Table of contents

Before reionization, the intergalactic medium (IGM) may have been sufficiently cold for low-mass "minihalos" to condense out of the gas and subsequently affect reionization. Previous work has shown that minihalos generate reasonably large 21 cm fluctuations. Here we consider this signal in its proper cosmological context and show that isolating minihalos from the rest of the IGM is extremely difficult. Using the well-known halo model, we compute the power spectrum of 21 cm fluctuations from minihalos and show that the signal decreases rapidly as feedback increases the Jeans mass. We then show that even a small Lyα background increases the 21 cm fluctuations of the diffuse IGM well beyond those of the minihalos; because the mass fraction in the IGM is much larger, minihalos will lie buried within the IGM signal. The distinctive signatures of nonlinear bias and minihalo structure emerge only at much smaller scales, well beyond the resolution of any upcoming instrument. Using simple but representative reionization histories, we then show that the required Lyα background level is most likely achieved at z ≳ 15, while minihalos are still rare, so that they are almost always degenerate with the diffuse IGM.

With a wariness of Occam's razor awakened by the discovery of cosmic acceleration, we abandon the usual assumption of zero mean curvature and ask how well it can be determined by planned surveys. We also explore the impact of uncertain mean curvature on forecasts for the performance of planned dark energy probes. We find that weak lensing and photometric baryon acoustic oscillation data, in combination with cosmic microwave background (CMB) data, can determine the mean curvature well enough that the residual uncertainty does not degrade constraints on dark energy. We also find that determinations of curvature are highly tolerant of photometric redshift errors.

Using a model for the self-regulated growth of supermassive BHs in mergers involving gas-rich galaxies, we study the relationship between quasars and the population of merging galaxies and predict the merger-driven SFR density of the universe. Mergers drive gas inflows, fueling starbursts and ``buried'' quasars until feedback disperses the gas, allowing the quasar to be briefly visible as a bright optical source. By simulating the evolution of such events, we demonstrate that the observed statistics of merger rates/fractions, luminosity and mass functions, SFR distributions, quasar (and quasar host) luminosity functions, and elliptical/red galaxy luminosity and mass functions are self-consistent. We use our simulations to deconvolve quasar and merger luminosity functions and determine the birthrate of BHs and merger rates as a function of mass. From this, we predict the merging galaxy luminosity function in various observed wavebands, color-magnitude relations, mass functions, SFR distributions and density, and quasar host galaxy luminosity function, as a function of redshift from z = 0 to 6. We invert this to predict quasar luminosity functions from observed merger luminosity functions or SFR distributions. Our results show good agreement with observations, but idealized models of quasar light curves give inaccurate estimates and are ruled out at >99.9% confidence, provided that quasars are triggered in mergers. Using only quasar observations, we estimate the contribution of mergers to the SFR density of the universe to high redshifts, z ~ 4, and constrain the evolution in the characteristic initial gas fractions of quasar and spheroid-producing mergers.

We compute the Type Ia, Ib/c, and II supernova (SN) rates as functions of the cosmic time for galaxies of different morphological types. We use four different chemical evolution models, each one reproducing the features of a particular morphological type: E/S0, S0a/b, Sbc/d, and Irr galaxies. We essentially describe the Hubble sequence by means of decreasing efficiency of star formation and increasing infall timescale. These models are used to study the evolution of the SN rates per unit luminosity and per unit mass as functions of cosmic time and as functions of the Hubble type. Our results indicate that (1) the observed increase of the SN rate per unit luminosity and unit mass from early to late galaxy types is accounted for by our models. Our explanation of this effect is related to the fact that the latest Hubble types have the highest star formation rate per unit mass. (2) By adopting a Scalo initial mass function in spiral disks, we find that massive (i.e., with initial mass >25 M_☉) single stars ending their lives as Wolf-Rayet objects are not sufficient to account for the observed Type Ib/c SN rate per unit mass. Less massive stars (i.e., with initial masses 12 < M/M_☉ < 20) in close binary systems can give instead a significant contribution to the local Ib/c SN rates. On the other hand, with the assumption of a Salpeter IMF for all galaxy types, single massive W-R stars are sufficient to account for the observed Type Ib/c SN rate. (3) Our models allow us to reproduce the observed Type Ia SN rate density up to redshift z ~ 1. At higher redshifts, our rates are higher than the few available data. In particular, we predict an increasing Type Ia SN rate density with redshift, reaching a peak at redshift z ≥ 3, because of the contribution of massive spheroids. (4) At z = 0, we reproduce the observed core-collapse (CC) SN rate density. Because of the few available observations, no firm conclusion can be drawn about the behavior of the CC SN rate at redshift z > 0.

The polytropic equation of state of an atomic hydrogen gas is examined for primordial halos with baryonic masses of M_h ~ 10⁷-10⁹M_☉. For roughly isothermal collapse around 10⁴ K, we find that line trapping of Lyα (H I and He II) photons causes the polytropic exponent to stiffen to values significantly above unity. Under the assumptions of zero H₂ abundance and very modest pollution by metals (<10^-4 solar), fragmentation is likely to be inhibited for such an equation of state. We argue on purely thermodynamic grounds that a single black hole of ~(0.02-0.003)M_h can form at the center of a halo for z = 10-20 when the free-fall time is less than the time needed for a resonantly scattered Lyα photon to escape from the halo. The absence of H₂ follows naturally from the high temperatures, >10⁴ K, that are attained when Lyα photons are trapped in the dense and massive halos that we consider. An H₂-dissociating UV background is needed if positive feedback effects on H₂ formation from X-rays occur. The black hole-to-baryon mass fraction is suggestively close to what is required for these intermediate-mass black holes, of mass M_BH ~ 10⁴-10⁶M_☉, to act as seeds for forming the supermassive black holes of mass ~0.001M_spheroid found in galaxies today.

We determine the shape, multiplicity, size, and radial structure of superclusters in the ΛCDM concordance cosmology from z = 0 to z = 2. Superclusters are defined as clusters of clusters in our large-scale cosmological simulation. We find that superclusters are triaxial in shape; many have flattened since early times to become nearly two-dimensional structures at present, with a small fraction of filamentary systems. The size and multiplicity functions are presented at different redshifts. Supercluster sizes extend to scales of ~100-200 h^-1 Mpc. The supercluster multiplicity (richness) increases linearly with supercluster size. The density profile in superclusters is approximately isothermal (~R^-2) and steepens on larger scales. These results can be used as a new test of the current cosmology when compared with upcoming observations of large-scale surveys.

We present gas mass fractions of 38 massive galaxy clusters at redshifts 0.14 ≤ z ≤ 0.89, derived from Chandra X-ray and OVRO/BIMA interferometric Sunyaev-Zel'dovich effect (SZE) measurements. We use three models for the gas distribution: (1) an isothermal β-model fit jointly to the X-ray data at radii beyond 100 kpc and to all of the SZE data, (2) a nonisothermal double β-model in hydrostatic equilibrium with a Navarro-Frenk-White (NFW) dark matter distribution, fit jointly to all of the X-ray and SZE data, and (3) an isothermal β-model fit only to the SZE spatial data. We show that the isothermal model well characterizes the intracluster medium outside the cluster core and provides good fits to clusters with a range of morphological properties. X-ray and SZE mean gas mass fractions for model 1 are f_gas(X-ray) = 0.110 ± 0.003 and f_gas(SZE) = 0.116 ± 0.005 assuming = ; uncertainties are statistical followed by systematic at 68% confidence. For model 2, f_gas(X-ray) = 0.119 ± 0.003 and f_gas(SZE) = 0.121 ± 0.005. For model 3, f_gas(SZE) = 0.120 ± 0.009. The agreement in the results shows that the core can be accounted for satisfactorily by either excluding it from fits to the X-ray data or modeling the intracluster gas with a nonisothermal double β-model. We find that the SZE is largely insensitive to core structure. Our results indicate that the ratio of gas mass fraction within r₂₅₀₀ to the cosmic baryon fraction, f_gas/ , is 0.68, where statistical and systematic uncertainties are included at 68% confidence. By assuming that cluster gas mass fractions are independent of redshift, we find that the results agree with standard ΛCDM cosmology and are inconsistent with a flat matter-dominated (Ω_M = 1) universe.

The galaxy cluster 1ES 0657-558 (z = 0.296) is remarkably well suited for addressing outstanding issues in both galaxy evolution and fundamental physics. We present a reconstruction of the mass distribution from both strong and weak gravitational lensing data. Multicolor, high-resolution HST ACS images allow detection of many more arc candidates than were previously known, especially around the subcluster. Using the known redshift of one of the multiply imaged systems, we determine the remaining source redshifts using the predictive power of the strong-lens model. Combining this information with shape measurements of "weakly" lensed sources, we derive a high-resolution, absolutely calibrated mass map, using no assumptions regarding the physical properties of the underlying cluster potential. This map provides the best available quantification of the total mass of the central part of the cluster. We also confirm the result from Clowe and colleagues that the total mass does not trace the baryonic mass.

Evidence for nonthermal activity in clusters of galaxies is well established from radio observations of synchrotron emission by relativistic electrons. New windows in the extreme-ultraviolet and hard X-ray ranges have provided more powerful tools for the investigation of this phenomenon. Detections of hard X-rays in the 20-100 keV range have been reported for several clusters of galaxies, notably from Coma and others. Based on these earlier observations we identified the relatively high-redshift cluster 1ES 0657-558 (also known as RX J0658-5557) as a good candidate for hard X-ray observations. This cluster, also known as the Bullet Cluster, has many other interesting and unusual features, most notably that it is undergoing a merger, clearly visible in the X-ray images. Here we present results from successful RXTE observations of this cluster. We summarize past observations and their theoretical interpretation that guided us in the selection process. We describe the new observations and present the constraints we can set on the flux and spectrum of the hard X-rays. Finally, we discuss the constraints one can set on the characteristics of accelerated electrons that produce the hard X-rays and the radio radiation.

We present the CXOCY J220132.8-320144 system, which is composed of an edge-on spiral galaxy at z = 0.32 lensing a z = 3.9 background quasar. Two images of the quasar are seen. The geometry of the system is favorable to separate the relative mass contribution of the disk and halo in the inner parts of the galaxy. We model the system with one elliptical mass component with the same ellipticity as the light distribution and manage to reproduce the quasar image positions and fluxes. We also model the system with two mass components, disk and halo. Again, we manage to reproduce the quasar image positions and fluxes. However, all models predict at least a third visible image close to the disk that is not seen in our images. We speculate that this is most likely due to extinction by the disk. We also measure the rotational velocity of the galaxy at 2.7 disk scale radius to be v_c = 130 ± 20 km s^-1 from the [O II] emission lines. When adding the rotational velocity constraint to the models, we find that the contribution to the rotational velocity of the disk is likely to be equal to or larger than the contribution of the halo at this radius. The detection of the third image and a more accurate measurement of the rotational velocity would help to set tighter constraints on the mass distribution of this edge-on spiral galaxy.

We used the deep, multiwavelength images obtained by the Great Observatories Origins Deep Survey (GOODS) to identify ~4700 Lyman break galaxies (LBGs) at z > 2.5, and 292 starburst galaxies at z ~ 1.2. We present the results from morphological analysis based on light profile shape and ellipticity for ~1333 of the most luminous LBGs. About 40% of LBGs at z ~ 3 have exponential profiles, ~30% of the galaxies have steep (r^1/4-like) profiles, and ~30% of LBGs have multiple cores of disturbed morphologies suggestive of close pairs or mergers. The fraction of spheriod-like LBGs decrease by about 15% from z ~ 5 to 3. A comparison of LBGs with the starburst galaxies at z ~ 1.2 shows that disklike and merger morphologies are dominant, but the fraction of spheroid-like profiles is about 20% higher among LBGs. The ellipticity distribution for LBGs exhibits a pronounced skew toward high ellipticities ( > 0.5), which cannot be explained by morphologies similar to the local disks and spheroids viewed at random orientations. The peak of the distribution evolves toward lower , from 0.7 at z = 4 to ~0.5 at z = 3. The ellipticity distribution for the z ~ 1.2 galaxies is relatively flat, similar to that seen for present-day galaxies. The dominance of elongated morphologies suggests that in a significant fraction of LBGs we may be witnessing star formation in clumps along gas-rich filaments, or the earliest gas-rich bars that encompass essentially the entire visible galaxy.

Applying the Kennicutt-Schmidt law to damped Lyα absorption systems (DLAs), we predict that 3% of the sky should be covered with extended objects brighter than μ_V ≈ 28.4 mag arcsec^-2, if DLAs at redshift z = [2.5, 3.5] undergo in situ star formation. We test this hypothesis by searching the Hubble Ultra Deep Field (UDF) F606W image for low surface brightness features of angular sizes, ranging between θ_DLA = 025 (d_DLA = 2 kpc) and 40 (d_DLA = 31 kpc). After convolving the F606W image with smoothing kernels of angular diameters θ_kern = θ_DLA, we find the number of detected objects to decrease rapidly to zero at θ_kern > 1''. Our search yields upper limits on the comoving SFR densities that are between factors of 30 and 100 lower than predictions, suggesting a reduction by more than a factor of 10 in star formation efficiency at z ~ 3. The lower star formation efficiencies may be due to the reduced molecular content of the DLA gas. We also find that the cosmological increase with redshift of the critical surface density for the Toomre instability may be sufficient to suppress star formation to the levels implied by the UDF observations. The upper limits on in situ star formation reduce the predicted metallicities at z ~ 3 to be significantly lower than observed, and reduce the heat input in the gas to be substantially lower than the inferred cooling rates. In contrast, the radiative output from compact Lyman break galaxies (LBGs) with R < 27 is sufficient to balance the comoving cooling rate. Thus, many DLAs may host more compact regions of active star formation, which may chemically enrich these DLAs. Such regions are likely to be LBGs.

