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This is the first paper of our series of high resolution (1") studies of the massive star forming region G10.6--0.4. We present the emission line observations of the hot core type tracers (O$^{13}$CS, OCS, SO$_{2}$) with $\sim$0$"$.5 resolution. By comparing the results to the high--resolution NH$_{3}$ absorption line observation, we confirm for the first time the rotationally flattened hot toroid in the central $, Comment: 26 pages, 5 figures, Accepted by ApJ

In this paper we discuss two approximate methods previously suggested for modeling hyperfine spectral line emission for molecules whose collisional transitions rates between hyperfine levels are unknown. Hyperfine structure is seen in the rotational spectra of many commonly observed molecules such as HCN, HNC, NH3, N2H+, and C17O. The intensities of these spectral lines can be modeled by numerical techniques such as Lambda-iteration that alternately solve the equations of statistical equilibrium and the equation of radiative transfer. However, these calculations require knowledge of both the radiative and collisional rates for all transitions. For most commonly observed radio frequency spectral lines, only the net collisional rates between rotational levels are known. For such cases, two approximate methods have been suggested. The first method, hyperfine statistical equilibrium (HSE), distributes the hyperfine level populations according to their statistical weight, but allows the population of the rotational states to depart from local thermodynamic equilibrium (LTE). The second method, the proportional method approximates the collision rates between the hyperfine levels as fractions of the net rotational rate apportioned according to the statistical degeneracy of the final hyperfine levels. The second method is able to model non-LTE hyperfine emission. We compare simulations of N2H+ hyperfine lines made with approximate and more exact rates and find that satisfactory results are obtained., Comment: 34 pages. Pages 22-34 are data tables. For ApJ.

In a previous paper we demonstrated that nonradial hydrodynamic oscillations of a thermally supported (Bonnor-Ebert) sphere embedded in a low-density, high-temperature medium persist for many periods. The predicted column density variations and molecular spectral line profiles are similar to those observed in the Bok globule B68, suggesting that the motions in some starless cores may be oscillating perturbations on a thermally supported equilibrium structure. Such oscillations can produce molecular line maps which mimic rotation, collapse, or expansion and, thus, could make determining the dynamical state from such observations alone difficult. However, while B68 is embedded in a very hot, low-density medium, many starless cores are not, having interior/exterior density contrasts closer to unity. In this paper we investigate the oscillation damping rate as a function of the exterior density. For concreteness we use the same interior model employed by Broderick et al., with varying models for the exterior gas. We also develop a simple analytical formalism, based on the linear perturbation analysis of the oscillations, which predicts the contribution to the damping rates due to the excitation of sound waves in the external medium. We find that the damping rate of oscillations on globules in dense molecular environments is always many periods, corresponding to hundreds of thousands of years and persisting over the inferred lifetimes of the globules.

In a previous paper, we modeled the oscillations of a thermally-supported (Bonnor-Ebert) sphere as non-radial, linear perturbations following a standard analysis developed for stellar pulsations. The predicted column density variations and molecular spectral line profiles are similar to those observed in the Bok globule B68 suggesting that the motions in some starless cores may be oscillating perturbations on a thermally supported equilibrium structure. However, the linear analysis is unable to address several questions, among them the stability, and lifetime of the perturbations. In this paper we simulate the oscillations using a three-dimensional numerical hydrodynamic code. We find that the oscillations are damped predominantly by non-linear mode-coupling, and the damping time scale is typically many oscillation periods, corresponding to a few million years, and persisting over the inferred lifetime of gobules., 7 pages, 7 figures, accepted by ApJ

The fine-structure lines of singly ([Ne II] 12.8 micron) and doubly ([Ne III] 15.6 micron) ionized neon are among the most prominent features in the mid-infrared spectra of star-forming regions, and have the potential to be a powerful new indicator of the star formation rate in galaxies. Using a sample of star-forming galaxies with measurements of the fine-structure lines available from the literature, we show that the sum of the [Ne II] and [Ne III] luminosities obeys a tight, linear correlation with the total infrared luminosity, over 5 orders of magnitude in luminosity. We discuss the formation of the lines and their relation with the Lyman continuum luminosity. A simple calibration between star formation rate and the [Ne II]+[Ne III] luminosity is presented., To appear in ApJ. 8 pages

