component changes alternately. The formation of magnetic multilayers is due to the initial condition in which the magnetic fields are inclined with respect to the rotation axis. The energy of the jet depends only weakly on the initial magnetic field assumed. When the initial magnetic fields are weaker, the time lag is longer between the PNS formation and jet ejection. It is also shown that the time lag is related to the Alfvén transit time. Although the nearly spherical prompt shock propagates outward in our simulations, it is an artifact due to our simplified equation of state and neglect of neutrino loss. The morphology of twisted magnetic field and associate jet ejection are, however, not affected by the simplification.

The ultracompact binary systems V407 Vul (RX J1914.4+2456) and HM Cnc (RX J0806.3+1527), a two-member subclass of the AM CVn stars, continue to generate interest because they defy unambiguous classification. Three proposed models remain viable at this time, but none of the three are significantly more compelling than the remaining two, and all three can satisfy the observational constraints if parameters in the models are tuned. One of the three proposed models is the direct impact model of Marsh & Steeghs, in which the accretion stream impacts the surface of a rapidly rotating primary white dwarf directly, but at a near-glancing angle. One requirement of this model is that the accretion stream have a high enough density to advect its specific kinetic energy below the photosphere for progressively more thermalized emission downstream, a constraint that requires an accretion spot size of ~1.2 × 10⁵ km² or smaller. Because we had at hand a smoothed particle hydrodynamics code optimized for cataclysmic variable accretion disk simulations, it was relatively straightforward for us to adapt it to calculate the footprint of the accretion stream at the nominal radius of the primary white dwarf and thus to test this constraint of the direct impact model. We find that the mass flux at the impact spot can be approximated by a bivariate Gaussian with a standard deviation of σ_ϕ = 164 km in the orbital plane and σ_θ = 23 km in the perpendicular direction. The area of the 2 σ ellipse into which ~86% of the mass flux occurs is roughly 47,400 km², or roughly half the size estimated by Marsh & Steeghs. We discuss the necessary parameters of a simple model of the luminosity distribution in the postimpact emission region.

Kepler's supernova, discovered in 1604 October, produced a remnant that has been well studied observationally in the radio, infrared, optical, and X-ray bands, and theoretically. Some models have predicted a TeV gamma-ray flux that is detectable with current Imaging Cerenkov Atmospheric Telescopes. We report on observations carried out in 2005 April with the CANGAROO-III Telescope. No statistically significant excess was observed, and limitations on the allowed parameter range in the model are discussed.

The accretion flow in the disk-dominated state of black hole binaries has a peak temperature and luminosity that vary together in such a way as to indicate an approximately constant emitting area. The association of this with the last stable orbit gives one of the few ways to estimate spin when the mass of the black hole is known. However, deriving this radius requires knowing how the disk spectrum is modified by radiative transfer through the vertical structure of the disk, as well as special and general relativistic effects on the propagation of this radiation. Here we investigate the extent to which differences in vertical structure change the derived disk spectra by calculating these for a range of different stress prescriptions. We find that at a given mass accretion rate, the spectra are almost identical for accretion rates of L/L_Edd≲ 0.1. The spectra are remarkably similar, even up to the highest luminosities considered (L/L_Edd ∼ 0.6), as long as the stresses do not dissipate more than about 10% of the gravitational energy above the effective photosphere. This is exceeded only by classic alpha disks with α ≳ 0.1, but these models yield spectral variation that is incompatible with existing data. Therefore, we conclude that disk spectral modeling can place interesting constraints on angular momentum transport, but still provide a robust estimate of the spin of the black hole.

We report an analysis of X-ray and γ-ray observations of PKS 1830–211, based on long-term campaigns carried out by INTEGRAL and COMPTEL. The INTEGRAL data currently available provide a 33 σ significance detection in the 20-100 keV band, while the COMPTEL 6 yr data provide a 5.2 σ significance detection in the 1-3 MeV energy band. In hard X-rays, INTEGRAL and supplementary Swift observations show flux variability on timescales of months. In γ-rays, the source shows persistent emission over years. The hard X-ray spectrum is well represented by a power-law model, with Γ ~ 1.3 in the 20-250 keV band. This photon index is consistent with a previous report of Γ ~ 1.3 obtained at E > 3.5 keV from XMM-Newton data fitted with a broken power law model. The joint XMM-Newton and INTEGRAL spectrum presented here is thus fitted with a broken power law model; the parameters are refined as compared with the previous fit. The results show that the photon index changes from ~1.0 to ~1.3 at a break energy of ~4 keV. At MeV energies, the spectrum softens to Γ ~ 2.2. These results, together with an EGRET measurement at E ≥ 100 MeV, constitute a broadband spectrum containing the peak of the power output at MeV energies, similar to most high-luminosity γ-ray blazars. The measured spectral characteristics are discussed in the framework of gravitational lensing effects.