We use 211 galaxy spectra from our survey for Lyman break galaxies (LBGs) associated with 11 damped Lyα systems (DLAs) to measure the three-dimensional LBG autocorrelation and DLA-LBG cross-correlation functions with the primary goal of inferring the mass of DLAs at z ~ 3. From every measurement and test in this work, we find evidence for an overdensity of LBGs near DLAs that is very similar to that of LBGs near other LBGs. Conventional binning of the data while varying both r₀ and γ parameters of the fiducial model of the correlation function ξ(r) = (r/r₀)^-γ resulted in the best-fit values and 1 σ uncertainties of r₀ = 2.65 ± 0.48, γ = 1.55 ± 0.40 for the LBG autocorrelation, and r₀ = 3.32 ± 1.25, γ = 1.74 ± 0.36 for the DLA-LBG cross-correlation function. To circumvent shortcomings found in binning small data sets, we perform a maximum likelihood analysis based on Poisson statistics. The best-fit values and 1 σ confidence levels were found to be r₀ = 2.91, γ = 1.21 for the LBG autocorrelation, and r₀ = 2.81, γ = 2.11 for the DLA-LBG cross-correlation function. We report a redshift spike of five LBGs with Δz = 0.015 of the z = 2.936 DLA in the PSS 0808+5215 field; the DLA-LBG clustering signal survives when omitting this field from the analysis. Using the correlation function measurements and uncertainties, we compute the z ~ 3 LBG galaxy bias b_LBG to be 1.5 < b_LBG < 3, corresponding to an average halo mass of 10^9.7 < ⟨M_LBG⟩ < 10^11.6M_☉, and the z ~ 3 DLA galaxy bias b_DLA to be 1.3 < b_DLA < 4, corresponding to an average halo mass of 10⁹ < ⟨M_LBG⟩ < 10¹²M_☉. Finally, two of the six QSOs discovered were found to lie within Δz = 0.0125 of two of the survey DLAs. We estimate that the probability of this occurring by chance is 1 in 940, indicating a possible relationship between the distribution of QSOs and DLAs at z ~ 3.

The optical afterglow spectrum of GRB 050401 (at z = 2.8992 ± 0.0004) shows the presence of a damped Lyα absorber (DLA), with log N = 22.6 ± 0.3. This is the highest column density ever observed in a DLA and is about 5 times larger than the strongest DLA detected so far in any QSO spectrum. From the optical spectrum, we also find a very large Zn column density, implying an abundance of [Zn/H] = -1.0 ± 0.4. These large columns are supported by the early X-ray spectrum from Swift XRT, which shows a column density (in excess of Galactic) of log N_H = 22.21 assuming solar abundances (at z = 2.9). The comparison of this X-ray column density, which is dominated by absorption due to α-chain elements, and the H I column density derived from the Lyα absorption line allows us to derive a metallicity for the absorbing matter of [α/H] = -0.4 ± 0.3. The optical spectrum is reddened and can be well reproduced with a power law with SMC extinction, where A_V = 0.62 ± 0.06. But the total optical extinction can also be constrained independent of the shape of the extinction curve: from the optical to X-ray spectral energy distribution, we find 0.5 ≲ A_V ≲ 4.5. However, even this upper limit, independent of the shape of the extinction curve, is still well below the dust column that is inferred from the X-ray column density, i.e., A_V = 9.1. This discrepancy might be explained by a small dust content with high metallicity (low dust-to-metals ratio). "Gray" extinction cannot explain the discrepancy, since we are comparing the metallicity to a measurement of the total extinction (without reference to the reddening). Little dust with high metallicity may be produced by sublimation of dust grains or may naturally exist in systems younger than a few hundred megayears.

We present an exact three-dimensional wave solution to the shearing-sheet equations of motion. The existence of this solution argues against transient amplification as a route to turbulence in unmagnetized disks. Moreover, because the solution covers an extensive dynamical range in wavenumber space, it is an excellent test of the dissipative properties of numerical codes.

We present an analysis of the observed broad iron line feature and putative warm absorber in the long 2001 XMM-Newton observation of the Seyfert 1.2 galaxy MCG -06-30-15. The new kerrdisk model we have designed for simulating line emission from accretion disk systems allows black hole spin to be a free parameter in the fit, enabling the user to formally constrain the angular momentum of a black hole, among other physical parameters of the system. In an important extension of previous work, we derive constraints on the black hole spin in MCG -06-30-15 using a self-consistent model for X-ray reflection from the surface of the accretion disk while simultaneously accounting for absorption by dusty photoionized material along the line of sight (the warm absorber). Even including these complications, the XMM-Newton EPIC pn data require extreme relativistic broadening of the X-ray reflection spectrum; assuming no emission from within the radius of marginal stability, we derive a formal constraint on the dimensionless black hole spin parameter of a = 0.989 at 90% confidence. The principal unmodeled effect that can significantly reduce the inferred black hole spin is powerful emission from within the radius of marginal stability. Although significant theoretical developments are required to fully understand this region, we argue that the need for a rapidly spinning black hole is robust to physically plausible levels of emission from within the radius of marginal stability. In particular, we show that a nonrotating black hole is strongly ruled out.

A microscopic analysis of the viscous energy gain of energetic particles in (gradual) nonrelativistic shear flows is presented. We extend previous work and derive the Fokker-Planck coefficients for the average rate of momentum change and dispersion in the general case of a momentum-dependent scattering time τ(p) ∝ p^α with α ≥ 0. We show that in contrast to diffusive shock acceleration, the characteristic shear acceleration timescale depends inversely on the particle mean free path, which makes the mechanism particularly attractive for high-energy seed particles. Based on an analysis of the associated Fokker-Planck equation we show that above the injection momentum p₀, power-law differential particle number density spectra n(p) ∝ p^-(1+α) are generated for α > 0 if radiative energy losses are negligible. We discuss the modifications introduced by synchrotron losses and determine the contribution of the accelerated particles to the viscosity of the background flow. Possible implications for the plasma composition in mildly relativistic extragalactic jet sources are addressed.

We report measurements of time delays of up to 8 minutes in the centimeter-wavelength variability patterns of the intrahour scintillating quasar PKS 1257-326, as observed between the VLA and the ATCA on three separate epochs. These time delays confirm interstellar scintillation as the mechanism responsible for the rapid variability, at the same time effectively ruling out the coexistence of intrinsic intrahour variability in this source. The time delays are combined with measurements of the annual variation in variability timescale exhibited by this source to determine the characteristic length scale and anisotropy of the quasar's intensity-scintillation pattern, as well as to attempt to fit for the bulk velocity of the scattering plasma responsible for the scintillation. We find evidence for anisotropic scattering and highly elongated scintillation patterns at both 4.9 and 8.5 GHz, with an axial ratio >10 : 1, extended in a northwest direction on the sky. The characteristic scale of the scintillation pattern along its minor axis is well determined, but the high anisotropy leads to degenerate solutions for the scintillation velocity. The decorrelation of the pattern over the baseline gives an estimate of the major-axis length scale of the scintillation pattern. We derive an upper limit on the distance to the scattering plasma of no more than 10 pc.

Using a new approach to modeling the magnetically dominated outflows from active galactic nuclei, we study the propagation of magnetic tower jets in gravitationally stratified atmospheres (such as a galaxy cluster environment) at large scales (more than tens of kiloparsecs) by performing three-dimensional MHD simulations. We present the detailed analysis of the MHD waves, the cylindrical radial force balance, and the collimation of magnetic tower jets. As magnetic energy is injected into a small central volume over a finite amount of time, the magnetic fields expand down the background density gradient, forming a collimated jet and an expanded "lobe" due to the gradually decreasing background density and pressure. Both the jet and lobes are magnetically dominated. In addition, the injection and expansion produce a hydrodynamic shock wave that moves ahead of and encloses the magnetic tower jet. This shock can eventually break the hydrostatic equilibrium in the ambient medium and cause a global gravitational contraction. This contraction produces a strong compression at the head of the magnetic tower front and helps to collimate the jet radially to produce a slender body. At the outer edge of the jet, the magnetic pressure is balanced by the background (modified) gas pressure, without any significant contribution from the hoop stress. On the other hand, along the central axis of the jet, hoop stress is the dominant force in shaping the central collimation of the poloidal current. The system, which possesses a highly wound helical magnetic configuration, never quite reaches a force-free equilibrium state, although the evolution becomes much slower at late stages. The simulations were performed without any initial perturbations, so the overall structures of the jet remain mostly axisymmetric.

We have analyzed a sample of nearby cool and warm infrared (IR) galaxies using photometric and structural parameters. The set of measures include far-infrared color [C = log(S_{60 μm}/S_{100 μm})], total IR luminosity (L_TIR), radio surface brightness, and radio, near-infrared, and optical sizes. In a given luminosity range cool and warm galaxies are considered as those sources that are found approximately 1 σ below and above the mean color in the far-infrared C-L_TIR diagram. We find that galaxy radio surface brightness is well correlated with color whereas size is less well correlated with color. Our analysis indicates that IR galaxies that are dominated by cool dust are large, massive spirals that are not strongly interacting or merging and presumably the ones with the least active star formation. Dust in these cool objects is less centrally concentrated than in the more typical luminous and ultraluminous IR galaxies that are dominated by warm dust. Our study also shows that low-luminosity early-type unbarred and transitional spirals are responsible for the large scatter in the C-L_TIR diagram. Among highly luminous galaxies, late-type unbarred spirals are predominately warm, and early-type unbarred and barred spirals are systematically cooler. We highlight the significance of the C-L_TIR diagram in terms of local and high-redshift submillimeter galaxies.

We derive composite luminosity functions (LF) for galaxies in groups and examine the behavior of the LF as a function of group luminosity (used as an indicator of group or halo mass). We consider both the entire galaxy population and galaxies split into red and blue (quiescent and star-forming) samples, in order to examine possible mechanisms behind observed variations of galaxy properties with environment. We find evidence that M* brightens and α steepens with group luminosity, until a threshold value where the LF parameters stabilize at those found in rich clusters. The effect is seen in the total LF and for the blue and red galaxies separately. The behavior of the quiescent and star-forming samples is qualitatively consistent with variations resulting from interactions and mergers, where mergers build the bright end of the luminosity function at the same time as dwarf irregulars have their star formation quenched and evolve into dwarf ellipticals. These processes appear to take place preferentially in low-luminosity groups and to be complete at a group luminosity of -22.5 in B, corresponding to a halo mass of order 10^13.5 _☉.

The number of detected baryons in the universe at z < 0.5 is much smaller than predicted by standard big bang nucleosynthesis and by the detailed observation of the Lyα forest at redshift z = 2. Hydrodynamic simulations indicate that a large fraction of the baryons is expected to be in a "warm-hot" (10⁵-10⁷ K) filamentary gas, distributed in the intergalactic medium. This gas, if it exists, should be observable only in the soft X-ray and UV bands. Using the predictions of a particular hydrodynamic model, we simulated the X-ray flux as a function of energy in the 0.1-2 keV band due to the warm-hot intergalactic medium (WHIM) and compared it with the flux from other diffuse components. Our results show that as much as 20% of the total diffuse X-ray background (DXB) in the energy range 0.37-0.925 keV could be due to X-rays from the WHIM, 70% of which from filaments at redshift between 0.1 and 0.6. Simulations done using a FOV of 3' show that in more than 20% of the observations we expect the WHIM flux to contribute to more than 20% of the DXB. These simulations also show that in about 10% of all the observations a single bright filament in the FOV accounts alone for more than 20% of the DXB flux. Redshifted oxygen lines should be clearly visible in these observations. We also investigate the expected angular distribution of the X-ray flux from the WHIM and found a characteristics angular scale of a few arcminutes.

We present the first low-luminosity [L_X > (5-10) × 10³⁶ ergs s^-1] X-ray luminosity functions (XLFs) of low-mass X-ray binaries (LMXBs) determined for two typical old elliptical galaxies, NGC 3379 and NGC 4278. Because both galaxies contain little diffuse emission from hot ISM and no recent significant star formation (hence no high-mass X-ray binary contamination), they provide two of the best homogeneous sample of LMXBs. With 110 and 140 ks Chandra ACIS S3 exposures, we detect 59 and 112 LMXBs within the D₂₅ ellipses of NGC 3379 and NGC 4278, respectively. The resulting XLFs are well represented by a single power law with a slope (in a differential form) of 1.9 ± 0.1. In NGC 4278, we can exclude the break at L_X ~ 5 × 10³⁷ ergs s^-1 that was recently suggested as being a general feature of LMXB XLFs. In NGC 3379, on the other hand, we find a localized excess over the power-law XLF at ~4 × 10³⁷ ergs s^-1, but with a marginal significance of ~1.6 σ. Because of the small number of luminous sources, we cannot constrain the high-luminosity break (at 5 × 10³⁸ ergs s^-1) found in a large sample of early-type galaxies. For our two galaxies, the ratios of the integrated LMXB X-ray luminosities to the optical luminosities differ by a factor of 4, but are consistent with the general trend of a positive correlation between the X-ray-to-optical luminosity ratio and the globular cluster specific frequency.