We present Atacama Large Millimeter/submillimeter Array (ALMA) line and continuum observations at 1.2mm with ~0.3" resolution that uncover a Keplerian-like disk around the forming O-type star AFGL 4176. The continuum emission from the disk at 1.21 mm (source mm1) has a deconvolved size of 870+/-110 AU x 330+/-300 AU and arises from a structure ~8 M_sun in mass, calculated assuming a dust temperature of 190 K. The first-moment maps, pixel-to-pixel line modeling, assuming local thermodynamic equilibrium (LTE), and position-velocity diagrams of the CH3CN J=13-12 K-line emission all show a velocity gradient along the major axis of the source, coupled with an increase in velocity at small radii, consistent with Keplerian-like rotation. The LTE line modeling shows that where CH3CN J=13-12 is excited, the temperatures in the disk range from ~70 to at least 300 K and that the H2 column density peaks at 2.8x10^24 cm^-2. In addition, we present Atacama Pathfinder Experiment (APEX) 12CO observations which show a large-scale outflow from AFGL 4176 perpendicular to the major axis of mm1, supporting the disk interpretation. Finally, we present a radiative transfer model of a Keplerian disk surrounding an O7 star, with a disk mass and radius of 12 M_sun and 2000 AU, that reproduces the line and continuum data, further supporting our conclusion that our observations have uncovered a Keplerian disk around an O-type star., 6 pages, 4 figures, accepted for publication in ApJL

We test the hypothesis that the starless cores may be gravitationally bound clouds supported largely by thermal pressure by comparing observed molecular line spectra to theoretical spectra produced by a simulation that includes hydrodynamics, radiative cooling, variable molecular abundance, and radiative transfer in a simple one-dimensional model. The results suggest that the starless cores can be divided into two categories: stable starless cores that are in approximate equilibrium and will not evolve to form protostars, and unstable pre-stellar cores that are proceeding toward gravitational collapse and the formation of protostars. The starless cores might be formed from the interstellar medium as objects at the lower end of the inertial cascade of interstellar turbulence. Additionally, we identify a thermal instability in the starless cores. Under par ticular conditions of density and mass, a core may be unstable to expansion if the density is just above the critical density for the collisional coupling of the gas and dust so that as the core expands the gas-dust coupling that cools the gas is reduced and the gas warms, further driving the expansion., Submitted to ApJ

We observed the nearby starburst galaxy M82 in CO in the higher frequency (2--1) transition to achieve an angular resolution below 1 arc second or 17 pc at the target. We resolved the molecular gas into a large number of compact clouds, with masses ranging from about 2x10^3 to 2x10^6 solar masses. The mass spectrum scales as N(M) ~ M^-1.5, similar to the mass spectra of young massive star clusters suggesting that individual molecular clouds are transformed in the starburst into individual star clusters. The larger clouds are surrounded by supernovae and HII regions suggesting that star formation proceeds from the outside of the clouds and progresses inward consistent with triggering by a sudden increase in external pressure. The clouds with internal star formation have velocity gradients and inverse P-Cygni spectral line profiles indicating inward motions of 35 kms consistent with shock driven compression. Diffuse free-free radio emission and X-ray emission around the clouds provides evidence for superheated ionized gas sufficient to drive the compression. Clouds with spectral lines indicating expansion show little internal star formation suggesting that the dynamics precedes and is responsible for the star formation rather than the inverse. M82 is known to be in interaction with neighboring M81. The overall picture is consistent with the formation of massive star clusters from individual giant molecular clouds crushed by a sudden galactic scale increase in external pressure generated by the changing dynamics that result from a near-collision with a neighboring galaxy. Present day globular clusters may have formed in a similar fashion in primordial galaxies., Submitted to ApJ

We present the first 865 μm continuum image with subarcsecond resolution obtained with the Submillimeter Array. These data resolve the Orion KL region into the hot core, the nearby radio source I, the submillimeter counterpart to the infrared source n (radio source L), and new submillimeter continuum sources. The radio to submillimeter emission from source I can be modeled as either the result of proton-electron free-free emission that is optically thick to ~100 GHz plus dust emission that accounts for the majority of the submillimeter flux, or H- free-free emission that gives rise to a power-law spectrum with a power-law index of ~1.6. The latter model would indicate similar physical conditions as found in the inner circumstellar environment of Mira variable stars. Future subarcsecond resolution observations at shorter submillimeter wavelengths should easily discriminate between these two possibilities. The submillimeter continuum emission toward source n can be interpreted in the framework of emission from an accretion disk.