We present phase-resolved low-resolution infrared spectra of AM Her and ST LMi, two low-field polars that we observed with SPEX on the IRTF. Optical/NIR light curves are also published to help constrain the viewing geometry and brightness of the objects at the time they were observed. Currently, only limited IR spectra have been published for these objects, and none with the phase-coverage presented here. In both cases, the resulting spectra are dominated by emission from the secondary star in the NIR. However, the emission regions are also self-eclipsed, allowing us to isolate the cyclotron emission through subtraction of the dim-phase spectrum. We use a constant-lambda prescription to model the changing cyclotron features seen in the resulting data. For AM Her, we find a best-fit model of B = 13.6 MG, kT = 4.0 keV, and log Λ = 5.0. The cyclotron derived accretion geometry is consistent with i = 50° and β = 85°. For ST LMi, B = 12.1 MG, kT = 3.3 keV, and log Λ = 5.7, with i = 55° and β = 128°.

Using the longest baselines of the CHARA Array, we have measured the angular diameter of the G5 V subdwarf μ Cas A, the first such determination for a halo population star. We compare this result to new diameters for the higher metallicity K0 V stars, σ Dra and HR 511, and find that the metal-poor star, μ Cas A, has an effective temperature (T_eff = 5297 ± 32 K), radius (R = 0.791 ± 0.008 R_☉), and absolute luminosity (L = 0.442 ± 0.014 L_☉) comparable to those of the other two stars with later spectral types. We show that stellar models show a discrepancy in the predicted temperature and radius for μ Cas A, and we discuss these results and how they provide a key to understanding the fundamental relationships for stars with low metallicity.

We present the analysis of 4.5 years of nearly continuous observations of the classical Cepheid Polaris, which comprise the most precise data available for this star. We have made spectroscopic measurements from ground and photometric measurements from the WIRE star tracker and the SMEI instrument on the Coriolis satellite. Measurements of the amplitude of the dominant oscillation (P = 4 days), which go back more than a century, show a decrease from A_V = 120 to 30 mmag around the turn of the millennium. It has been speculated that the reason for the decrease in amplitude is the evolution of Polaris toward the edge of the instability strip. However, our new data reveal an increase in the amplitude by ~30% from 2003 to 2006. It now appears that the amplitude change is cyclic rather than monotonic and most likely the result of a pulsation phenomenon. In addition, previous radial velocity campaigns have claimed the detection of long-period variation in Polaris (P > 40 days). Our radial velocity data are more precise than previous data sets, and we find no evidence for additional variation for periods in the range 3-50 days with an upper limit of 100 m s⁻¹. However, in the WIRE data we find evidence of variation on timescales of 2-6 days, which we interpret as being due to granulation.

Spectral energy distributions for models of arbitrarily rotating stars are computed using two-dimensional rotating stellar models, NLTE plane-parallel model atmospheres, and a code to integrate the appropriately weighted intensities over the visible surface of the stellar disk. The spectral energy distributions depend on the inclination angle between the observer and the rotation axis of the model. We use these curves to deduce what one would infer the model's luminosity and effective temperature to be assuming the object was nonrotating.

Using the Far Ultraviolet Spectroscopic Explorer satellite, we obtained series of spectra for two of the three known WC + O binaries in the LMC, Br 22, and Br 32 (HD 36521). Compared to Br 22, we detect a higher ratio of C IV to He II lines in Br 32, which could indicate a more advanced evolutionary stage for the latter. The orbit of the O star in Br 32 has been determined from its P V absorption lines. We find that continuum fluxes in both systems are substantially diluted by a third-light source. The maximum extension of the black absorption troughs in P Cygni profiles provides the terminal velocities of the WC winds: 3775 ± 125 km s⁻¹ for Br 22 and 4400 ± 150 km s⁻¹ for Br 32. From the phase-dependent displacements of the blue absorption edges of prominent emission lines we estimate the half-opening angles of the wind-wind collision zones and their Coriolis deflections. In both binaries we fitted, via iterative procedure, the phase-dependent changes in the O VI λ1032-1037 and C III λ1175 profiles as a function of the wind, stellar, and orbital parameters. This allowed us to isolate the excess emission produced in the wind-wind collision zone and reproduce profile changes caused by atmospheric eclipses. A strong extra emission component is observed in Br 22 (P = 14.9 days), while it is negligible in Br 32 (P = 1.9 days).