We present a comparison of bar strength Q_b and circumnuclear dust morphology for 75 galaxies in order to investigate how bars affect the centers of galaxies. We trace the circumnuclear dust morphology and amount of dust structure with structure maps generated from visible-wavelength HST data, finding that tightly wound nuclear dust spirals are primarily found in weakly barred galaxies. While strongly barred galaxies sometimes exhibit grand-design structure within the central 10% of D₂₅, this structure rarely extends to within ~10 pc of the galaxy nucleus. In some galaxies, these spiral arms terminate at a circumnuclear starburst ring. Galaxies with circumnuclear rings are generally more strongly barred than galaxies lacking rings. Within these rings, the dust structure is fairly smooth and usually in the form of a loosely wound spiral. These data demonstrate that multiple nuclear morphologies are possible in the most strongly barred galaxies: chaotic central dust structure inconsistent with a coherent nuclear spiral, a grand-design spiral that loses coherence before reaching the nucleus, or a grand-design spiral that ends in a circumnuclear ring. These observations may indicate that not all strong bars are equally efficient at fueling material to the centers of their host galaxies. Finally, we investigate the long-standing hypothesis that SB(s) galaxies have weak bars and SB(r) galaxies have strong bars, finding the opposite to be the case: namely, SB(r) galaxies are less strongly barred and have less dust structure than SB(s) galaxies. In general, more strongly barred galaxies tend to have higher nuclear dust contrast.

One possible way for spiral galaxies to internally evolve would be for gas to flow to the center and form stars in a central disk (pseudo-bulge). If the inflow rate is faster than the rate of star formation, a central concentration of gas will form. In this paper we present radial profiles of stellar and 8 μm emission from polycyclic aromatic hydrocarbons (PAHs) for 11 spiral galaxies to investigate whether the interstellar medium in these galaxies contains a central concentration above that expected from the exponential disk. In general, we find that the two-dimensional CO and PAH emission morphologies are similar, and that they exhibit similar radial profiles. We find that in 6 of the 11 galaxies there is a central excess in the 8 μm and CO emission above the inward extrapolation of an exponential disk. In particular, all four barred galaxies in the sample have strong central excesses in both 8 μm and CO emission. These correlations suggest that the excess seen in the CO profiles is, in general, not simply due to a radial increase in the CO emissivity. In the inner disk, the ratio of the stellar to the 8 μm radial surface brightness is similar for 9 of the 11 galaxies, suggesting a physical connection between the average stellar surface brightness and the average gas surface brightness at a given radius. We also find that the ratio of the CO to 8 μm PAH surface brightness is consistent over the sample, implying that the 8 μm PAH surface brightness can be used as an approximate tracer of the interstellar medium.

The huge star formation events that occur at some galactic centers do not provide enough clues as to their origin, since the morphological signatures of the triggering mechanism are smeared out in the timescale of a few orbital revolutions of the galaxy core. Our high spatial resolution three-dimensional near-infrared spectroscopy for the first time reveals that a previously known hidden mass concentration is located exactly at the youngest end of a giant star-forming arc. This location, the inferred average cluster ages, and the dynamical times clearly indicate that the interloper has left behind a spur of violent star formation in M83, in a transient event lasting less than one orbital revolution. The study of the origin (bar funneling or cannibalized satellite) and fate (black hole merging or giant stellar cluster) of this system could provide clues to the question of core growing and morphological evolution in grand-design spiral galaxies. In particular, our TreeSPH numerical modeling suggests that the two nuclei could coalesce, forming a single massive core in about 60 million years or less. This work is based on observations made at the Gemini South Telescope.

We derive and interpret some relations between the luminosity, mass, and age distributions of star clusters, denoted here by ϕ(L), ψ(M), and χ(τ), respectively. Of these, ϕ(L) is the easiest to determine observationally, whereas ψ(M) and χ(τ) are more informative about formation and disruption processes. For a population of young clusters, with a relatively wide range of ages, ϕ(L) depends on both ψ(M) and χ(τ) and thus cannot serve as a proxy for ψ(M) in general. We demonstrate this explicitly by four illustrative examples with specific forms for either ψ(M) or χ(τ). In the special case in which ψ(M) is a power law and is independent of χ(τ), however, ϕ(L) is also a power law with the same exponent as ψ(M). We conclude that this accounts for the observed similarity between ϕ(L) and ψ(M) for the young clusters in the Antennae galaxies. This result reinforces our picture in which clusters form with ψ(M) ∝ M^-2 and are then disrupted rapidly at a rate roughly independent of their masses. The most likely disruptive process in this first stage is the removal of interstellar matter by the energy and momentum input from young stars (by photoionization, winds, jets, and supernovae). The few clusters that avoid this "infant mortality" are eventually disrupted in a second stage by the evaporation of stars driven by two-body relaxation, a process with a strong dependence on mass. We suspect this picture may apply to many, if not all, populations of star clusters, but this needs to be verified observationally by determinations of ψ(M) and χ(τ) in more galaxies.

We present initial results from a time series BVI survey of two fields in NGC 4258 using the HST ACS. This galaxy was selected because of its accurate maser-based distance, which is anticipated to have a total uncertainty of ~3%. The goal of the HST observations is to provide an absolute calibration of the Cepheid distance scale and to measure its dependence on chemical abundance (the so-called metallicity effect). We carried out observations of two fields at different galactocentric distances with a mean abundance difference of 0.5 dex. We discovered a total of 281 Cepheids with periods ranging from 4 to 45 days (the duration of our observing window). We determine a Cepheid distance modulus for NGC 4258 (relative to the LMC) of Δμ₀ = 10.88 ± 0.04 (random) ± 0.05 (systematic) mag. Given the published maser distance to the galaxy, this implies μ₀(LMC) = 18.41 ± 0.10_r ± 0.13_s mag or D(LMC) = 48.1 ± 2.3_r ± 2.9_s kpc. We measure a metallicity effect of γ = -0.29 ± 0.09_r ± 0.05_s mag dex^-1. We see no evidence for a variation in the slope of the period-luminosity relation as a function of abundance. We estimate a Hubble constant of H₀ = 74 ± 3_r ± 6_s km s^-1 Mpc^-1 using a recent sample of four well-observed Type Ia SNe and our new calibration of the Cepheid distance scale. It may soon be possible to measure the value of H₀ with a total uncertainty of 5%, with consequent improvement in the determination of the equation of state of dark energy.

The effect of a barred potential (such as the one of the Milky Way) on the Galactic orbits of 48 globular clusters for which absolute proper motions are known is studied. The orbital characteristics are compared with those obtained for the case of an axisymmetric Galactic potential. Tidal radii are computed and discussed for both the better known axisymmetric case and that including a bar. The destruction rates due to bulge and disk shocking are calculated and compared in both Galactic potentials.

We present Spitzer imaging of the metal-deficient (Z ≃ 30% Z_☉) Local Group dwarf galaxy NGC 6822. On spatial scales of ~130 pc, we study the nature of IR, Hα, H I, and radio continuum emission. Nebular emission strength correlates with IR surface brightness; however, roughly half of the IR emission is associated with diffuse regions not luminous at Hα (as found in previous studies). The global ratio of dust to H I gas in the ISM, while uncertain at the factor of ~2 level, is ~25 times lower than the global values derived for spiral galaxies using similar modeling techniques; localized ratios of dust to H I gas are about a factor of 5 higher than the global value in NGC 6822. There are strong variations (factors of ~10) in the relative ratios of Hα and IR flux throughout the central disk; the low dust content of NGC 6822 is likely responsible for the different Hα/IR ratios compared to those found in more metal-rich environments. The Hα and IR emission is associated with high column density (≳10²¹ cm^-2) neutral gas. Increases in IR surface brightness appear to be affected by both increased radiation field strength and increased local gas density. Individual regions and the galaxy as a whole fall within the observed scatter of recent high-resolution studies of the radio-far-IR correlation in nearby spiral galaxies; this is likely the result of depleted radio and far-IR emission strengths in the ISM of this dwarf galaxy.

We present a method for isolating a clean sample of red giant branch stars in the outer regions of M31. Our study is based on an ongoing spectroscopic survey using the DEIMOS instrument on the Keck II 10 m telescope. The survey aims to study the kinematics, (sub)structure, and metallicity of M31's halo. Although most of our spectroscopic targets were photometrically screened to reject foreground Milky Way dwarf star contaminants, dwarf stars still constitute a substantial fraction of the observed spectra in the sparse outer halo. Our likelihood-based method for isolating M31 red giants uses five criteria: (1) radial velocity, (2) photometry in the intermediate-width DDO51 band to measure the strength of the MgH/Mg b absorption features, (3) strength of the Na I λ8190 absorption line doublet, (4) location within an (I, V - I) color-magnitude diagram, and (5) comparison of photometric (color-magnitude diagram based) versus spectroscopic (Ca II λ8500 triplet based) metallicity estimates. We also discuss other potential giant/dwarf separation criteria: the strength of the K I absorption lines at 7665 and 7699 Å and the TiO bands at 7100, 7600, and 8500 Å. Training sets consisting of definite M31 red giants and Galactic dwarf stars are used to derive empirical probability distribution functions for each diagnostic. These functions are used to calculate the likelihood that a given star is a red giant in M31 versus a Milky Way dwarf star. Using our diagnostic method, we isolate 40 M31 red giants beyond a projected distance of R = 60 kpc from the galaxy's center, including three red giants at R ~ 165 kpc. The ability to identify individual M31 red giant stars gives us an unprecedented level of sensitivity in studying the properties of the galaxy's outer halo.

We present a measurement of the systemic proper motion of the Small Magellanic Cloud (SMC) made using the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope (HST). We tracked the SMC's motion relative to four background QSOs over a baseline of approximately 2 yr. The measured proper motion is μ_W = -1.16 ± 0.18 mas yr^-1, μ_N = -1.17 ± 0.18 mas yr^-1. This is the best measurement yet of the SMC's proper motion. We combine this new result with our prior estimate of the proper motion of the Large Magellanic Cloud (LMC) from the same observing program to investigate the orbital evolution of both Clouds over the past 9 Gyr. The current relative velocity between the Clouds is 105 ± 42 km s^-1. Our investigations of the past orbital motions of the Clouds in a simple model for the dark halo of the Milky Way imply that the Clouds could be unbound from each other. However, our data are also consistent with orbits in which the Clouds have been bound to each other for approximately a Hubble time. Smaller proper-motion errors and better understanding of the LMC and SMC masses would be required to constrain their past orbital history and their bound versus unbound nature unambiguously. The new proper-motion measurements should be sufficient to allow the construction of improved models for the origin and properties of the Magellanic Stream. In turn, this will provide new constraints on the properties of the Milky Way dark halo.

We have carried out a blind search in the general direction of the Galactic anticenter for absorption of the cosmic microwave background (CMB) radiation near 4.83 GHz by molecular clouds containing gaseous ortho-formaldehyde (H₂CO). The observations were done using the 25 m radio telescope at Onsala in Sweden and covered strips in Galactic latitude -1° ≤ b ≤ +1° at several longitudes in the region 170° ≤ l ≤ 190°. Spectra were obtained in these strips with a grid spacing corresponding to the telescope resolution of 10'. We have detected H₂CO CMB absorption at ≈10% of the survey pointings. This detection rate is likely to increase with further improvements in sensitivity and may become comparable to the detection rate expected from a blind CO survey with a corresponding sensitivity limit. We have mapped some of these detections in more detail and compared the H₂CO absorption to existing maps of CO(1-0) emission in the same regions. There appears to be a rough correlation between the velocity-integrated line strength of the CO(1-0) emission and that of the H₂CO absorption. However, the scatter in this correlation is significantly larger than the measurement errors, indicating differences of detail at and below the linear resolution of our observations (≈4-9 pc). Although these two tracers are expected to have similar excitation requirements on the microscopic level characteristic of warm (T_K > 10 K) dense (10³ cm^-3 < n < 10⁵ cm^-3) condensations in molecular clouds, the CO(1-0) line is expected to be optically thick, whereas the H₂CO line is not. This latter difference is likely to be responsible for a significant part of the scatter in the correlation we have found.

We investigate the suggestion that there are stellar populations in some globular clusters with enhanced helium (Y ~ 0.28-0.40) compared to the primordial value. We assume that a previous generation of massive asymptotic giant branch (AGB) stars have polluted the cluster. Two independent sets of AGB yields are used to follow the evolution of helium and CNO using a Salpeter initial mass function (IMF) and two top-heavy IMFs. In no case are we able to produce the postulated large Y ~ 0.35 without violating the observational constraint that the CNO content is nearly constant.