We develop a method of analyzing radio frequency spectral line observations to derive data on the temperature, density, velocity, and molecular abundance of the emitting gas. The method incorporates a radiative transfer code with a new technique for handling overlapping hyperfine emission lines within the accelerated lambda iteration algorithm and a heuristic search algorithm based on simulated annnealing. We apply this method to new observations of N_2H^+ in three Lynds clouds thought to be starless cores in the first stages of star formation and determine their density structure. A comparison of the gas densities derived from the molecular line emission and the millimeter dust emission suggests that the required dust mass opacity is about kappa_{1.3mm}=0.04 cm^2/g, consistent with models of dust grains that have opacities enhanced by ice mantles and fluffy aggregrates., Comment: 42 pages, 17 figures, to appear in ApJ

The classic model for the pressure-driven expansion of H II regions is reevaluated to include the gravitational force of the star responsible for the H II region. The model shows that the gravitational attraction of the star maintains a steep density gradient and accretion flow within the ionized gas and prevents the H II region from expanding hydrodynamically unless the radius of ionization equilibrium is beyond the radius where the sound speed of the ionized gas approximates the escape velocity. Once past this critical radius, the H II region will expand rapidly and the accretion flow through the H II region is quickly reduced. However, in contrast to the model without gravity in which the velocity of the ionized gas is everywhere outward, in the model with gravity, the velocity within the H II region is always inward. Newly formed massive stars within dense molecular cores may initially form very small H II regions that at first evolve slowly through an increase in ionizing flux, as would be caused by an increase in the mass or number of stars through continuing accretion through the H II region.

Analysis of the H66α radio recombination line velocities in the ultracompact H II region G10.6-0.4 indicates inward motion of the ionized gas within the H II region. Comparison with the velocities of molecular lines from the surrounding neutral gas suggests that a molecular accretion flow passes through the ionization boundary of the H II region and continues inward as an ionized accretion flow. Interpreted in the context of a model for star formation in hot molecular cores, these observations indicate that G10.6-0.4 is at a stage of development just after the massive star or stars at the center of the accretion flow have first formed an ionized H II region within the hot molecular core.

We present single-dish observations of the L1689-SMM16 core in the Ophiuchus molecular cloud in NH3 (1, 1) and (2, 2) emission using the Green Bank Telescope, in N2H+ (1-0) emission using the Nobeyama Radio Observatory, and in NH2D (), HCN (1-0), HNC (1-0), H 13CO+ (1-0), and HCO+ (1-0) emission using the Mopra telescope. The morphologies of the integrated NH3 (1, 1) and N2H+ (1-0) emission well match that of 250 μm continuum emission. Line widths of NH3 (1, 1) and N2H+ (1-0) show the presence of transonic turbulence across the core. Jeans and virial analyses made using updated measurements of core mass and size confirm that L1689-SMM16 is prestellar, i.e., gravitationally bound. It also has accumulated more mass compared to its corresponding Jeans mass in the absence of magnetic fields and therefore is a "super-Jeans" core. The high levels of X(NH 3)/X(N2H+) and deuterium fractionation reinforce the idea that the core has not yet formed a protostar. Comparing the physical parameters of the core with those of a Bonnor-Ebert sphere reveals the advanced evolutionary stage of L1689-SMM16 and shows that it might be unstable to collapse. We do not detect any evidence of infall motions toward the core. Instead, red asymmetry in the line profiles of HCN (1-0) and HNC (1-0) indicates the expansion of the outer layers of the core at a speed of 0.2 km s -1 to 0.3 km s-1. For a gravitationally bound core, expansion in the outer layers might indicate that the core is experiencing oscillations.