We present and interpret new spectropolarimetric observations of the magnetic white dwarf WD 1953–011. Circular polarization and intensity spectra of the Hα spectral line demonstrate the presence of two-component magnetic field in the photosphere of this star. The geometry consists of a weak, large-scale component, and a strong, localized component. Analyzing the rotationally modulated low-field component, we establish a rotation period P_rot = 1.4480 ± 0.0001 days. Modeling the measured magnetic observables, we find that the low-field component can be described by the superposition of a dipole and quadrupole. According to the best-fit model, the inclination of the stellar rotation axis with respect to the line of sight is i ≈ 20°, and the angle between the rotation axis and the dipolar axis is β ≈ 10°. The dipole strength at the pole is about 180 kG, and the quadrupolar strength is about 230 kG. These data suggest a fossil origin of the low-field component. In contrast, the strong-field component exhibits a peculiar, localized structure ("magnetic spot") that confirms the conclusions of Maxted and coworkers. The mean field modulus of the spot (|B_spot| = 520 ± 7 kG) together with its variable longitudinal magnetic field having a maximum of about +400 kG make it difficult to describe it naturally as a high-order component of the star's global poloidal field. Instead, we suggest that the observed strong-field region has a geometry similar to a magnetic flux tube.

This paper is one in a series presenting results obtained within the Formation and Evolution of Planetary Systems (FEPS) Legacy Science Program on the Spitzer Space Telescope. Here we present a study of dust processing and growth in seven protoplanetary disks. Our spectra indicate that the circumstellar silicate dust grains have grown to sizes at least 10 times larger than observed in the interstellar medium and show evidence for a non-negligible (~5% in mass fractions) contribution from crystalline species. These results are similar to those of other studies of protoplanetary disks. In addition, we find a correlation between the strength of the amorphous silicate feature and the shape of the spectral energy distribution. This latter result is consistent with the growth and subsequent gravitational settling of dust grains toward the disk midplane. Furthermore, we find a change in the relative abundance of the different crystalline species: more enstatite than forsterite is observed in the inner warm dust population at ~1 AU, while forsterite dominates in the colder outer regions at ~5-15 AU. This change in the relative abundances argues for a localized crystallization process rather than a radial mixing scenario in which crystalline silicates are being transported outwards from a single formation region in the hot inner parts of the disk. Finally, we report the detection of emission from polycyclic aromatic hydrocarbon (PAH) molecules in five out of seven sources. We find a tentative PAH band at 8.2 μm that was previously undetected in the spectra of disks around low-mass pre-main-sequence stars.

We analyze the long-term variation of the solar wind helium abundance, both in and out of the ecliptic, using stackplot displays to compare these in situ observations with derived coronal parameters. The coronal source regions are identified and their magnetic properties characterized by applying a current-free extrapolation, with source surface located at heliocentric distance r = 2.5 R_☉, to magnetograph measurements. The density ratio A_He of α-particles to protons is found to correlate best with the source-surface field strength B_ss, which tends to be enhanced in high-speed flows and in the slow wind at sunspot maximum, but to be weak in the low-speed wind that originates from the polar coronal-hole boundaries around sunspot minimum. A much weaker correlation exists between A_He and the proton flux density at the source surface. These results are consistent with acceleration of the α-particles by ion cyclotron resonance in the outer corona. However, we are unable to explain why the helium abundance was depressed in the recurrent high-speed streams observed in the ecliptic during 1999-2000 and 2003-2004.

We have examined the preflare activity of an M1.2 flare that occurred in NOAA active region 8440 on 1999 January 16, using images from the Soft X-Ray Telescope (SXT) on board Yohkoh, 1600 Å UV images from the Transition Region and Coronal Explorer (TRACE), X-ray flux data from the GOES satellite, and magnetograms from Big Bear Solar Observatory (BBSO). During the preflare phase, we note a weak GOES X-ray flux enhancement just 4 minutes before the main flare begins. The SXT images show that this enhancement occurs at one footpoint of a soft X-ray loop bundle, which exactly coincides with the kernel of the major flare. The series of TRACE images provides the following pieces of evidence for small-scale magnetic reconnections associated with the preflare activity. (1) A small-scale UV sigmoid is seen at the X-ray loop footpoint before the preflare activity, and it is located along the polarity inversion line. (2) The brightest among the UV brightenings is exactly coincident and cospatial with the soft X-ray brightening observed by the Yohkoh SXT and GOES. (3) There were several interactions and brightenings among small UV loops. After these brightenings, the connectivity of the UV loops was apparently changed. As a result, a large rising loop structure was formed, with a maximum rising speed of about 40 km s⁻¹. (4) The main flare occurred in this structure. In the aspects of the overall configuration and morphological change of UV loops, the preflare activity is quite consistent with the tether-cutting model with a single-bipole magnetic explosion. We suggest that the preflare activity and the main flare in this event not only have similar physical mechanisms, but also have a causal relation.