We introduce a Monte Carlo model of nonlinear diffusive shock acceleration that allows for the generation of large-amplitude magnetic turbulence, i.e., ΔB ≫ B₀, where B₀ is the ambient magnetic field. The model is the first to include strong wave generation, efficient particle acceleration to relativistic energies in nonrelativistic shocks, and thermal particle injection in an internally self-consistent manner. We find that the upstream magnetic field B₀ can be amplified by large factors and show that this amplification depends strongly on the ambient Alfvén Mach number. We also show that, in the nonlinear model, large increases in B do not necessarily translate into a large increase in the maximum particle momentum a particular shock can produce, a consequence of high-momentum particles diffusing in the shock precursor where the large amplified field converges to the low ambient value. To deal with the field growth rate in the regime of strong fluctuations, we extend to strong turbulence a parameterization that is consistent with the resonant quasi-linear growth rate in the weak turbulence limit. We believe our parameterization spans the maximum and minimum range of the fluctuation growth, and within these limits we show that the nonlinear shock structure, acceleration efficiency, and thermal particle injection rates depend strongly on the yet to be determined details of wave growth in strongly turbulent fields. The most direct application of our results will be to estimate magnetic fields amplified by strong cosmic-ray modified shocks in supernova remnants.

We present observations of two LMC supernova remnants (SNRs), DEM L238 and DEM L249, with the Chandra and XMM-Newton X-ray satellites. Bright central emission, surrounded by a faint shell, is present in both remnants. The central emission has an entirely thermal spectrum dominated by strong Fe L-shell lines, with the deduced Fe abundance in excess of solar and not consistent with the LMC abundance. This Fe overabundance leads to the conclusion that DEM L238 and DEM L249 are remnants of thermonuclear (Type Ia) explosions. The shell emission originates in gas swept up and heated by the blast wave. A standard Sedov analysis implies about 50 M_☉ in both swept-up shells, SNR ages between 10,000 and 15,000 yr, low (≲0.05 cm^-3) preshock densities, and subluminous explosions with energies of 3 × 10⁵⁰ ergs. The central Fe-rich supernova ejecta are close to collisional ionization equilibrium. Their presence is unexpected, because standard Type Ia SNR models predict faint ejecta emission with short ionization ages. Both SNRs belong to a previously unrecognized class of Type Ia SNRs characterized by bright interior emission. Denser than expected ejecta and/or a dense circumstellar medium around the progenitors are required to explain the presence of Fe-rich ejecta in these SNRs. Substantial amounts of circumstellar gas are more likely to be present in explosions of more massive Type Ia progenitors. DEM L238, DEM L249, and similar SNRs could be remnants of "prompt" Type Ia explosions with young (~100 Myr old) progenitors.

Sub-TeV gamma-ray emission from the northwest rim of the supernova remnant RX J0852.0-4622 was detected with the CANGAROO II telescope and recently confirmed by the HESS group. In addition, the HESS data revealed a very wide (up to 2° in diameter), shell-like profile of the gamma-ray emission. We carried out CANGAROO III observations in 2005 January and February with three telescopes and show here the results of threefold coincidence data. We confirm the HESS results about the morphology and the energy spectrum and find that the energy spectrum in the NW rim is consistent with that of the whole remnant.

We report on a Chandra observation of the Crab Nebula that gives the first clear view of the faint boundary of the Crab's X-ray-emitting pulsar wind nebula. There is structure in all directions. Fingers, loops, bays, and the south pulsar jet all indicate that either filamentary material or the magnetic field is controlling the relativistic electrons. In general, spectra soften as distance from the pulsar increases but do not change rapidly along linear features. This is particularly true for the pulsar jet. The termination of the jet is abrupt; the east side is close to an [O III] optical filament, which may be blocking propagation on this side. We argue that linear features have ordered magnetic fields and that the structure is determined by the synchrotron lifetime of particles diffusing perpendicular and parallel to the magnetic field. We find no significant evidence for thermal X-rays inside the filamentary envelope.

We present spectral line observations of the ground-state transitions of hydroxyl (OH) toward supernova remnant (SNR) IC 443 carried out with the Green Bank Telescope. Weak, extended OH (1720 MHz) maser emission with OH (1667, 1665, 1612 MHz) absorption is detected along the southern extent of the remnant where no bright compact maser sources have been previously observed. These newly detected SNR-type masers are coincident with well-known molecular clumps and a ridge of shocked H₂ emission indicative of the SNR shock front interacting with the adjacent molecular cloud. Simultaneous observation of all four ground-state transitions of OH permits us to fit physical conditions of the shocked gas at the interaction site. A simple two-component model for the line profiles yields the physical parameters for detected regions of maser emission, including excitation temperature, OH column density, and filling factor. Observed line profiles suggest the shock is largely propagating toward the line of sight in the region of these newly identified weak masers. The implications of shock geometry and physical parameters in producing spatially extended OH maser emission in SNRs are explored. We also present VLA radio continuum observations at 330 MHz for comparison with OH line observations of the remnant.

We argue and demonstrate by particle-in-cell simulations that the synchrotron maser instability could develop at the front of a relativistic, magnetized shock. The instability generates strong low-frequency electromagnetic waves propagating both upstream and downstream of the shock. Upstream of the shock, these waves make electrons lag behind ions so that a longitudinal electric field arises and the electrons are accelerated up to the ion kinetic energy. Then thermalization at the shock front results in a plasma with equal temperatures of electrons and ions. Downstream of the shock, the amplitude of the maser-generated wave may exceed the strength of the shock-compressed background magnetic field. In this case the shock-accelerated particles radiate via nonlinear Compton scattering rather than via a synchrotron mechanism. The spectrum of the radiation differs, in the low-frequency band, from that of the synchrotron radiation, providing possible observational tests of the model.

This paper investigates to what extent the numerical scheme regularized smoothed particle hydrodynamics (RSPH) is able to accurately describe multidimensional MHD shocks. The scheme can be viewed as an extension to smoothed particle hydrodynamics (SPH), which is widely used for astrophysical applications. In the first of two previous papers, the basic idea behind the RSPH scheme was introduced and tested, primarily on one-dimensional MHD shock problems. A new formulation of the momentum equation was also proposed to secure stability in the low-β regime. A two-dimensional, linear stability analysis of this formulation was presented in the second paper. The second paper also utilized recent developments of the RSPH scheme that improve the overall description of multidimensional problems in general. Based on the results from the linear stability analysis, adjustments to the momentum equation are made in the present work, which are also applicable to the nonlinear regime. These adjustments address the problem of asymmetries in the momentum equation, which in nonlinear problems can lead to small, yet systematic errors in postshock conditions. In addition, this paper describes the first application of the improved RSPH scheme to multidimensional MHD shocks. Comparisons are made with existing methods, in particular the related SPH method. Special attention is given to the scheme's ability to maintain the ∇ = 0 constraint and to what extent redefining the particle distribution affects the conservation of kinetic energy and angular momentum.

We calculate the scattering of X-rays by interstellar dust for a dust model that reproduces the observed wavelength-dependent extinction and polarization of starlight. On interstellar sight lines that produce appreciable starlight polarization, we predict that the dust-scattered X-ray halo around point sources will have measurable azimuthal asymmetry due to scattering by partially aligned nonspherical grains. We calculate the expected halo asymmetry. X-ray halo asymmetry provides a new test of interstellar dust models.

We analyze the structure and stability in plane-parallel geometry of the transition layer (or front) that connects the cold neutral medium and the warm neutral medium. Such fronts appear in recent numerical simulations of a thermally bistable interstellar medium. The front becomes an evaporation or condensation front depending on the surrounding pressure. The stability analysis is performed in both long- and short-wavelength approximations. We find that the plane-parallel evaporation front is unstable under corrugational deformations, whereas the condensation front seems to be stable. The instability is analogous to the Darrieus-Landau instability in combustion fronts. The growth rate of the instability is proportional to the speed of the evaporation flow and the corrugation wavenumber for modes with wavelength much longer than the thickness of the front, and it is suppressed at scales approximately equal to the thickness of the front. The timescale of the instability is smaller than the cooling timescale of the warm neutral medium (~1 Myr) and can be as small as the cooling timescale of the cold neutral medium (~0.01-0.1 Myr). Thus, this instability should be one of the processes that drives interstellar turbulence.

We present new high-resolution (100'') neutral hydrogen (H I) self-absorption images of the Riegel-Crutcher cloud obtained with the Australia Telescope Compact Array and the Parkes Radio Telescope. The Riegel-Crutcher cloud lies in the direction of the Galactic center at a distance of 125 ± 25 pc. Our observations resolve the very large, nearby sheet of cold hydrogen into a spectacular network of dozens of hairlike filaments. Individual filaments are remarkably elongated, being up to 17 pc long with widths of less than ~0.1 pc. The strands are reasonably cold, with spin temperatures of ~40 K and in many places appearing to have optical depths larger than 1. Comparing the H I images with observations of stellar polarization, we show that the filaments are very well aligned with the ambient magnetic field. We argue that the structure of the cloud has been determined by its magnetic field. In order for the cloud to be magnetically dominated the magnetic field strength must be >30 μG.

We discuss a new technique for studying astrophysical turbulence that utilizes the statistics of Doppler-broadened spectral lines. The technique relates the power velocity coordinate spectrum (VCS), i.e., the spectrum of fluctuations measured along the velocity axis in position-position-velocity data cubes available from observations, to the underlying power spectra of the velocity/density fluctuations. Unlike the standard spatial spectra, which are a function of angular wavenumber, the VCS is a function of the velocity wavenumber k_v ~ 1/v. We show that absorption affects the VCS to a higher degree for small k_v and obtain the criteria for disregarding the absorption effects for turbulence studies at large k_v. We consider the retrieval of turbulence spectra from observations for high and low spatial resolution observations and find that the VCS allows one to study turbulence even when the emitting turbulent volume is not spatially resolved. This opens interesting prospects for using the technique for extragalactic research. We show that, while thermal broadening interferes with the turbulence studies using the VCS, it is possible to separate thermal and nonthermal contributions. This allows a new way of determining the temperature of the interstellar gas using emission and absorption spectral lines.

If the split, asymmetric molecular spectral line profiles that are seen in many starless cores are interpreted as indicative of global collapse or expansion of the core, then one possible implication is that most starless cores have short lifetimes, on the order of the collapse or sound crossing timescale. An alternative interpretation of the line profiles, as indicative of perturbations on an underlying equilibrium structure, leads to the opposite implication, that many cores have long lifetimes. While evidence suggests that some cores are collapsing on a free-fall timescale, we show that observations of some other starless cores can be reproduced by a model of nonradial oscillations about the equilibrium configuration of a pressure-bounded, thermally-supported sphere (Bonnor-Ebert sphere). We model the oscillations as linear perturbations following a standard analysis developed for stellar pulsations and compare the column densities and molecular spectral line profiles predicted from a particular model to observations of the Bok globule B68.

We investigate the structure of the core surrounding the recently identified deeply embedded young stellar object Barnard 1c. B1c lies within the Perseus molecular cloud at a distance of 250 pc. It is a deeply embedded core of 2.4 M_☉ (Kirk et al.) and a luminosity of 4 ± 2 L_☉. Observations (and resolutions) of ¹²CO J = 1-0 (92 × 59), ¹³CO J = 1-0, C¹⁸O J = 1-0 (143 × 67), HCO⁺J = 1-0 (76 × 58), and N₂H⁺J = 1-0 (59 × 46) were obtained with the BIMA array, together with the continuum at 3.3 mm (64 × 49) and 2.7 mm (95 × 63). Single-dish measurements of N₂H⁺J = 1-0 and HCO⁺J = 1-0 with FCRAO reveal the larger scale emission in these lines with ~60 resolution. The ¹²CO and HCO⁺ emission traces the outflow extending over the full field of view (21), which coincides in detail with the S-shaped jet recently found in Spitzer IRAC imaging. The N₂H⁺ emission, which anticorrelates spatially with the C¹⁸O emission, originates from a rotating envelope with effective radius ~2400 AU and mass 2.1-2.9 M_☉, as derived from the 3.3 mm continuum emission. N₂H⁺ emission is absent from a 600 AU diameter region around the young star, offset from the continuum peak. The remaining N₂H⁺ emission may lie in a coherent torus of dense material. With its outflow and rotating envelope, B1c closely resembles the previously studied object L483 mm, and we conclude that it is a protostar in an early stage of evolution, i.e., Class 0 or in transition between Class 0 and Class I. We hypothesize that heating by the outflow and star has desorbed CO from grains, which has destroyed N₂H⁺ in the inner region, and surmise that the presence of grains without ice mantles in this warm inner region can explain the unusual polarization signature observed from B1c.

Even with the renaissance in gamma-ray burst (GRB) research fostered by the Swift satellite, few bursts have both contemporaneous observations at long wavelengths and exquisite observations at later times across the electromagnetic spectrum. We present here contemporaneous imaging with the KAIT robotic optical telescope, dense optical sampling with Lulin, supplemented with infrared data from PAIRITEL and radio to gamma-ray data from the literature. For the first time, we can test the constancy of microphysical parameters in the internal-external shock paradigm and carefully trace the flow of energy from the GRB to the surrounding medium. KAIT data taken ≲1 minute after the start of GRB 051111 and coinciding with the fading gamma-ray tail of the prompt emission indicate a smooth reinjection of energy into the shock. No color change is apparent in observations beginning ~1.5 minutes after the GRB and lasting for the first hour after the burst. There are achromatic flux modulations about the best-fit model at late (t ≈ 10⁴ s) times, possibly due to variations in the external density. We find that the host galaxy extinction is well fit by a curve similar to that of the Small Magellanic Cloud. Low visual extinction, A_V ≈ 0.2 mag, combined with high column densities determined from the X-ray and optical spectroscopy (N_H > 10²¹ cm^-2), indicate a low dust-to-metals ratio and a possible overabundance of the light metals. An apparent small ratio of total to selective extinction (R_V ≈ 2) argues against dust destruction by the GRB. Time constancy of both the IR/optical/UV spectral energy distribution and the soft X-ray absorption suggests that the absorbing material is not local to the GRB.