We examine the role of the gravitational instability in an isothermal, self-gravitating layer threaded by magnetic fields on the formation of filaments and dense cores. Using numerical simulation we follow the non-linear evolution of a perturbed equilibrium layer. The linear evolution of such a layer is described in the analytic work of Nagai et al (1998). We find that filaments and dense cores form simultaneously. Depending on the initial magnetic field, the resulting filaments form either a spiderweb-like network (for weak magnetic fields) or a network of parallel filaments aligned perpendicular to the magnetic field lines (for strong magnetic fields). Although the filaments are radially collapsing, the density profile of their central region (up to the thermal scale height) can be approximated by a hydrodynamical equilibrium density structure. Thus, the magnetic field does not play a significant role in setting the density distribution of the filaments. The density distribution outside of the central region deviates from the equilibrium. The radial column density distribution is then flatter than the expected power law of $r^{-4}$ and similar to filament profiles observed with Herschel. Our results does not explain the near constant filament width of $\sim 0.1$pc. However, our model does not include turbulent motions. It is expected that accretion-driven amplification of these turbulent motions provides additional support within the filaments against gravitational collapse. Finally, we interpret the filamentary network of the massive star forming complex G14.225-0.506 in terms of the gravitational instability model and find that the properties of the complex are consistent with being formed out of an unstable layer threaded by a strong, parallel magnetic field., Comment: Accepted by ApJ, 14 pages, 22 figures

Massive stars ($M > 8$ \msun) typically form in parsec-scale molecular clumps that collapse and fragment, leading to the birth of a cluster of stellar objects. We investigate the role of magnetic fields in this process through dust polarization at 870 $��$m obtained with the Submillimeter Array (SMA). The SMA observations reveal polarization at scales of $\lsim$ 0.1 pc. The polarization pattern in these objects ranges from ordered hour-glass configurations to more chaotic distributions. By comparing the SMA data with the single dish data at parsec scales, we found that magnetic fields at dense core scales are either aligned within $40^\circ$ of or perpendicular to the parsec-scale magnetic fields. This finding indicates that magnetic fields play an important role during the collapse and fragmentation of massive molecular clumps and the formation of dense cores. We further compare magnetic fields in dense cores with the major axis of molecular outflows. Despite a limited number of outflows, we found that the outflow axis appears to be randomly oriented with respect to the magnetic field in the core. This result suggests that at the scale of accretion disks ($\lsim 10^3$ AU), angular momentum and dynamic interactions possibly due to close binary or multiple systems dominate over magnetic fields. With this unprecedentedly large sample massive clumps, we argue on a statistical basis that magnetic fields play an important role during the formation of dense cores at spatial scale of 0.01 - 0.1 pc in the context of massive star and cluster star formation., Accepted for publication in Astrophysical Journal

We observed the nearby starburst galaxy NGC 253 in the mid-infrared to obtain a three-dimensional data set with arcsecond angular resolution and 0.2 km spectral resolution. The observations show the major spectral features in the upper half of the mid-IR window: the 11.3 km polycyclic aromatic hydro- carbon (PAH) line and the 12.8 km (Ne II) line as well as the broad silicate absorption feature at 9.7 km. We use the (Ne II) line to determine the emission measure of the ionized gas, and in combination with radio observations to predict the thermal and nonthermal contributions to the radio continuum. The amount of ionized gas is related to the rate of star formation. Based on the mid-IR spectra, we identify three major components in the nucleus of NGC 243: an AGN in the center of the galaxy, a superstar cluster also seen in optical images, and a larger scale diUuse envelope composed of an older population of supernova remnants and lower mass stars. Subject headings: galaxies: individual (NGC 253) ¨ galaxies: starburstgalaxies: star clusters

We present arcsecond angular-resolution mid-IR images of three IR-bright galaxies, NGC 6240, IRAS 0518-25, and the NGC 3690-IC 694 (Arp 299) system, and we compare the sizes of their emission regions in the mid-IR and near-IR. In all these galaxies, most ({approximately}75{percent}) of the mid-IR flux, as measured in the IRAS 12 {mu}m observations, is found in our images to be concentrated within the central few hundred parsecs. In the two galaxies which show a single IR source in the nucleus, IRAS 0518-25 and IC 694, the mid-IR emission is more compact than the near-IR. This suggests that the mid-IR source, warm dust heated by an active galactic nucleus or an unresolved starburst, is more compact than the population of cooler, older stars. In the two galaxies which show double sources in the nucleus, the reverse is the case. That is, the mid-IR is more extended than the near-IR. Similar results are obtained for a handful of other galaxies for which we have high-resolution mid-IR data. Thus in our small sample, the double source nuclei contain starbursts which are more extended than the older stars and the single source nuclei do not. Some of the double sources seen in themore » IR bright galaxies, for example NGC 3690, are probably starbursts in conjunction with a galactic nucleus rather than twin galactic nuclei. {copyright} {ital 1997} {ital The American Astronomical Society}« less