There is a growing body of evidence that the plasma loops seen with current instrumentation (SOHO, TRACE, and Hinode) may consist of many subresolution elements or strands. Thus, the overall plasma evolution we observe in these features could be the cumulative result of numerous individual strands undergoing sporadic heating. This paper presents a short (10⁹ cm ≡ 10 Mm ) "global loop" as 125 individual strands, where each strand is modeled independently by a one-dimensional hydrodynamic simulation. The energy-release mechanism across the strands consists of localized, discrete heating events (nanoflares). The strands are "coupled" together through the frequency distribution of the total energy input to the loop, which follows a power-law distribution with index α. The location and lifetime of each energy event is random. Although a typical strand can go through a series of well-defined heating/cooling cycles, when the strands are combined, the overall quasi-static emission-measure-weighted thermal profile for the global loop reproduces a hot apex/cool base structure. Localized cool plasma blobs are seen to travel along individual strands, which could cause the loop to "disappear" from coronal emission and to appear in transition or chromospheric emission. As α increases (from 0 to 2.29 to 3.29), more weight is given to the smallest heating episodes. Consequently, the overall global loop apex temperature increases, while the variation of the temperature around that value decreases. Any further increase in α saturates the loop apex temperature variations at the current simulation resolution. The effect of increasing the number of strands and the loop length, as well as the implications of these results for possible future observing campaigns for TRACE and Hinode, are discussed.

In this paper we introduce a new approach to study the interaction of solar eigenoscillations, with particular emphasis on the f-mode, with random inhomogeneities caused by flows and magnetic field near the solar surface. We present an initial value method to derive a general dispersion relation for a class of models where the magnetic atmosphere is overlying an arbitrary static solar interior. In these models the interior part is treated parametrically and does not need to be specified before we obtain the dispersion relation. In order to demonstrate the applicability of the proposed method, an analytical solution of the dispersion relation is given for an incompressible interior with constant density.

The magnetic solar cycle consists of two components: the well-known main cycle with period T₀ = 11 yr, and a high-frequency component with period close to 2 yr, whose origin is not yet understood. Here we use proper orthogonal decomposition (POD) analysis on the green coronal emission line at 530.3 nm to investigate the spatio-temporal behavior of the solar cycle. We find that very few POD modes suffice to describe the cycle. In particular, the first most energetic mode describes the main 11 yr cycle, while a 2 yr periodic signal dominates the second POD mode only for high solar latitudes. The quasi-biennial signal is confined to the fourth mode for equatorial data, thus suggesting that, within the dynamo effect, the high-frequency cycle should be due to some phenomenon that is different from the emergence of solar active regions.

A big challenge in solar and stellar physics in the coming years will be to decipher the magnetism of the solar outer atmosphere (chromosphere and corona) along with its dynamic coupling with the magnetic fields of the underlying photosphere. To this end, it is important to develop rigorous diagnostic tools for the physical interpretation of spectropolarimetric observations in suitably chosen spectral lines. Here we present a computer program for the synthesis and inversion of Stokes profiles caused by the joint action of atomic level polarization and the Hanle and Zeeman effects in some spectral lines of diagnostic interest, such as those of the He I 10830 Å and 5876 Å (or D₃) multiplets. It is based on the quantum theory of spectral line polarization, which takes into account in a rigorous way all the relevant physical mechanisms and ingredients (optical pumping, atomic level polarization, level crossings and repulsions, Zeeman, Paschen-Back, and Hanle effects). The influence of radiative transfer on the emergent spectral line radiation is taken into account through a suitable slab model. The user can either calculate the emergent intensity and polarization for any given magnetic field vector or infer the dynamical and magnetic properties from the observed Stokes profiles via an efficient inversion algorithm based on global optimization methods. The reliability of the forward modeling and inversion code presented here is demonstrated through several applications, which range from the inference of the magnetic field vector in solar active regions to determining whether or not it is canopy-like in quiet chromospheric regions. This user-friendly diagnostic tool called "HAZEL" (from HAnle and ZEeman Light) is offered to the astrophysical community, with the hope that it will facilitate new advances in solar and stellar physics.

Astronomical applications of recent advances in the field of nonastronomical image processing are presented. These innovative methods, applied to multiscale astronomical images, increase signal-to-noise ratio, do not smear point sources or extended diffuse structures, and are thus a highly useful preliminary step for detection of different features including point sources, smoothing of clumpy data, and removal of contaminants from background maps. We show how the new methods, combined with other algorithms of image processing, unveil fine diffuse structures while at the same time enhance detection of localized objects, thus facilitating interactive morphology studies and paving the way for the automated recognition and classification of different features. We have also developed a new application framework for astronomical image processing that implements some recent advances made in computer vision and modern image processing, along with original algorithms based on nonlinear partial differential equations. The framework enables the user to easily set up and customize an image-processing pipeline interactively; it has various common and new visualization features and provides access to many astronomy data archives. Altogether, the results presented here demonstrate the first implementation of a novel synergistic approach based on integration of image processing, image visualization, and image quality assessment.

Measurements of the opacity of silicon at high temperature and high density are reported. A silicon dioxide foam was heated by eight nanosecond laser beams while a backlighter X-ray source was produced with a picosecond laser. Absorptions of the 1-2 transitions of Si XII through Si VI were observed in the wavelength range from 6.6 to 7.1 Å. The experimental results are simulated with theoretical calculations under local thermodynamic equilibrium using a detailed level accounting model and can be reproduced in general when the effects of the oxygen in the SiO₂ are taken into account.