We present a physical framework that can account for most of the observed spectral properties of prompt gamma-ray burst emission. This includes the variety of spectral shapes and shape evolutions, and spectral correlations between flux and spectral peaks within bursts, described by Borgonovo & Ryde, and among bursts described by Amati and Ghirlanda. In our proposed model the spectral peak is given by the photospheric emission from a relativistic outflow for which the horizon length is much smaller that the radial width. The observed duration of the thermal flash is given by the radial light-crossing time. This then gives the typical emission site at ~10¹¹ cm with a Lorentz factor of ~300. This emission is accompanied by nonthermal emission from dissipation locations outside the photosphere. The relative strengths of these two components depend on injection effects at the central engine, leading to varying relative locations of the saturation and photospheric radii. The total emission can then reproduce the observed variety. The spectral correlations are found by assuming that the amount of energy dissipated depends nonlinearly on the averaged particle density. Besides the spectral correlations, this also gives a description of how the relative strength of the thermal component varies with temperature within a burst.

Very early observations with the Swift satellite of γ-ray burst (GRB) afterglows reveal that the optical component is not detected in a large number of cases. This is in contrast to the bright optical flashes previously discovered in some GRBs (e.g., GRB 990123 and GRB 021211). Comparisons of the X-ray afterglow flux to the optical afterglow flux and prompt γ-ray fluence is used to quantify the seemingly deficient optical, and in some cases X-ray, light at these early epochs. This comparison reveals that some of these bursts appear to have higher than normal γ-ray efficiencies. We discuss possible mechanisms and their feasibility for explaining the apparent lack of early optical emission. The mechanisms considered include, foreground extinction, circumburst absorption, Lyα blanketing and absorption due to high-redshift, low-density environments, rapid temporal decay, and intrinsic weakness of the reverse shock. Of these, foreground extinction, circumburst absorption, and high redshift provide the best explanations for most of the nondetections in our sample. There is tentative evidence of suppression of the strong reverse shock emission. This could be because of a Poynting flux-dominated flow or a pure nonrelativistic hydrodynamic reverse shock.

The statistical properties of a complete, flux-limited sample of 197 long gamma-ray bursts (GRBs) detected by BATSE are studied. In order to bring forth their main characteristics, care was taken to define a representative set of 10 parameters. A multivariate analysis gives that ~70% of the total variation in parameter values is driven by only three principal components. The variation of the temporal parameters is clearly distinct from that of the spectral ones. A close correlation is found between the half-width of the autocorrelation function (τ) and the emission time (₅₀); most importantly, this correlation is self-similar in the sense that the mean values and dispersions of both τ and ₅₀ scale with the duration of the burst (T₉₀). It is shown that the Amati relation can be derived from the sample and that the scatter around this relation is correlated with the value of τ. Hence, τ has a role similar to that of the break in the afterglow light curve (t_b) in the Ghirlanda-relation. In the standard GRB-scenario, the close relation between a global parameter (t_b) and a local one (τ) indicates that some of the jet-properties do not vary much for different lines of sight. Finally, it is argued that the basic temporal and spectral properties are associated with individual pulses, while the overall properties of a burst is determined mainly by the number of pulses.

A toy model is analyzed in order to evaluate the linear stability of the gain region immediately behind a stalled accretion shock, after core bounce. This model demonstrates that a negative entropy gradient is not sufficient to warrant linear instability. The stability criterion is governed by the ratio χ of the advection time through the gain region divided by the local timescale of buoyancy. The gain region is linearly stable if χ < 3. The classical convective instability is recovered in the limit χ ≫ 3. For χ > 3, perturbations are unstable in a limited range of horizontal wavelengths centered around twice the vertical size H of the gain region. The threshold horizontal wavenumbers k_min and k_max follow simple scaling laws such that Hk_min ∝ 1/χ and Hk_max ∝ χ. The convective stability of the l = 1 mode in spherical accretion is discussed, in relation with the asymmetric explosion of core-collapse supernovae. The advective stabilization of long-wavelength perturbations weakens the possible influence of convection alone on a global l = 1 mode.

We reinvestigate the generation and accumulation of magnetic flux in optically thin accretion flows around active gravitating objects. The source of the magnetic field is the azimuthal electric current associated with the Poynting-Robertson drag on the electrons of the accreting plasma. This current generates magnetic field loops that open up because of the differential rotation of the flow. We show through simple numerical simulations that what regulates the generation and accumulation of magnetic flux near the center is the value of the plasma conductivity. Although the conductivity is usually considered to be effectively infinite for the fully ionized plasmas expected near the inner edge of accretion disks, the turbulence of those plasmas may actually render them much less conducting due to the presence of anomalous resistivity. We have discovered that if the resistivity is sufficiently high throughout the turbulent disk while it is suppressed inside its inner edge, an interesting steady state process is established: accretion carries and accumulates magnetic flux of one polarity inside the inner edge of the disk, whereas magnetic diffusion releases magnetic flux of the opposite polarity to large distances. In this scenario, magnetic flux of one polarity grows and accumulates at a steady rate in the region inside the inner edge and up to the point of equipartition when it becomes dynamically important. We argue that this inward growth and outward expulsion of oppositely directed magnetic fields that we propose could account for the ~30 minute cyclic variability observed in the Galactic microquasar GRS 1915+105.

We present a possible explanation for the high-frequency quasi-periodic oscillations (QPOs) of microquasars by a magnetohydrodynamic (MHD) instability that combines the physics developed, in different contexts, for the accretion-ejection instability, the Rossby wave instability, and the normal modes of diskoseismic models (which rely on the properties of the relativistic rotation curve in the vicinity of the marginally stable orbit). This instability can appear as modes of azimuthal wavenumbers m = 2, 3,... that have very similar pattern speeds ω/m, while the m = 1 mode, which would appear as the fundamental of this discrete spectrum, is less unstable. This would readily explain the 2 : 3 (and sometimes higher) frequency ratio observed between these QPOs. These instabilities form eigenmodes, i.e., standing wave patterns at a constant frequency in the disk; they are strongly unstable and thus do not need an external excitation mechanism to reach high amplitudes. Furthermore, they have the property that a fraction of the accretion energy can be emitted toward the corona; this would explain why these QPOs are seen in a spectral state where Comptonized emission from the corona is always present. Their existence depends critically on the existence of a magnetic structure, formed by poloidal flux advected in the accretion process, in the central region between the disk and the black hole.

A general superposed solution between a Schwarzschild black hole and one of the first family of Lemos-Letelier disks with two opposite dipoles is given. For the Newtonian core-disk system, it is integrable in the region z > 0 or <0, but it might be nonintegrable over the global interval of z since the Newtonian potential from the disk has discontinuous derivatives. On the other hand, Poincaré sections reveal that the dynamics of test particles in the relativistic core-disk system depends on some specified dynamical parameters. The system goes from regular to chaotic with increasing disk parameter. In addition, other parameters such as energy and angular momentum have an impact on chaos. The larger the energy gets, or the smaller the angular momentum becomes, the more dramatic chaos the system produces. If the interaction term between the black hole and the disk is dropped, chaos seems to die out. This fact illustrates sufficiently that the nonlinearity of the Einstein field equations is a crucial factor for the onset of chaos. Note, as a crucial point, that there is a much thinner chaotic domain trapped in a larger regular region in the case of larger angular momenta, unlike the case of smaller angular momenta. This can also be described by the spectral analysis and the invariant Lyapunov exponents.

We investigate the self-consistent electrodynamic structure of a particle accelerator in the Crab pulsar magnetosphere on the two-dimensional poloidal plane, solving the Poisson equation for the electrostatic potential together with the Boltzmann equations for electrons, positrons, and γ-rays. If the transfield thickness of the gap is thin, the created current density becomes sub-Goldreich-Julian, giving the traditional outer-gap solution but with negligible γ-ray luminosity. As the thickness increases, the created current increases to become super-Goldreich-Julian, giving a new gap solution with substantially screened acceleration electric field in the inner part. In this case, the gap extends toward the neutron star with a small-amplitude positive acceleration field, extracting ions from the stellar surface as a space-charge-limited flow. The acceleration field is highly unscreened in the outer magnetosphere, resulting in a γ-ray spectral shape that is consistent with the observations.

We discuss axisymmetric force-free pulsar magnetospheres with magnetically collimated jets and a disk wind obtained by numerical solution of the pulsar equation. This solution represents an alternative to the quasi-spherical wind solutions in which a major part of the current flow is in a current sheet that is unstable to magnetic field annihilation.

We have conducted a radio pulsar survey of 56 unidentified γ-ray sources from the third EGRET catalog that are at intermediate Galactic latitudes (5° < |b| < 73°). For each source, four interleaved 35 minute pointings were made with the 13 beam, 1400 MHz multibeam receiver on the Parkes 64 m radio telescope. This covered the 95% error box of each source at a limiting sensitivity of ~0.2 mJy to pulsed radio emission for periods P ≳ 10 ms and dispersion measures ≲50 pc cm^-3. Roughly half of the unidentified γ-ray sources at |b| > 5° with no proposed active galactic nucleus counterpart were covered in this survey. We detected nine isolated pulsars and four recycled binary pulsars, with three from each class being new discoveries. Timing observations suggest that only one of the pulsars has a spin-down luminosity that is even marginally consistent with the inferred luminosity of its coincident EGRET source. Our results suggest that population models, which include the Gould Belt as a component, overestimate the number of isolated pulsars among the midlatitude Galactic γ-ray sources, and that it is unlikely that Gould Belt pulsars make up the majority of these sources. However, the possibility of steep pulsar radio spectra and the confusion of terrestrial radio interference with long-period pulsars (P ≳ 200 ms) having very low dispersion measures (≲10 pc cm^-3, expected for sources at a distance of less than about 1 kpc) prevent us from strongly ruling out this hypothesis. Our results also do not support the hypothesis that millisecond pulsars make up the majority of these sources. Nonpulsar source classes should therefore be further investigated as possible counterparts to the unidentified EGRET sources at intermediate Galactic latitudes.

We use the relativistic hydrodynamics code Cosmos++ to model the evolution of the radio nebula triggered by the 2004 December 27 giant flare event of SGR 1806-20. We primarily focus on the rebrightening and centroid motion occurring subsequent to day 20 following the flare event. We model this period as a mildly relativistic (γ ~ 1.07-1.67) jetted outflow expanding into the ISM. We demonstrate that a jet with total energy ~10⁴⁶ ergs confined to a half opening angle ~20° fits the key observables of this event, e.g., the flux light curve, emission map centroid position, and aspect ratio. In particular, we find excellent agreement with observations if the rebrightening is due to the jet, moving at 0.5c and inclined ~0°-40° toward the observer, colliding with a density discontinuity in the ISM at a radius of several times 10¹⁶ cm. We also find that a jet with a higher velocity, ≳0.7c, and larger inclination, ≳70°, moving into a uniform ISM can fit the observations in general but tends to miss the details of rebrightening. The latter, uniform ISM model predicts an ISM density more than 100 times lower than that of the former model and thus suggests an independent test that might discriminate between the two. One of the strongest constraints of both models is that the data seem to require a nonuniform jet in order to be well fitted.

"The Duck" is a complicated nonthermal radio system, consisting of the energetic radio pulsar B1757-24, its surrounding pulsar wind nebula G5.27-0.90, and the adjacent supernova remnant (SNR) G5.4-1.2. PSR B1757-24 was originally claimed to be a young (≈15,000 yr) and extreme-velocity (≳1500 km s^-1) pulsar, which had penetrated and emerged from the shell of the associated SNR G5.4-1.2; but recent upper limits on the pulsar's motion have raised serious difficulties with this interpretation. We here present 8.5 GHz interferometric observations of the nebula G5.27-0.90 over a 12 yr baseline, doubling the time span of previous measurements. These data correspondingly allow us to halve the previous upper limit on the nebula's westward motion to 14 mas yr^-1 (5 σ), allowing a substantive reevaluation of this puzzling object. We rule out the possibility that the pulsar and SNR were formed from a common supernova explosion ≈15,000 yr ago, as implied by the pulsar's characteristic age, but conclude that an old (≳70,000 yr) pulsar/SNR association, or a situation in which the pulsar and SNR are physically unrelated, are both still viable explanations.