We present narrowband images at selected wavelengths in the 8-13 μm window of four carbon-rich, proto-planetary nebulae that have an unusual 21 μm dust feature: IRAS 04296+3429, IRAS 22272+5435, IRAS 07134+1005, and IRAS 19500-1709. We observe axially symmetric dust emission structures in IRAS 22272+5435 and IRAS 07134+1005, and tentatively in IRAS 19500-1709, while IRAS 04296+3429 is unresolved with our ~1'' angular resolution. The well-resolved morphology of IRAS 07134+1005 shows an elliptical outer shell surrounding two aligned peaks that we interpret as limb-brightened peaks of an optically thin, elliptical shell with an equatorial density enhancement. This mid-IR morphology contrasts with that observed in the better studied carbon-rich proto-planetary nebulae, AFGL 2688, AFGL 915, and AFGL 618, which show bright, unresolved cores, probably created by optically thick inner regions, and bipolar extensions that align with their optical reflection nebulosities. Using an axially symmetric dust code and assuming that the dust is composed of 0.01 μm amorphous carbon grains, we model the dust emission images and the spectral energy distributions of these four proto-planetary nebulae and of the young, carbon-rich planetary nebula IRAS 21282+5050, which also has an axially symmetric dust shell and other similarities with the proto-planetary nebulae that have the 21 μm dust feature. Marginally resolved mid-infrared images constrain the dust shell's inner radius, while well-resolved mid-infrared images additionally constrain other geometric parameters of the model (e.g., inclination angles and pole-to-equator mass-loss rate ratios). The modeling reveals that the observed axial symmetry in the dust shells of these objects coincides with an enhanced mass-loss phase (~3 × 10-5 M☉ yr-1) during which the equatorial mass-loss rate was a factor of 18-90 higher than the polar mass-loss rate, i.e., an axially symmetric superwind. Our dynamical age estimates indicate that these stars left the asymptotic giant branch approximately 300-1400 years ago, just after the superwind phase. For each object, the size and structure of the dust shell is the same for the sampled wavelengths, with the exception of IRAS 22272+5435 for which the 11.8 μm emission is larger than either the 8.2 or the 9.7 μm emission. IRAS 22272+5435's spectrum has a larger dust feature-to-dust continuum ratio than found in the other objects, and hence its 11.8 μm image is probably dominated by the 11.8 μm feature emission that has different optical properties than the underlying continuum.

If star formation proceeds by thermal fragmentation and the subsequent gravitational collapse of the individual fragments, how is it possible to form fragments massive enough for O and B stars in a typical star-forming molecular cloud where the Jeans mass is about 1Msun at the typical densities (10^4 cm^-3) and temperatures (10K)? We test the hypothesis that a first generation of low-mass stars may heat the gas enough that subsequent thermal fragmentation results in fragments >=10Msun, sufficient to form B stars. We combine ATCA and SMA observations of the massive star-forming region G8.68-0.37 with radiative transfer modeling to derive the present-day conditions in the region and use this to infer the conditions in the past, at the time of core formation. Assuming the current mass/separation of the observed cores equals the fragmentation Jeans mass/length and the region's average density has not changed, requires the gas temperature to have been 100K at the time of fragmentation. The postulated first-generation of low-mass stars would still be around today, but the number required to heat the cloud exceeds the limits imposed by the observations. Several lines of evidence suggest the observed cores in the region should eventually form O stars yet none have sufficient raw material. Even if feedback may have suppressed fragmentation, it was not sufficient to halt it to this extent. To develop into O stars, the cores must obtain additional mass from outside their observationally defined boundaries. The observations suggest they are currently fed via infall from the very massive reservoir (~1500Msun) of gas in the larger pc scale cloud around the star-forming cores. This suggests that massive stars do not form in the collapse of individual massive fragments, but rather in smaller fragments that themselves continue to gain mass by accretion from larger scales., 23 pages, 14 figures. Accepted for publication in ApJ