Recently, the WMAP group published their 5 year data and considered the constraints on the time-evolving equation of state of dark energy for the first time from the WMAP distance information. In this Letter, we study the effectiveness of the usage of this distance information and find that the compressed CMB information can give similar constraints on dark energy parameters compared with the full CMB power spectrum if dark energy perturbations are included; however, if the dark energy perturbations are incorrectly neglected, the difference of the results is sizable.

While the high-z frontier of star formation rate (SFR) studies has advanced rapidly, direct measurements beyond z ∼ 4 remain difficult, as shown by significant disagreements among different results. Gamma-ray bursts, owing to their brightness and association with massive stars, offer hope of clarifying this situation, provided that the GRB rate can be properly related to the SFR. The Swift GRB data reveal an increasing evolution in the GRB rate relative to the SFR at intermediate z; taking this into account, we use the highest-z GRB data to make a new determination of the SFR at z = 4–7. Our results exceed the lowest direct SFR measurements and imply that no steep drop exists in the SFR up to at least z ∼ 6.5. We discuss the implications of our result for cosmic reionization, the efficiency of the universe in producing stellar-mass black holes, and "GRB feedback" in star-forming hosts.

It is now recognized that long-duration gamma-ray Bursts (GRBs) are linked to the collapse of massive stars, based on the association between (low redshift) GRBs and (Type Ic) core-collapse supernovae (SNe). The census of massive stars and GRBs reveals, however, that not all massive stars produce a GRB. Only ~1% of core-collapse SNe are able to produce a highly relativistic collimated outflow, and hence a GRB. The extra crucial parameter has long been suspected to be metallicity and/or rotation. We find observational evidence strongly supporting that both ingredients are necessary in order to make a GRB out of a core-collapsing star. A detailed study of the absorption pattern in the X-ray spectrum of GRB 060218 reveals evidence of material highly enriched in low-atomic-number metals ejected before the SN/GRB explosion. We find that, within the current scenarios of stellar evolution, only a progenitor star characterized by a fast stellar rotation and subsolar initial metallicity could produce such a metal enrichment in its close surrounding.

The outer regions of disk galaxies show a drop-off in optical and Hα emission, suggesting a star formation threshold radius, assumed to owe to a critical surface density below which star formation does not take place. Signs of filamentary star formation beyond this threshold radius have been observed in individual galaxies in the Hα, and recent GALEX surveys have discovered that 30% of disk galaxies show UV emission out to 2-3 times the optical radius of the galaxy. We run smooth particle hydrodynamics simulations of disk galaxies with constant-density extended gas disks to test whether overdensities owing to spiral structure in the outer disk can reproduce the observed star formation. We indeed find that spiral density waves from the inner disk propagate into the outer gas disk and raise local gas regions above the star formation density threshold, yielding features similar to those observed. Because the amount of star formation is low, we expect to see little optical emission in outer disks, as observed. Our results indicate that XUV disks can be simulated simply by adding an extended gas disk with a surface density near the threshold density to an isolated galaxy and evolving it with fiducial star formation parameters.

We present multiwavelength observations of the brightest galaxies in four X-ray-luminous groups at z ∼ 0.37 that will merge to form a cluster comparable in mass to Coma. Ordered by increasing stellar mass, the four brightest group galaxies (BGGs) present a time sequence where BGG-1, 2, and 3 are in merging systems and BGG-4 is a massive remnant (M_* = 6.7 × 10¹¹M_☉). BGG-1 and 2 have bright, gravitationally bound companions and BGG-3 has two nuclei separated by only 2.5 kpc; thus, merging at z < 0.5 increases the BGG mass by ≳40% (t_MGR < 2 Gyr) and V-band luminosity by ~0.4 mag. The BGGs' rest-frame (B − V) colors correspond to stellar ages of >3 Gyr, and their tight scatter in (B − V) color (σ_BV = 0.032) confirms that they formed the bulk of their stars at z > 0.9. Optical spectroscopy shows no signs of recent (< 1.5 Gyr) or ongoing star formation. Only two BGGs are weakly detected at 24 μm, and X-ray and optical data indicate that the emission in BGG-2 is due to an AGN. All four BGGs and their companions are early-type (bulge-dominated) galaxies, and they are embedded in diffuse stellar envelopes up to ~140 kpc across. The four BGG systems must evolve into the massive, red, early-type galaxies dominating local clusters. Our results show that (1) massive galaxies in groups and clusters form via dissipationless merging and (2) the group environment is critical for this process.