We report on more than 7 yr of monitoring of PSR J0537-6910, the 16 ms pulsar in the LMC, using data acquired with RXTE. During this campaign the pulsar experienced 23 sudden increases in frequency ("glitches") amounting to a total gain of over 6 ppm of rotation frequency superposed on its gradual spin-down of = -2 × 10^-10 Hz s^-1. The time interval from one glitch to the next obeys a strong linear correlation to the amplitude of the first glitch, with a mean slope of about 400 days ppm^-1 (6.5 days μHz^-1), such that these intervals can be predicted to within a few days, an accuracy that has never before been seen in any other pulsar. There appears to be an upper limit of ~40 μHz for the size of glitches in all pulsars, with the 1999 April glitch of PSR J0537-6910 the largest so far. The change of its spin-down across the glitches, Δ, appears to have the same hard lower limit, -1.5 × 10^-13 Hz s^-1, as that observed in all other pulsars. The spin-down continues to increase in the long term, = -10^-21 Hz s^-2, and thus the timing age of PSR J0537-6910 (-0.5ν^-1) continues to decrease at a rate of nearly 1 yr every year, consistent with movement of its magnetic moment away from its rotational axis by 1 rad every 10,000 yr, or about 1 m yr^-1. PSR J0537-6910 was likely to have been born as a nearly aligned rotator spinning at 75-80 Hz, with a || considerably smaller than its current value of 2 × 10^-10 Hz s^-1. Its pulse profile consists of a single pulse that is found to be flat at its peak for at least 0.02 cycles.

The reconstruction of three-dimensional (3D) Doppler tomograms based on observational data has been accomplished for the first time. The distribution of the Hα emission intensity I(V_x,V_y,V_z) of the interacting Algol binary system U Coronae Borealis has been restored in 3D velocity space with resolutions of 30 km s^-1 in V_x and V_y and 110 km s^-1 in V_z. The reconstruction was based on 47 Hα spectra from 1994 by applying the developed radioastronomical approach (RA) to few-projections tomography. The comparison between the previous 2D and our 3D Doppler tomograms shows similarities with the main structural features of the gas flows in the orbital plane. Specifically, the gas stream along the ballistic trajectory and equatorial emission centered on the primary star are displayed on the 3D Doppler tomogram as distinct emission sources. A high-velocity stream (V_z ~ 200 km s^-1) with strong emission intensity has also been discovered moving in the direction across the orbital plane of the system.

We present a study of the iron abundance pattern in hot, hydrogen-rich (DA) white dwarfs. The study is based on new and archival far-ultraviolet spectroscopy of a sample of white dwarfs in the temperature range 30,000 K ≲ T_eff ≲ 64,000 K. The spectra obtained with the Far Ultraviolet Spectroscopic Explorer, along with spectra obtained with the Hubble Space Telescope Imaging Spectrograph and the International Ultraviolet Explorer, sample Fe III-Fe VI absorption lines, enabling a detailed iron abundance analysis over a wider range of effective temperatures than previously afforded. The measurements reveal abundance variations in excess of 2 orders of magnitude between the highest and the lowest temperatures probed, but also show considerable variations (over 1 order of magnitude) between objects with similar temperatures and surface gravities. Such variations in cooler objects may be imputed to accretion from unseen companions or so-called circumstellar debris, although the effect of residual mass loss and selective radiation pressure in the hottest objects in the sample remain dominant.

We continue to explore the accretion model of the massive binary system η Car by studying the anomalously high He II λ4686 line. The line appears just before periastron and disappears immediately thereafter. Based on the He II λ4686 line emission from O stars and their modeling in the literature, we postulate that the He II λ4686 line comes from the acceleration zone of the secondary stellar wind. We attribute the large increase in the line intensity to a slight increase in the density of the secondary stellar wind in its acceleration zone. The increase in density could be due to the ionization and subsequent deceleration of the wind by the enhanced X-ray emission arising from the shocked secondary wind farther downstream or to accretion of the primary stellar wind. Accretion around the secondary equatorial plane gives rise to collimation of the secondary wind, which increases its density, hence enhancing the He II λ4686 emission line. In contrast with previous explanations, the model proposed in this paper does not require a prohibitively high X-ray flux to directly photoionize the He.

We have carried out a sensitive high-resolution imaging survey of stars in the young (6-8 Myr), nearby (97 pc) compact cluster around η Chamaeleontis to search for stellar and substellar companions. Our data were obtained using the NACO adaptive optics system on the ESO Very Large Telescope (VLT). Given its youth and proximity, any substellar companions are expected to be luminous, especially in the near-infrared, and thus easier to detect next to their parent stars. Here, we present VLT NACO adaptive optics imaging with companion detection limits for 17 η Cha cluster members, and follow-up VLT ISAAC near-infrared spectroscopy for companion candidates. The widest binary detected is ~02, corresponding to the projected separation 20 AU, despite our survey being sensitive down to substellar companions outside 03, and planetary-mass objects outside 05. This implies that the stellar companion probability outside 03 and the brown dwarf companion probability outside 05 are less than 0.16 with 95% confidence. We compare the wide binary frequency of η Cha to that of the similarly aged TW Hydrae association and estimate the statistical likelihood that the wide binary probability is equal in both groups to be less than 2 × 10^-4. Even though the η Cha cluster is relatively dense, stellar encounters in its present configuration cannot account for the relative deficit of wide binaries. We thus conclude that the difference in wide binary probability in these two groups provides strong evidence for multiplicity properties being dependent on environment. In two appendices we derive the projected separation probability distribution for binaries, used to constrain physical separations from observed projected separations, and summarize statistical tools useful for multiplicity studies.

We present a sample of 1777 bright (9 < B < 14) metal-poor candidates selected from the Hamburg/ESO Survey (HES). Despite saturation effects present in the red portion of the HES objective-prism spectra, the data were recoverable and quantitative selection criteria could be applied to select the sample. Analyses of medium-resolution (~2 Å) follow-up spectroscopy of the entire sample, obtained with several 2-4 m class telescopes, yielded 145 new metal-poor stars with metallicity [Fe/H] < -2.0, of which 79 have [Fe/H] < -2.5 and 17 have [Fe/H] < -3.0. We also obtained C/Fe estimates for all of these stars. From this, we find a frequency of C-enhanced ([C/Fe] > 1.0) metal-poor ([Fe/H] < -2.0) giants of 9% ± 2%, which is lower than previously reported. However, the frequency rises to similar (>20%) and higher values with increasing distance from the Galactic plane. Although the numbers of stars at low metallicity are falling rapidly at the lowest metallicities, there is evidence that the fraction of carbon-enhanced metal-poor stars is increasing rapidly as a function of declining metallicity. For ~60 objects, high-resolution data have already been obtained; one of these, HE 1327-2326, is the new record holder for the most iron-deficient star known.

We describe a method for accurately determining M dwarf metallicities with spectral synthesis based on abundance analyses of visual binary stars. We obtained high-resolution, high-signal-to-noise ratio spectra of each component of five visual binary pairs at McDonald Observatory. The spectral types of the components range from F7 to K3 V for the primaries and from M0.5 to M3.5 V for the secondaries. We have determined the metallicities of the primaries differentially with respect to the Sun by fitting synthetic spectra to Fe I line profiles in the observed spectra. In the course of our analysis of the M dwarf secondaries, we have made significant improvements to the PHOENIX cool-star model atmospheres and the spectrum analysis code MOOG. Our analysis yields an rms deviation of 0.11 dex in metallicity values between the binary pairs. We estimate the uncertainties in the derived stellar parameters for the M dwarfs to be 48 K, 0.10 dex, 0.12 dex, 0.15 km s^-1, and 0.20 km s^-1 for T_eff, log g, [M/H], ξ, and η, respectively. Accurate stellar evolutionary models are needed to progress further in the analysis of cool-star spectra; the new model atmospheres warrant recalculation of the evolutionary models.

We present optical WBVR and infrared JHKL photometric observations of the Be binary system δ Sco obtained in 2000-2005, and mid-infrared (10 and 18 μm) photometry and optical (λλ3200-10500) spectropolarimetry obtained in 2001. Our optical photometry confirms the results of a frequent visual monitoring being done by amateurs. The 2001 spectral energy distribution and polarization are successfully modeled with a three-dimensional non-LTE Monte Carlo code that self-consistently calculates the hydrogen level populations, electron temperature, and gas density for hot star disks. Our disk model is hydrostatically supported in the vertical direction and radially controlled by viscosity. Such a disk model has essentially only two free parameters, viz., the equatorial mass-loss rate and the disk outer radius, if one assumes a prescription for the viscosity. We find that the primary companion is surrounded by a small (7R_⋆), geometrically-thin disk, which is highly nonisothermal and fully ionized. Our model requires an average equatorial mass-loss rate of 1.5 × 10^-9M_☉ yr^-1 to successfully explain the observations. In 2005, we detected a significant simultaneous decrease in the object's optical and near-infrared brightness, which is associated with a continuous rise in the hydrogen line equivalent widths. We discuss possible causes for this unusual phenomenon, which is difficult to explain in view of current models of Be star disks.

This paper reports the results of a small imaging survey of eight evolved stars including two AGB stars (IRC +10216 and Mira), five proto-planetary nebula (PPN) candidates (AFGL 2688, IRAS 22272+5435, HD 161796, 89 Her, and HD 179821), and a planetary nebula (PN, NGC 7027). We present high-resolution ¹²CO J = 1 → 0 maps of their full molecular envelopes made by combining BIMA Millimeter Array and NRAO 12 m telescope observations. For the PPNe and PN, the neutral molecular envelopes are compared with images taken at optical, near-IR, and mid-IR wavelengths. Drawing from the literature, we augmented our BIMA survey sample to 38 well-studied sources with CO emission maps. We classified this sample of sources based on the kinematics and morphologies of the CO emission into three types: spherical/elliptical/shell sources, disk sources, and structured outflow sources. Confirming previous studies, we find strong evidence for the photodissociation of the molecular envelope as an object evolves from the AGB to PN stages. While the spherical AGB stars follow theoretical expectations for mass-loss rate versus envelope size, the post-AGB structured outflow sources have significantly higher mass-loss rates than expected probably because of their recent superwinds. We find evidence that the structured outflows are clearly younger than the AGB wind. The disk sources have little correlation between mass-loss rate and envelope size because their properties are determined more by the properties of the central stars and disk evolution than by the mass-loss rate history that shapes the spherical and structured-outflow sources.

There have been several investigations of the evolution of the mid-infrared (IR) dust features in carbon star spectra based on IRAS LRS data, but these studies are somewhat contradictory. In order to understand these differences in interpretations and to develop an understanding of the carbon star dust sequence, we have reexamined 26 IRAS LRS spectra of carbon stars that have also been observed spectroscopically by ISO SWS. The low resolution and narrow wavelength coverage of the IRAS LRS data hinder determination of the effect of molecular absorptions in these spectra. This has led to incorrect estimations of the continuum levels in these spectra, which has a huge effect on the continuum-divided and continuum-subtracted spectra used to analyze trends in the shape, strength, and position of the mid-IR features. The higher resolution and broader wavelength coverage of the ISO data allow more accurate fitting of the underlying continuum. We have reassessed the trends in shape, strength, and position of the ~11 μm silicon carbide (SiC) feature and the apparent emergence of the ~9 μm feature. We find that there are no correlations between the spectral parameters. We also investigate whether any of these parameters correlate with the strength of the molecular bands; no correlation was found. Moreover, we show that the apparent 9 μm feature is probably an artifact. We discuss the implications of this study in terms of both a carbon star condensation sequence and the application of this study to the larger IRAS data set.

Using the MIPS instrument on Spitzer, we have searched for infrared excesses around a sample of 82 stars, mostly F, G, and K main-sequence field stars, along with a small number of nearby M stars. These stars were selected for their suitability for future observations by a variety of planet-finding techniques. These observations provide information on the asteroidal and cometary material orbiting these stars, data that can be correlated with any planets that may eventually be found. We have found significant excess 70 μm emission toward 12 stars. Combined with an earlier study, we find an overall 70 μm excess detection rate of 13% ± 3% for mature cool stars. Unlike the trend for planets to be found preferentially toward stars with high metallicity, the incidence of debris disks is uncorrelated with metallicity. By newly identifying four of these stars as having weak 24 μm excesses (fluxes ~10% above the stellar photosphere), we confirm a trend found in earlier studies wherein a weak 24 μm excess is associated with a strong 70 μm excess. Interestingly, we find no evidence for debris disks around 23 stars cooler than K1, a result that is bolstered by a lack of excess around any of the 38 K1-M6 stars in two companion surveys. One motivation for this study is the fact that strong zodiacal emission can make it hard or impossible to detect planets directly with future observatories such as the Terrestrial Planet Finder (TPF). The observations reported here exclude a few stars with very high levels of emission, >1000 times the emission of our zodiacal cloud, from direct planet searches. For the remainder of the sample, we set relatively high limits on dust emission from asteroid belt counterparts.

Hostile tidal forces may inhibit the formation of Jovian planets in binaries with semimajor axes of ≲50 AU, binaries that might be called "close" in this context. As an alternative to in situ planet formation, a binary can acquire a giant planet when one of its original members is replaced in a dynamical interaction with another star that hosts a planet. Simple scaling relations for the structure and evolution of star clusters, coupled with analytic arguments regarding binary-single and binary-binary scattering, indicate that dynamical processes can deposit Jovian planets in <1% of close binaries. If ongoing and future exoplanet surveys measure a much larger fraction, it may be that giant planets do somehow form frequently in such systems.