We present observations of newly discovered 24 micron circumstellar structures detected with the Multiband Imaging Photometer for Spitzer (MIPS) around three evolved stars in the Cygnus-X star forming region. One of the objects, BD+43 3710, has a bipolar nebula, possibly due to an outflow or a torus of material. A second, HBHA 4202-22, a Wolf-Rayet candidate, shows a circular shell of 24 micron emission suggestive of either a limb-brightened shell or disk seen face-on. No diffuse emission was detected around either of these two objects in the Spitzer 3.6-8 micron Infrared Array Camera (IRAC) bands. The third object is the luminous blue variable candidate G79.29+0.46. We resolved the previously known inner ring in all four IRAC bands. The 24 micron emission from the inner ring extends ~1.2 arcmin beyond the shorter wavelength emission, well beyond what can be attributed to the difference in resolutions between MIPS and IRAC. Additionally, we have discovered an outer ring of 24 micron emission, possibly due to an earlier episode of mass loss. For the two shell stars, we present the results of radiative transfer models, constraining the stellar and dust shell parameters. The shells are composed of amorphous carbon grains, plus polycyclic aromatic hydrocarbons in the case of G79.29+0.46. Both G79.29+0.46 and HBHA 4202-22 lie behind the main Cygnus-X cloud. Although G79.29+0.46 may simply be on the far side of the cloud, HBHA 4202-22 is unrelated to the Cygnus-X star formation region., Accepted by AJ

Accretion flows onto massive stars must transfer mass so quickly that they are themselves gravitationally unstable, forming dense clumps and filaments. These density perturbations interact with young massive stars, emitting ionizing radiation, alternately exposing and confining their H II regions. As a result, the H II regions are predicted to flicker in flux density over periods of decades to centuries rather than increase monotonically in size as predicted by simple Spitzer solutions. We have recently observed the Sgr B2 region at 1.3 cm with the Very Large Array in its three hybrid configurations (DnC, CnB, and BnA) at a resolution of ~025. These observations were made to compare in detail with matched continuum observations from 1989. At 025 resolution, Sgr B2 contains 41 ultracompact (UC) H II regions, 6 of which are hypercompact. The new observations of Sgr B2 allow comparison of relative peak flux densities for the H II regions in Sgr B2 over a 23 year time baseline (1989-2012) in one of the most source-rich massive star forming regions in the Milky Way. The new 1.3 cm continuum images indicate that four of the 41 UC H II regions exhibit significant changes in their peak flux density, with one source (K3) dropping in peak flux density, and the other three sources (F10.303, F1, and F3) increasing in peak flux density. The results are consistent with statistical predictions from simulations of high mass star formation, suggesting that they offer a solution to the lifetime problem for UC H II regions.

Water is a crucial molecule in molecular astrophysics as it controls much of the gas/grain chemistry, including the formation and evolution of more complex organic molecules in ices. Pre-stellar cores provide the original reservoir of material from which future planetary systems are built, but few observational constraints exist on the formation of water and its partitioning between gas and ice in the densest cores. Thanks to the high sensitivity of the Herschel Space Observatory, we report on the first detection of water vapor at high spectral resolution toward a dense cloud on the verge of star formation, the pre-stellar core L1544. The line shows an inverse P-Cygni profile, characteristic of gravitational contraction. To reproduce the observations, water vapor has to be present in the cold and dense central few thousand AU of L1544, where species heavier than Helium are expected to freeze-out onto dust grains, and the ortho:para H2 ratio has to be around 1:1 or larger. The observed amount of water vapor within the core (about 1.5x10^{-6} Msun) can be maintained by Far-UV photons locally produced by the impact of galactic cosmic rays with H2 molecules. Such FUV photons irradiate the icy mantles, liberating water wapor in the core center. Our Herschel data, combined with radiative transfer and chemical/dynamical models, shed light on the interplay between gas and solids in dense interstellar clouds and provide the first measurement of the water vapor abundance profile across the parent cloud of a future solar-type star and its potential planetary system., Comment: The Astrophysical Journal Letters, in press