Supermassive black holes ejected from galaxy nuclei by gravitational wave recoil will carry a retinue of bound stars, even in the absence of an accretion disk. We discuss the observable signatures related to these stars, with an emphasis on electromagnetic flares from stars which are tidally disrupted by the black hole. We calculate disruption rates for the bound, and the unbound, stars. The rates are smaller than, but comparable to, rates for nonrecoiling black holes. A key observational consequence is the existence of powerful off-nuclear and intergalactic X-ray flares. We also discuss other observable signatures associated with the bound stars, including episodic X-ray emission from accretion due to stellar mass loss, intergalactic supernovae, and feedback trails.

Type Ia supernovae (SNe Ia) occur in both old, passive galaxies and active, star-forming galaxies. This fact, coupled with the strong dependence of SN Ia rate on star formation rate, suggests that SNe Ia form from stars with a wide range of ages. Here we show that the rate of SN Ia explosions is about 1% of the stellar death rate, independent of star formation history. The dependence of SN Ia rate on star formation rate implies a delay time distribution proportional to t^{−0.5 ± 0.2}. The single-degenerate channel for SNe Ia can be made to match the observed SN Ia rate-SFR relation, but only if white dwarfs are converted to SNe Ia with a uniform efficiency of ~1%, independent of mass. Since low-mass progenitors are expected to have lower conversion efficiencies than high-mass progenitors, we conclude that some other progenitor scenario must be invoked to explain some, or perhaps all, SNe Ia.

We present multiband photometric and optical spectroscopic observations of SN 2007ax, the faintest and reddest Type Ia supernova (SN Ia) yet observed. With M_B = − 15.9 and (B − V)_max = 1.2, this SN is over half a magnitude fainter at maximum light than any other SN Ia. Similar to subluminous SN 2005ke, SN 2007ax also appears to show excess in UV emission at late time. Traditionally, Δ m₁₅(B) has been used to parameterize the decline rate for SNe Ia. However, the B-band transition from fast to slow decline occurs sooner than 15 days for faint SNe Ia. Therefore we suggest that a more physically motivated parameter, the time of intersection of the two slopes, be used instead. Only by explaining the faintest (and the brightest) supernovae can we thoroughly understand the physics of thermonuclear explosions. We suggest that future surveys should carefully design their cadence, depth, pointings, and follow-up to find an unbiased sample of extremely faint members of this subclass of faint SNe Ia.

The paradigmatic luminous blue variable R127 in the Large Magellanic Cloud has been found in the intermediate, peculiar early-B state, and substantially fainter in visual light, signaling the final decline from its major outburst that began between 1978 and 1980. This transformation was detected in 2008 January, but archival data show that it began between early 2005 and early 2007. In fact, significant changes from the maximum, peculiar A-type spectrum, which was maintained from 1986 through 1998, had already begun the following year, coinciding with a steep drop in visual light. We show detailed correspondences between the spectrum and light, in which the decline mimics the rise. Moreover, these trends are not monotonic but are characterized by multiple spikes and dips, which may provide constraints on the unknown outburst mechanism. Intensive photometric and spectroscopic monitoring of R127 should now resume, to follow the decline presumably back to the quiescent Ofpe/WN9 state, in order to fully document the remainder of this unique observational opportunity.

The recent identification of one or two subparsec disks of young, massive stars orbiting the ~4 × 10⁶M_☉ black hole Sgr A* has prompted an in situ scenario for star formation in disks of gas formed from a cloud captured from the Galactic center environment. To date there has been no explanation given for the low angular momentum of the disks relative to clouds passing close to the center. Here we show that the partial accretion of extended Galactic center clouds, such as the 50 km s⁻¹ giant molecular cloud, that temporarily engulf Sgr A* during their passage through the central region of the Galaxy provide a natural explanation for the angular momentum and surface density of the observed stellar disks. The captured cloud material is gravitationally unstable and forms stars as it circularizes, potentially explaining the large eccentricity and range of inclinations of the observed stellar orbits. The application of this idea to the formation of the circumnuclear ring is also discussed.

Electric currents j flow along the open magnetic field lines from the polar caps of neutron stars. Activity of a polar cap depends on the ratio α = j/cρ_GJ, where ρ_GJ is the corotation charge density. The customary assumption α ≈ 1 is not supported by recent global simulations of pulsar magnetospheres, and we study polar caps with arbitrary α. We argue that no significant activity is generated on field lines with 0 < α < 1. Charges are extracted from the star and flow along such field lines with low energies. By contrast, if α > 1 or α < 0, a high voltage is generated, leading to unsteady e^± discharge on a scale height smaller than the size of the polar cap. The discharge can power observed pulsars. Voltage fluctuations in the discharge imply unsteady twisting of the open flux tube and generation of Alfvén waves. These waves are ducted along the tube and converted to electromagnetic waves, providing a new mechanism for pulsar radiation.