We investigate the interaction between a giant planet and a viscous circumstellar disk by means of high-resolution, two-dimensional hydrodynamic simulations. We consider planetary masses that range from 1 to 3 Jupiter masses (M_J) and initial orbital eccentricities that range from 0 to 0.4. We find that a planet can cause eccentricity growth in a disk region adjacent to the planet's orbit, even if the planet's orbit is circular. Disk-planet interactions lead to growth in a planet's orbital eccentricity. The orbital eccentricities of a 2M_J and a 3M_J planet increase from 0 to 0.11 within about 3000 orbits. Over a similar time period, the orbital eccentricity of a 1M_J planet grows from 0 to 0.02. For a case of a 1M_J planet with an initial eccentricity of 0.01, the orbital eccentricity grows to 0.09 over 4000 orbits. Radial migration is directed inward but slows considerably as a planet's orbit becomes eccentric. If a planet's orbital eccentricity becomes sufficiently large, e ≳ 0.2, migration can reverse and so be directed outward. The accretion rate toward a planet depends on both the disk and the planetary orbital eccentricity and is pulsed over the orbital period. Planetary mass growth rates increase with planetary orbital eccentricity. For e ~ 0.2, the mass growth rate of a planet increases by ~30% above the value for e = 0. For e ≳ 0.1, most of the accretion within the planet's Roche lobe occurs when the planet is near the apocenter. Similar accretion modulation occurs for flow at the inner disk boundary, which represents accretion toward the star.

We present RIz photometry of four consecutive transits of the newly discovered exoplanet XO-1b. We improve on the estimates of the transit parameters, finding the planetary radius to be R_P = 1.184R_J, and the stellar radius to be R_⋆ = 0.928R_☉, assuming a stellar mass of M_⋆ = (1.00 ± 0.03) M_☉. The uncertainties in the planetary and stellar radii are dominated by the uncertainty in the stellar mass. These uncertainties increase by a factor of 2-3 if a more conservative uncertainty of 0.10 M_☉ is assumed for the stellar mass. Our estimate of the planetary radius is smaller than that reported by McCullough and coworkers, and the resulting estimate for the mean density of XO-1b is intermediate between that of the low-density planet HD 209458b and the higher density planets TrES-1 and HD 189733b. The timings of the transits have an accuracy ranging from 0.2 to 2.5 minutes and are marginally consistent with a uniform period.

We report the detection of a Jupiter-mass planet in a 6.838 day orbit around the 1.28 M_☉ subgiant HD 185269. The eccentricity of HD 185269b (e = 0.30) is unusually large compared to other planets within 0.1 AU of their stars. Photometric observations demonstrate that the star is constant to ±0.0001 mag on the radial velocity period, strengthening our interpretation of a planetary companion. This planet was detected as part of our radial velocity survey of evolved stars located on the subgiant branch of the H-R diagram—also known as the Hertzsprung gap. These stars, which have masses between 1.2 and 2.5 M_☉, play an important role in the investigation of the frequency of extrasolar planets as a function of stellar mass.

We present the results of a coronagraphic imaging search for extrasolar planets around the young main-sequence stars Eri and Vega. Concentrating the stellar light into the core of the point-spread function by the adaptive optic system and blocking the core by the occulting mask in the coronagraph, we have achieved the highest sensitivity for point sources in close vicinity of the both central stars. Nonetheless, we had no secure detection of a point source around the stars. The observations give the upper limits on the masses of the planets to M_J and M_J at a few arcseconds from Eri and Vega, respectively. Diffuse structures are also not detected around both stars.

Statistical properties of flares are a powerful tool for addressing the upper solar atmosphere heating problem. We simulate time series of synthetic flares by means of a dynamic model of the atmospheric magnetic field in which magnetic loop footpoints are controlled by photospheric flows computed through a n-body algorithm. The n-body simulation reproduces the behavior of a system where large spatial organization scales (i.e., mesogranulation) occur from the interaction of small-scale advection flows (i.e., granulation). The frequency function of the emitted magnetic energies obtained from the simulation is well approximated by a power law with index α ~ 2.4, while the frequency function of the waiting times between emissions shows a Poisson-like behavior with a deviation for longer times. The flare model yields a fairly intuitive interpretation of magnetic reconnection processes as magnetic field reconfigurations triggered by passive advection of magnetic footpoints through photospheric space-temporal correlated flows.

A simple parameterization of a three-dimensional flux rope is used to determine a "typical flux-rope geometry" that corresponds to observed flux-rope coronal mass ejection (CME) morphologies (average apparent angular widths) at a leading-edge height of about 5.5 R☉. The parameterized flux rope, the curved axis of which is assumed to trace out an ellipse, is described in terms of the eccentricity of the ellipse, the width (minor diameter d) of the flux rope at the apex, and the height of the apex above the solar surface 2R₁. Assuming self-similar expansion, there are only two geometrical parameters to be determined: the eccentricity and the axial aspect ratio Λ_a ≡ 2R₁/d. For each pair of geometrical parameters, an ensemble of 72 orientations is considered, with each being specified in terms of a latitude angle, a longitude angle, and a rotation about the direction of motion. The resulting ensemble of synthetic coronagraph images is used to produce statistical measures of the morphology for comparison to corresponding observational measures from St. Cyr et al. (2004). We find that a typical flux-rope CME has = 0.7 ± 0.2 and Λ_a = 1.1 ± 0.3.

Optical flow is a powerful image processing tool for measuring motion in digital images. The optical flow algorithm provides an estimate of the velocity vector at every pixel from a pair of successive images. Here we present an application of this method to images of coronal mass ejections (CMEs). The technique is first tested and validated on a simulated CME. It is then applied to several CMEs observed with the LASCO C2 coronagraph to derive their velocity fields. The resulting velocity measurements allow us to visualize the evolution of the CME plasma and to separate the "bulk" velocity from the expansion velocity of a given CME. To our knowledge, this is the first time that such information has been extracted from CME observations. We discuss the limitations and accuracy of our optical flow method and propose further improvements.

We construct a simple model for radioisotopic enrichment of the protosolar nebula by injection from a nearby supernova, based on the inverse square law for ejecta dispersion. In this parameter study, the presolar radioisotopic abundances (i.e., in solar masses) demand a nearby supernova: its distance D can be no larger than 66 times the radius of the protosolar nebula, at a 90% confidence level, assuming 1 M_☉ of protosolar material. The relevant size of the nebula depends on its state of evolution at the time of radioactivity injection. In one scenario, a collection of low-mass stars, including our Sun, formed in a group or cluster with a high-mass star that ended its life as a supernova while our Sun was still a protostar, a starless core, or perhaps a diffuse cloud. Using recent observations of protostars to estimate the size of the protosolar nebula constrains the distance of the supernova to D ~ 0.02-1.6 pc. This supernova distance limit is consistent with the scales of low-mass star formation around one or more massive stars, but it is closer than expected were the Sun formed in an isolated, solitary state. Consequently, if any presolar radioactivities originated via supernova injection, we must conclude that our Sun was a member of such a group or cluster that has since dispersed; thus, solar system formation should be understood in this context. The temporal choreography from supernova ejecta to meteorites is important, as the modeled timescale is ≤1.8 Myr. Finally, the model does not distinguish between progenitor masses from 15 to 25 M_☉, although the 20 M_☉ model is somewhat preferred.

A search was conducted for grains of the potential core-collapse supernova (SN) condensate minerals corundum (Al₂O₃), hibonite (CaAl₁₂O₁₉), and spinel (MgAl₂O₄) among grains filtered from the 308.6 m Guliya ice core recovered from the Qinghai-Tibetan plateau in China. Simple models are developed and calculations are presented to estimate the number of Al₂O₃ grains that would be deposited per cm² on the Earth by a nearby core-collapse SN and to estimate the number of presolar oxide grains that could be contained in micrometeorite (MM) grains that are accreted by the Earth. A total of 698 candidate SN condensate grains were identified in six Guliya ice core grain samples from the following time periods: ~2-10, ~25-27, ~34-36, ~53-57, ~59-62, and ~68-72 kyr. A procedure developed at the University of Chicago to identify presolar grains in meteoric samples was used to find these candidate grains. Nanometer-scale secondary ion mass spectrometry (NanoSIMS) analysis, performed at Washington University on 37 grains from the ~34-36, ~53-57, and ~59-62 kyr samples, indicated that none possessed the extreme oxygen-16 enhancements expected from a SN source. However, nine of the 37 grains did possess the oxygen-16 enhancements consistent with calcium-aluminum-rich inclusions (CAIs).

We present laboratory experiments on the formation of macroscopic dust aggregates. The centimeter-sized highly porous bodies are produced by random ballistic deposition from individual micrometer-sized dust particles. We find packing densities between 0.07 and 0.15 for uncompressed samples, dependent on the shape and size distribution of the constituent dust grains. Impacts into these bodies are simulated by uniaxial compression experiments. We find that the maximum compression, equivalent to the highest protoplanetary impact velocities of ~50 m s^-1, increases the packing density to 0.20-0.33. Tensile strength measurements with our laboratory samples yield values in the range 200-1100 Pa for slightly compressed samples. We review packing densities and tensile strengths found for primitive solar system bodies, e.g., for comets, primitive meteorites, and meteoroids. We find a consistency between packing densities and tensile strengths of our laboratory samples with those from cometary origin.

Consider a perfect adaptive optics (AO) system with a very fine wavefront sampling interval and a very small actuator interval. If this AO system senses wavefront at a wavelength, λ_WFS, and does science imaging at another wavelength, λ_SCI, the light paths through the turbulent atmosphere at these two wavelengths are slightly different for a finite zenith distance, z. The error in wavefront reconstruction of the science channel associated with this non-common path effect, or so-called chromatic shear, is uncorrectable and sets an upper bound for the system performance. We evaluate the wavefront variance, σ²(λ_WFS,λ_SCI,z) for typical seeing conditions at Mauna Kea and find that this effect is not negligible at large z. If we require that the Strehl ratio be greater than 99% or 95%, z must be less than about 50° or 60°, respectively, for the combination of visible wavefront sensing and infrared science imaging.

A combined laboratory and astronomical investigation has been conducted on the methyl sugar hydroxyacetone (CH₃COCH₂OH). Rotational transitions of this species in the ground torsional state (v_t = 0) were recorded using both millimeter-wave direct absorption techniques and Fourier transform microwave spectroscopy. A total of 1145 lines of CH₃COCH₂OH were analyzed in the frequency range 4 to 180 GHz, including transitions arising from both A- and E-symmetry species. A modified rho-axis method Hamiltonian was needed for the analysis because of the presence of perturbations resulting from the torsional motion of the methyl group in this molecule. Assignment of the E-species was particularly problematic as a consequence of significant mixing between the ground and torsionally excited levels. The complete data set was fitted using 21 spectroscopic parameters and had a global rms of 90 kHz; the barrier to internal rotation was established to be 65.3560(22) cm^-1. An astronomical search was subsequently conducted for hydroxyacetone at 2 and 3 mm using the 12 m telescope of the Arizona Radio Observatory. Twenty-eight favorable transitions arising from both A- and E-species, each consisting of collapsed quartets, were searched for toward Sgr B2(N). Although credible features were detected at several frequencies of hydroxyacetone, there were a sufficient number of missing lines to rule out an interstellar detection. An upper limit to the column density of N_tot < 5 × 10¹² cm^-2 was derived for CH₃COCH₂OH in Sgr B2(N), indicating that this species is an order of magnitude less abundant than glycolaldehyde (CH₂OHCHO).

We analyze the Millennium Run semianalytic galaxy catalog in order to explore quantitatively the gravitational pancaking effect on the orientation of the galaxy velocity field. We first calculate the probability density distribution of the cosine of the angle between the velocity of a field galaxy and the direction normal to a local pancake plane that is determined using the two nearest neighbor field galaxies. A clear signal of alignment is detected for the case in which the pancake scale is in the range of 5-8 h^-1 Mpc. This tendency of velocity-pancake alignment is found to still exist when the pancakes are determined using three nearest neighbor galaxies, indicating that it has a spatial coherence. The degree of the velocity-pancake alignment is shown to increase with the velocity magnitude and the local density, while it decreases with the separation distance from the galaxy to the pancake and disappears when the pancake has a filamentary shape. A final conclusion is that our work may provide another clue to understanding the large-scale structure in the universe.

We investigate the orientation of the axes and angular momentum of dark matter halos with respect to their neighboring voids using high-resolution N-body cosmological simulations. We find that the minor axis of a halo tends to be aligned along the line joining the halo with the center of the void and that the major axis tends to lie in the plane perpendicular to this line. However, we find that the angular momentum of a halo does not have any particular orientation. These results may provide information about the mechanisms whereby the large-scale structure of the universe affects galaxy formation, and they may cast light on the issue of the orientation of galaxy disks with respect to their host halos.

In order to study the evolution of the relative fraction of obscured active galactic nuclei (AGNs), we constructed the largest sample to date of AGNs selected in hard X-rays. The full sample contains 2341 X-ray-selected AGNs, roughly 4 times the largest previous samples studied in this connection. Of these, 1229 (53%) have optical counterparts for which redshifts are available; these span the redshift range z = 0-4. The observed fraction of obscured AGNs declines only slightly with redshift. Correcting for selection bias, we find that the intrinsic fraction of obscured AGNs must actually increase with redshift, as (1 + z)^α, with α ≃ 0.4 ± 0.1. This evolution is consistent with the integrated X-ray background, which provides the strongest constraints at relatively low redshift, z ~ 1. Summing over all AGNs, we estimate the bolometric AGN light to be 3.8 nW m^-2 sr^-1, or ≲8% of the total extragalactic light. Together with the observed black hole mass density in the local universe, this implies an accretion efficiency of η ~ 0.1-0.2, consistent with the values typically assumed.