We report a spectral-line survey of the extreme carbon star IRC+10216 carried out between 293.9 and 354.8 GHz with the Submillimeter Array. A total of 442 lines were detected, more than 200 for the first time; 149 are unassigned. Maps at an angular resolution of ~3" were obtained for each line. A substantial new population of narrow lines with an expansion velocity of ~4 km/s (i.e. ~ 30% of the terminal velocity) was detected. Most of these are attributed to rotational transitions within vibrationally excited states, emitted from energy levels above the v=0, J=0 ground state with excitation energy of 1000-3000 K. Emission from these lines appears to be centered on the star with an angular extent of, 90 pages, 13 figures. Accepted for publication in ApJSS

We present images at 8.5, 10.5, and 12.5 {mu}m which reveal that AFGL 2343 (HD 179821) is surrounded by a dusty nebula 4{double_prime}{minus}5{double_prime} in diameter at 10.5 and 12.5 {mu}m. This G5 Ia supergiant has long been suspected of having a detached dusty envelope because of its optical brightness and large IR excess. The nebula is ringlike and slightly elongated, with bright patches or clumps to the northeast. An 8.5 {mu}m image is dominated by the photosphere of the central star, which also appears to a lesser extent in the 10.5 and 12.5 {mu}m images. A full radiative transfer model of the nebula correctly predicts the observed inner (1{close_quote}{close_quote}.1) and outer (1{close_quote}{close_quote}.9) radii of the shell and the spectral energy distribution of the shell. With the aid of this model, we find that AFGL 2343 is almost certainly an extremely massive star at a distance of about 6 kpc, which has recently undergone an enormous mass-loss event, during which {dot {ital M}}=10{sup {minus}2} {ital M}{sub {circle_dot}} yr{sup {minus}1}, while the current mass-loss rate is less than 2{times}10{sup {minus}6} {ital M}{sub {circle_dot}} yr{sup {minus}1}. {copyright} {ital 1995 The American Astronomical Society.}

We present high angular resolution (1″.2), narrow-band (Δλ/λ=0.1) images of the nucleus of NGC 253 at three wavelengths in the mid-infrared (8.5, 10.0, and 12.5 μm). We find that most of the mid-IR flux in the nucleus of NGC 253 derives from a very small region ≤120 pc in diameter. Within this small region there are three spatially and spectrally distinct IR components: two bright compact sources, and a surrounding envelope of low-level, diffuse emission. The mid-IR and 6 cm radio are loosely correlated in position but not in brightness. The implication is that the mid-IR-radio correlation may begin to break down on small spatial scales relevant to individual star-forming regions and large individual sources

We present 3.3, 8.5, 10.0, 11.3, and 12.5 μm narrow-band (Δλ/λ ≃3%-10%) and K-band (2.2 μm) high spatial resolution images of IRAS 21282+5050 and a 6 cm flux density measurement. The new infrared images reveal a toroidal dust nebula, 4″.5×6″, P.A. 165°, with two prominent emission peaks aligned almost E-W and separated by 1″.7-2″.1; a remarkably similar structure to that observed in NGC 7027. From the 6 cm flux density and the dust nebula size, we derive an electron density of 3.6× 10 3 cm −3 for the central ionized region. Comparison of these images with a published CO 1-0 map suggests that the structure of this dusty, ionization-bounded planetary nebula may reflect the density inhomogeneities of the progenitor's wind

Young late-type stars in massive-star-forming regions, though much fainter than their early-type neighbors, are detectable in brightened molecular line emission, IR continuum, and H 2 O maser emission. The stars are found deeply embedded in high-temperature, high-density gas and near newly formed massive stars. Because the time scales in massive-star-forming regions are so short, these lower mass stars must be in very early evolutionary stages relative to their much longer lifetimes

The first subarcsecond resolution mid-IR images of the nuclei of two galaxies from the IRAS bright sample, NGC 1614 and NGC 7469, are reported. These results show that the starburst galaxy NGC 1614 contains a double structure in the nuclear region. Most of the flux, 78% of 650 ± 50 mJy, is emitted in two unresolved regions of 0.6″ FWHM (190 pc at 64 Mpc) of nearly equal strength separated by 0.6″ (200 pc). Most of the remaining flux is derived from a small arm 1″ in length (310 pc). The Seyfert I galaxy NGC 7469 shows an elongated structure, 1.4″ × 0.7″ (450 × 220 pc at 66 Mpc)