We present direct upper limits on gravitational wave emission from the Crab pulsar using data from the first 9 months of the fifth science run of the Laser Interferometer Gravitational-wave Observatory (LIGO). These limits are based on two searches. In the first we assume that the gravitational wave emission follows the observed radio timing, giving an upper limit on gravitational wave emission that beats indirect limits inferred from the spin-down and braking index of the pulsar and the energetics of the nebula. In the second we allow for a small mismatch between the gravitational and radio signal frequencies and interpret our results in the context of two possible gravitational wave emission mechanisms.

The Galactic black hole X-ray binary XTE J1650–500 entered a quiescent regime following the decline from the 2001-2002 outburst that led to its discovery. Here we report on the first detection of its quiescent counterpart in a 36 ks observation taken in 2007 July with the Chandra X-Ray Observatory. The inferred 0.5-10 keV unabsorbed flux is in the range (2.5-5.0) × 10⁻¹⁵ erg s⁻¹ cm⁻². Notwithstanding large distance uncertainties, the measured luminosity is comparable to that of the faintest detected black hole X-ray binaries, all having orbital periods close to the expected bifurcation period between j- and n-driven low-mass X-ray binaries. This suggests that a few 10³⁰ erg s⁻¹ might be a limiting luminosity value for quiescent black holes.

We study the superorbital modulation present in the Cygnus X-1 X-ray data, usually attributed to the precession of the accretion disk and relativistic jets. We find a new, strong, 326 ± 2 day period modulation starting in 2005, in Swift BAT and RXTE ASM light curves (LCs). We also investigate Vela 5B ASM and Ariel 5 ASM archival data and confirm the previously reported ~290 day periodic modulation, therefore confirming that the superorbital period is not constant. Finally, we study RXTE ASM LC before 2005 and find that the previously reported ~150 day period is most likely an artifact due to the use of a Fourier-power-based analysis under the assumption that the modulation has a constant period along the whole data sample. Instead, we find strong indications of several discrete changes of the precession period, happening in coincidence with soft and failed state transition episodes. We also find a hint of correlation between the period and the amplitude of the modulation. The detection of gamma rays above 100 GeV with MAGIC in 2006 September happened in coincidence with a maximum of the superorbital modulation. The next maximum will happen between 2008 July 2 and 14, when the observational conditions of Cygnus X-1 with ground-based Cerenkov telescopes, such as MAGIC and VERITAS, are optimal.

We present high-precision radial velocity observations of HD 17156 during a transit of its eccentric Jovian planet. In these data, we detect the Rossiter-McLaughlin effect, which is an apparent perturbation in the velocity of the star due to the progressive occultation of part of the rotating stellar photosphere by the transiting planet. This system had previously been reported by Narita et al. in 2008 to exhibit a λ = 62°± 25° misalignment of the projected planetary orbital axis and the stellar rotation axis. We model our data, along with the Narita et al. data, and obtain λ = 9.4°± 9.3° for the combined data set. We thus conclude that the planetary orbital axis is actually very well aligned with the stellar rotation axis.

We announce the discovery of a twin of Jupiter orbiting the slightly metal-poor ([Fe/H] = − 0.1) nearby (d = 18 pc) G8 dwarf HD 154345. This planet has a minimum mass of 0.95 M_Jup and a 9.2 year, circular orbit with radius 4.2 AU. There is currently little or no evidence for other planets in the system, but smaller or exterior planets cannot yet be ruled out. We also detect a ~ 9 year activity cycle in this star photometrically and in chromospheric emission. We rule out activity cycles as the source of the radial velocity variations by comparison with other cycling late G dwarfs.

The extrasolar system OGLE-2006-BLG-109L is the first multiple-planet system to be discovered by gravitational microlensing (reported by Gaudi et al. in 2008); the two large planets that have been detected have mass ratios, semimajor axis ratios, and equilibrium temperatures that are similar to those of Jupiter and Saturn; the mass of the host star is only 0.5 M_☉, and the system is more compact than our own solar system. We find that in the habitable zone of the host star, the two detected planets resonantly excite large orbital eccentricities on a putative Earth-mass planet, driving such a planet out of the habitable zone. We show that an additional inner planet of ≳0.3 M_⊕ at ≲0.1 AU would suppress the eccentricity perturbation and greatly improve the prospects for habitability of the system. Thus, the planetary architecture of a potentially habitable OGLE-2006-BLG-109L planetary system—with two "terrestrial" planets and two Jovian planets—could bear very close resemblance to our own solar system.

We measured organic volatiles (CH₄, CH₃OH, C₂H₆, H₂CO), CO, and water in comet 8P/Tuttle, a comet from the Oort Cloud reservoir now in a short-period Halley-type orbit. We compare its composition with two other comets in Halley-type orbits, and with comets of the "organics-normal" and "organics-depleted" classes. Chemical gradients are expected in the comet-forming region of the protoplanetary disk, and an individual comet should reflect its specific heritage. If Halley-type comets came from the inner Oort Cloud as proposed, we see no common characteristics that could distinguish such comets from those that were stored in the outer Oort Cloud.