We present diffraction-limited (FWHM ~ 03) Gemini/T-ReCS mid-infrared (MIR; N-band or narrowband at 8.7 μm) imaging of four luminous infrared galaxies (LIRGs) drawn from a representative local sample. The MIR emission in the central few kiloparsecs is strikingly similar to that traced by Paα and generally consists of bright nuclear emission and several compact circumnuclear and/or extranuclear H II regions. The central MIR emission is dominated by these powerful H II regions, consistent with the majority of active galactic nuclei in this local sample of LIRGs contributing a minor part of the MIR emission. The luminous circumnuclear H II regions detected in LIRGs follow the extrapolation of the 8 μm versus Paα relation found for M51 H II knots. The integrated central 3-7 kpc of galaxies, however, present elevated 8 μm/Paα ratios with respect to individual H II regions, similar to the integrated values for star-forming galaxies. Our results show that the diffuse 8 μm emission, not directly related to the ionizing stellar population, can be as luminous as that from the resolved H II regions. Therefore, calibrations of the star formation rate for distant galaxies should be based on the integrated 8 μm emission of nearby galaxies, not that of the H II regions alone.

The high end of the stellar mass function of galaxies is observed to have little evolution since z ~ 1. This represents a stringent constraint for merger-based models, aimed at explaining the evolution of the most massive galaxies in the concordance ΛCDM cosmology. In this Letter we show that it is possible to remove the tension between the above observations and model predictions by allowing a fraction of stars to be scattered to the diffuse stellar component (DSC) of galaxy clusters at each galaxy merger, as recently suggested by the analysis of N-body hydrodynamical simulations. To this purpose, we use the MORGANA model of galaxy formation in a minimal version, in which gas cooling and star formation are switched off after z = 1. In this way, any predicted evolution of the galaxy stellar mass function is purely driven by mergers. We show that, even in this extreme case, the predicted degree of evolution of the high end of the stellar mass function is larger than that suggested by data. Instead, the assumption that a significant fraction, ~30%, of stars are scattered in the DSC at each merger event leads to a significant suppression of the predicted evolution, in better agreement with observational constraints, while providing a total amount of DSC in clusters, which is consistent with recent observational determinations.

We report the serendipitous discovery of a peculiar main sequence in archived Hubble Space Telescope Wide Field Planetary Camera 2 (WFPC2) observations of the young star cluster NGC 2011 in the Large Magellanic Cloud. The bright part of this main sequence exhibits a prominent double, forklike feature, as if it consists of twin main sequences, one of them being redder. The color-magnitude diagram, constructed from the stars found in the only available WFPC2 field of the cluster, is used to distinguish the stars according to their membership to each of these sequences and to study their spatial distribution. We find that there are two well-distinguished populations in the sense that the redder main sequence is dominated by stars that belong to the main body of the cluster, while the stars of the bluer main sequence belong to the surrounding region. Providing that NGC 2011 is a verified binary cluster, with the second companion unfortunately not observed, and taking into account the general region where this cluster is located, we discuss the possible scenarios from both star formation and an early dynamical evolution point of view that might explain this unique discovery.

We report on the identification of the lens responsible for microlensing event MACHO-LMC-20. As part of a Spitzer IRAC program conducting mid-infrared follow-up of the MACHO Large Magellanic Cloud microlensing fields, we discovered a significant flux excess at the position of the source star for this event. These data, in combination with high-resolution near-infrared Magellan PANIC data, have allowed us to classify the lens as an early M dwarf in the thick disk of the Milky Way, at a distance of ~2 kpc. This is only the second microlens to have been identified, the first also being a M dwarf star in the disk. Together, these two events are still consistent with the expected frequency of nearby stars in the Milky Way thin and thick disks acting as lenses.

We present near-infrared spectra of late-phase (>200 days) Type Ia supernovae (SNe Ia) taken at the Subaru Telescope. The [Fe II] line of SN 2003hv shows a clear flat-topped feature, while that of SN 2005W shows a less prominent flatness. In addition, a large shift in their line center, varying from -3000 to 1000 km s^-1 with respect to the host galaxies, is seen. Such a shift suggests the occurrence of an off-center, nonspherical explosion in the central region and provides important, new constraints on the explosion models of SNe Ia.

We have analyzed the archival XMM-Newton data of the bright ultraluminous X-ray source M82 X-1 with a 105 ks exposure when the source was in the steady state. Thanks to the high photon statistics from the large effective area and long exposure, we were able to discriminate different X-ray continuum spectral models. Neither the standard accretion disk model [where the radial dependency of the disk effective temperature is T(r) ∝ r^-3/4] nor a power-law model gives a satisfactory fit. In fact, observed curvature of the M82 X-1 spectrum was just between those of the two models. When the exponent of the radial dependence [p in T(r) ∝ r^-p] of the disk temperature is allowed to be free, we obtained p = 0.61. Such a reduction of p from the standard value under extremely high mass accretion rates is predicted from the accretion disk theory as a consequence of the radial energy advection. Thus, the accretion disk in M82 X-1 is considered to be in the slim-disk state, where an optically thick advection-dominated accretion flow is taking place. We have applied a theoretical slim-disk spectral model to M82 X-1 and estimated the black hole mass ≈19-32 M_☉. We propose that M82 X-1 is a relatively massive stellar black hole that has been produced through evolution of an extremely massive star, shining at a super-Eddington luminosity by several times the Eddington limit.

We present high-resolution X-ray images taken with the Chandra X-Ray Observatory of the field that contains the unidentified TeV gamma-ray source HESS J1804-216. A total of 11 discrete sources were detected with a posteriori significance of >5 σ over the entire field of view. Among them, only one, designated as CXOU J180351.4-213707, is significantly extended. The source is about 40'' away from the radio pulsar PSR J1803-2137, which was the target of the Chandra observation but was not detected in X-rays. A natural question is whether the two sources are physically related. While it is conceivable that CXOU J180351.4-213707 could be associated with a previously unknown supernova remnant (SNR), in which the pulsar was born, it seems equally plausible that it might be a pulsar wind nebula (PWN) that is powered by a different pulsar whose emission is beamed away from us. In either case, we argue that CXOU J180351.4-213707 is likely the X-ray counterpart of HESS J1804-216, based on the fact that the Galactic TeV gamma-ray sources are predominantly SNRs or PWNe. The X-ray spectrum of the source can be well fitted with a power law, although the model is not well constrained due to large statistical uncertainties. The spectrum seems to be very hard, with a best-fit photon index of about 1.2. Under the assumption that CXOU J180351.4-213707 is the X-ray counterpart of HESS J1804-216, we attempted to model the X-ray and TeV emission as synchrotron and inverse Compton scattered radiation from relativistic electrons. We briefly discuss the results.

We report on simultaneous XMM-Newton and RXTE observations of the stellar mass black hole candidate SWIFT J1753.5-0127. The source was observed in the "low-hard" state, during the decline of a hard outburst. The inner accretion disk is commonly assumed to be radially truncated in the low-hard state, and it has been suggested that this property may be tied to the production of steady, compact jets. Fits to the X-ray spectra of SWIFT J1753.5-0127 with a number of simple models clearly reveal a cool (kT ≃ 0.2 keV) accretion disk. The disk component is required at more than the 8 σ level of confidence. Although estimates of inner disk radii based on continuum spectroscopy are subject to considerable uncertainty, fits with a number of models suggest that the disk is observed at or close to the innermost stable circular orbit. Recently, an observation of GX 339-4 revealed a disk extending to the innermost stable circular orbit at L_X/L_Edd ≃ 0.05; our results from SWIFT J1753.5-0127 may extend this finding down to L_X/L_Edd ≃ 0.003(d/8.5 kpc)²(M/10 M_☉). We discuss our results within the context of low-luminosity accretion flow models and disk-jet connections.

Recent Chandra observations of an outflowing gas in GRO J1655-40 resulted in a suggestion by Miller et al. that the wind in this system must be powered by a magnetic process that can also drive accretion through the disk around the black hole. The alternative explanations, of radiation pressure or thermally driven flows, were considered unsatisfactory because of the highly ionized level of the gas and because of the derived small distance from the black hole, well inside the minimum distance required for an efficient X-ray-heated wind. The present Letter shows that there is a simple photoionized wind solution for this system where the gas is much farther out than assumed by Miller et al., at r/r_g = 10^4.7-10^5.7. The expected wind velocity, as well as the computed equivalent widths of more than 50 absorption lines in this single-component one-dimensional model, are all in good agreement with the Chandra observations.

We have used a set of archived Hubble Space Telescope Advanced Camera for Surveys images to probe the evolved populations of the globular cluster 47 Tucanae. We find an excess of asymptotic giant branch (AGB) stars in the cluster core. We interpret this feature as the signature of an extrapopulation likely made by the progeny of massive stars originated by the evolution of binary systems. Indeed, the comparison with theoretical tracks suggests that the AGB population of 47 Tuc can be significantly contaminated by more massive stars currently experiencing the first ascending red giant branch.

We report the first detection of the 205 μm ³P₁P₀ [N II] line from a ground-based observatory using a direct detection spectrometer. The line was detected from the Carina star formation region using the South Pole Imaging Fabry-Perot Interferometer (SPIFI) on the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO) at the South Pole. The [N II] 205 μm line strength indicates a low-density (n ~ 32 cm^-3) ionized medium, similar to the low-density ionized halo previously reported in its [O III] 52 and 88 μm line emission. When compared with the Infrared Space Observatory [C II] observations of this region, we find that 27% of the [C II] line emission arises from this low-density ionized gas, but the large majority (~73%) of the observed [C II] line emission arises from the neutral interstellar medium. This result supports and underpins prior conclusions that most of the observed [C II] 158 μm line emission from Galactic and extragalactic sources arises from the warm, dense photodissociated surfaces of molecular clouds. The detection of the [N II] line demonstrates the utility of Antarctic sites for THz spectroscopy.

Gravitational interactions in very young high-density stellar clusters can to some degree change the angular momentum in the circumstellar disks initially surrounding the majority of stars. However, for most stars, the cluster environment alters the angular momentum only slightly. For example, in simulations of the Orion Nebula cluster (ONC), encounters reduce the angular momentum of the disks by 3%-5%, on average, and in the higher density region of the Trapezium by 15%-20%—still a minor loss process. However, in this Letter, it is demonstrated that the situation is very different if one considers high-mass stars only (M* > 10 M_☉). Assuming an age of 2 Myr for the ONC, their disks have, on average, a 50%-90% lower angular momentum than primordially. This enormous loss in angular momentum in the disk should result in an equivalent increase in accretion, implying that the cluster environment boosts accretion for high-mass stars, thus making them even more massive.

In the core-accretion model, gas-giant planets form solid cores that then accrete gaseous envelopes. Tidal interactions with disk gas cause a core to undergo inward type I migration in 10⁴-10⁵ yr. Cores must form faster than this to survive. Giant planets clear a gap in the disk and undergo inward type II migration in <10⁶ yr if observed disk accretion rates apply to the disk as a whole. Type II migration times exceed typical disk lifetimes if viscous accretion occurs mainly in the surface layers of disks. Low turbulent viscosities near the midplane may allow planetesimals to form by coagulation of dust grains. The radius r of such planetesimals is unknown. If r < 0.5 km, the core formation time is shorter than the type I migration timescale, and cores will survive. Migration is substantial in most cases, leading to a wide range of planetary orbits, consistent with the observed variety of extrasolar systems. When r ~ 100 m and the midplane α ~ 3 × 10^-5, giant planets similar to those in the solar system can form.

Theoretical studies predict that Trojans are likely a frequent by-product of planet formation and evolution. We present a novel method of detecting Trojan companions to transiting extrasolar planets that involves comparing the midtime of eclipse with the time of the stellar reflex velocity null. We demonstrate that this method offers the potential to detect terrestrial-mass Trojans using existing ground-based observatories. This method rules out Trojan companions to HD 209458b and HD 149026b more massive than ≃13 and ≃25 M_⊕ at a 99.9% confidence level. Such a Trojan would be dynamically stable, would not yet have been detected by photometric or spectroscopic monitoring, and would be unrecognizable from radial velocity observations alone. We outline the future prospects for this method and show that the detection of a "Hot Trojan" of any mass would place a significant constraint on theories of orbital migration.

The negative molecular ion C₆H^- has been detected in the radio band in the laboratory and has been identified in the molecular envelope of IRC +10216 and in the dense molecular cloud TMC-1. The spectroscopic constants derived from laboratory measurements of 17 rotational lines between 8 and 187 GHz are identical to those derived from the astronomical data, establishing unambiguously that C₆H^- is the carrier of the series of lines with rotational constant 1377 MHz first observed by K. Kawaguchi et al. in IRC +10216. The column density of C₆H^- toward both sources is 1%-5% that of neutral C₆H. These surprisingly high abundances for a negative ion imply that if other molecular anions are similarly abundant with respect to their neutral counterparts, they may be detectable both in the laboratory at high resolution and in interstellar molecular clouds.