We have obtained images of the compact radio and infrared nebula G5.89-0.39 with the Berkeley/IGPP mid-infrared array camera. The images have a resolution of 1″.1 and cover five wavelength bands between 8 and 13 μm. On the basis of the structure and spectral energy distribution, we concur with previously published assertions that G5.89-0.39 is an ultracompact H II region. The infrared emission is extended over 6″×4″.5, very similar to the object's size in published high-resolution radio continuum maps

The process of gravitational fragmentation of molecular clouds is investigated through a comparison of numerical and observational results. The expected millimeter wave molecular line emission from a model fragmenting cloud generated by a numerical hydrodynamic simulation is calculated and compared with observations of HCO(+) from the star-forming region W49A. This investigation suggests that the rotating ring of H II regions, the necklace in W49A, may have formed in the gravitational fragmentation of a flattened rotating molecular cloud.

A high angular resolution (1″) mid-IR image of the IR-bright galaxy NGC 34 is reported. The observation shows the mid-IR emission to be very compact, ≤ 2.3″ × 1.7″. This and previous observations indicate sizes smaller than predicted by starburst models for the emission. The achieved sensitivity, 15 mJy minutes −1/2 arcsec −2 , is the highest sensitivity ever obtained for any observation with a mid-IR array camera

Comparison of high angular resolution interferometric and lower resolution single-dish observations of NH 3 in molecular cloud cores around ultracompact H II regions suggests that the spectral line shapes observed at lower angular resolution may be used to determine the nature of the interaction between the newly formed stars, represented by the H II region, and the surrounding molecular gas. For example, VLA observations of NH 3 around NGC 7538-IRS 1 are reported. The single-dish observations show that the absorption lines which arise from gas in front of the H II region are blueshifted with respect to the emission which arise from the surrounding cloud.

Une procedure de simulation d'images de raies spectrales de nuages moleculaires 3D est presentee et appliquee a l'ecoulement d'accretion associe a une formation stellaire de grande masse dans la region G10.6-0.4. Les comparaisons observations-simulations sont realisees pour les temperatures, densites et champ de vitesse. La nature de l'ecoulement est analysee.

Les observations VLA des raies d'inversion (J,K)=(1,1) et (3,3) de NH 3 de la region de formation stellaire DR 21 sont presentees. Les deux nuages denses isoles situes au sud et au nord-est de l'amas DR 21 de regions H II sont analyses et semble avoir subi un effondrement gravitationnel aux premieres etapes de la formation stellaire

High-resolution aperture synthesis observations of the molecular gas surrounding the ultracompact H II region G10.6-0.4 in the (J,K) = (1,1) and (J,K) = (3,3) inversion lines of NH3 are reported. The ultracompact region is embedded in a dense molecular core whose average n(H2) density over the central 0.1 pc is 4 x 10 to the 6th/cu cm. The H II region is heating the molecular gas. Temperatures in the central 0.1 pc are in the range 95-140 K and scale outward as 1/sq rt R. The molecular core appears to be gravitationally collapsing onto the ultracompact H II region. Rotational gradient measurements suggest that between 1.5 and 0.35 pc the dynamics is consistent with Keplerian rotation and the mass distribution estimated from maps of the column density. 24 references.

Observations of NH3(1, 1) obtained with 10-arcsec resolution in the molecular cores surrounding two ultracompact H II regions, W3(OH) and G34.3+0.2, are reported. Both sources show extended emission components with velocity gradients consistent with rotation and absorption components that are redshifted with respect to the emission, suggesting gravitational infall. The velocity structure in both sources, and the density and temperature structure in W3(OH), appear similar to that around the previously observed H II region, G10.6-0.4, which is well modeled by a centrally condensed, and centrally heated, molecular core gravitationally collapsing onto the H II region. It is suggested that velocity and temperature structures similar to those seen in W3(OH), G34.3+0.2, and G10.6-0.4 may be a useful signature of gravitational collapse. 34 references.