Establishing the sources of the fast and slow solar wind is important for understanding their drivers and their subsequent interaction in interplanetary space. Although coronal holes continue to be viewed as the main source of the fast solar wind, there is recent evidence that the quiet Sun provides other spatially concentrated sources. To identify the underlying physical characteristics of the outflow from coronal holes, solar disk observations from the Solar and Heliospheric Observatory (SOHO) are considered. These observations encompass photospheric line-of-sight magnetic field measurements from the Michelson Doppler Imager (MDI), Fe X 171 Å passband imaging from the Extreme-ultraviolet Imaging Telescope (EIT), and Ne VIII 770 Å spectral observations with outflows inferred from their corresponding Doppler blueshifts, at solar minimum and maximum and at different latitudes, from the Solar Ultraviolet Measurement of Emitted Radiation (SUMER) instrument. The sharp variations of outflows within the SUMER field of view, referred to as velocity gradients, are introduced as a new diagnostic. It is shown that, in general, coronal holes are indistinguishable from the quiet Sun, whether in their outflows or their gradients. Surprisingly, however, when enhanced unbalanced magnetic flux from active regions extends into neighboring coronal holes, both outflows and their gradients become significantly enhanced within the coronal holes and along their boundaries. The same effect is observed in the quiet Sun, albeit to a lesser extent. These findings point to the possibility that active regions can lead to enhanced plasma outflows in neighboring coronal holes.

Long-baseline observations of interplanetary scintillation (IPS) provide a unique source of information on solar wind speed and meridional direction across the inner regions of the solar system. We report the results of a series of coordinated IPS observations of an Earth-directed CME. A significant development in the interpretation of these data is the use of 3D tomographic reconstructions of solar wind structure derived from STELab IPS data to better constrain the analysis of extremely long baseline observations from EISCAT and MERLIN. The combination of these two approaches leads to a significantly better understanding of the interaction of the CME with the background solar wind than would be possible with either technique alone, revealing a significant rotation in the meridional flow direction of the background wind associated with the passage of the CME. The CME itself is decelerated significantly between its emergence through the corona and its arrival in the IPS ray path, with comparatively little change in speed from then until arrival at ACE.

Hinode discovered a beautiful giant jet with both cool and hot components at the solar limb on 2007 February 9. Simultaneous observations by the Hinode SOT, XRT, and TRACE 195 Å satellites revealed that hot (~5 × 10⁶ K) and cool (~10⁴ K) jets were located side by side and that the hot jet preceded the associated cool jet (~1-2 minutes). A current-sheet-like structure was seen in optical (Ca II H), EUV (195 Å), and soft X-ray emissions, suggesting that magnetic reconnection is occurring in the transition region or upper chromosphere. Alfvén waves were also observed with Hinode SOT. These propagated along the jet at velocities of ~200 km s⁻¹ with amplitudes (transverse velocity) of ~5-15 km s⁻¹ and a period of ~200 s. We performed two-dimensional MHD simulation of the jets on the basis of the emerging flux-reconnection model, by extending Yokoyama and Shibata's model. We extended the model with a more realistic initial condition (~10⁶ K corona) and compared our model with multiwavelength observations. The improvement of the coronal temperature and density in the simulation model allowed for the first time the reproduction of the structure and evolution of both the cool and hot jets quantitatively, supporting the magnetic reconnection model. The generation and the propagation of Alfvén waves are also reproduced self-consistently in the simulation model.

Prompted by high-resolution observations, I propose an explanation for the 40+ year old problem of structure and energy balance in the solar transition region. The ingredients are simply cross-field diffusion of neutral atoms from cool threads extending into the corona, and the subsequent excitation, radiation, and ionization of these atoms via electron impact. The processes occur whenever chromospheric plasma is adjacent to coronal plasma, and are efficient even when ion gyrofrequencies exceed collision frequencies. Cool threads—fibrils and spicules perhaps—grow slowly in thickness as a neutral, ionizing front expands across the magnetic field into coronal plasma. Radiative intensities estimated for H Lyα are within an order of magnitude of those observed, with no ad hoc parameters; only thermal parameters and geometric considerations are needed. I speculate that the subsequent dynamics of the diffused material might also explain observed properties of trace elements.

Photospheric granulation may excite transverse kink pulses in anchored vertical magnetic flux tubes. The pulses propagate upward along the tubes with the kink speed, while oscillating wakes are formed behind the wave front in a stratified atmosphere. The wakes oscillate at the kink cutoff frequency of stratified medium and gradually decay in time. When two or more consecutive kink pulses with different polarizations propagate in the same thin tube, then the wakes corresponding to different pulses may be superimposed. The superposition sets up helical motions of magnetic flux tubes in the photosphere/chromosphere as seen in recent Hinode movies. The energy carried by the pulses is enough to heat the solar chromosphere/corona and accelerate the solar wind.