Ronald L. Snell

A Study of the Physics and Chemistry of TMC-1

We present a comprehensive study of the physical and chemical conditions along the TMC-1 ridge. Temperatures were estimated from observations of CH3CCH, NH3, and CO. Densities were obtained from a multitransition study of HC3N. The values of the density and temperature allow column densities for 13 molecular species to be estimated from statistical equilibrium calculations, using observations of rarer isotopomers where possible, to minimize opacity effects. The most striking abundance variations relative to HCO+ along the ridge were seen for HC3N, CH3CCH, and SO, while smaller variations were seen in CS, C2H, and HCN. On the other hand, the NH3, HNC, and N2H+ abundances relative to HCO+ were determined to be constant, indicating that the so-called NH3 peak in TMC-1 is probably a peak in the ammonia column density rather than a relative abundance peak. In contrast, the well-studied cyanopolyyne peak is most likely due to an enhancement in the abundance of long-chain carbon species. Comparisons of the derived abundances to the results of time-dependent chemical models show good overall agreement for chemical timescales around 10(5) yr. We find that the observed abundance gradients can be explained either by a small variation in the chemical timescale from 1.2 x 10(5) to 1.8 x 10(5) yr or by a factor of 2 change in the density along the ridge. Alternatively, a variation in the C/O ratio from 0.4 to 0.5 along the ridge produces an abundance gradient similar to that observed.

The equilibrium state of molecular regions in the outer Galaxy

A summary of global properties and an evaluation of the equilibrium state of molecular regions in the outer Galaxy are presented from the decomposition of the FCRAO Outer Galaxy Survey and targeted 12CO and 13CO observations of four giant molecular cloud complexes. The ensemble of identified objects includes both small, isolated clouds and clumps within larger cloud complexes. The 12CO luminosity function and size distribution of a subsample of objects with well-defined distances are determined such that ΔN/ΔLCO = L and ΔN/Δre = r. 12CO velocity dispersions show little variation with cloud sizes for radii less than 10 pc. It is demonstrated that the internal motions of regions with MCO = XCOLCO > 104 M☉ are bound by self-gravity, yet the constituent clumps of cloud complexes and isolated molecular clouds with MCO < 103 M☉ are not in self-gravitational equilibrium. The required external pressures to maintain the equilibrium of this population are (1-2) × 104 cm-3 K.

The Submillimeter Wave Astronomy Satellite: Science Objectives and Instrument Description

The Submillimeter Wave Astronomy Satellite (SWAS), launched in 1998 December, is a NASA mission dedicated to the study of star formation through direct measurements of (1) molecular cloud composition and chemistry, (2) the cooling mechanisms that facilitate cloud collapse, and (3) the large-scale structure of the UV-illuminated cloud surfaces. To achieve these goals, SWAS is conducting pointed observations of dense [n(H2) > 103 cm-3] molecular clouds throughout our Galaxy in either the ground state or a low-lying transition of five astrophysically important species: H2O, H218O, O2, C I, and 13CO. By observing these lines SWAS is (1) testing long-standing theories that predict that these species are the dominant coolants of molecular clouds during the early stages of their collapse to form stars and planets and (2) supplying previously missing information about the abundance of key species central to the chemical models of dense interstellar gas. SWAS carries two independent Schottky barrier diode mixers—passively cooled to ~175 K—coupled to a 54 × 68 cm off-axis Cassegrain antenna with an aggregate surface error ~11 μm rms. During its baseline 3 yr mission, SWAS is observing giant and dark cloud cores with the goal of detecting or setting an upper limit on the water and molecular oxygen abundance of 3 × 10-6 (relative to H2). In addition, advantage is being taken of SWASs relatively large beam size of 33 × 45 at 553 GHz and 35 × 50 at 490 GHz to obtain large-area (~1° × 1°) maps of giant and dark clouds in the 13CO and C I lines. With the use of a 1.4 GHz bandwidth acousto-optical spectrometer, SWAS has the ability to simultaneously observe either the H2O, O2, C I, and 13CO lines or the H218O, O2, and C I lines. All measurements are being conducted with a velocity resolution less than 1 km s-1.

THE RELATION BETWEEN GAS AND DUST IN THE TAURUS MOLECULAR CLOUD

We report a study of the relation between dust and gas over a 100 deg 2 area in the Taurus molecular cloud. We compare the H2 column density derived from dust extinction with the CO column density derived from the 12 CO and 13 CO J = 1 → 0 lines. We derive the visual extinction from reddening determined from 2MASS data. The comparison is done at an angular size of 200 �� corresponding to 0.14 pc at a distance of 140 pc. We find that the relation between visual extinction AV and N (CO) is linear between AV � 3 and 10 mag in the region associated with the B213-L1495 filament. In other regions, the linear relation is flattened for AV 4 mag. We find that the presence of temperature gradients in the molecular gas affects the determination of N (CO) by ∼30%–70% with the largest difference occurring at large column densities. Adding a correction for this effect and accounting for the observed relation between the column density of CO and CO2 ices and AV, we find a linear relationship between the column of carbon monoxide and dust for observed visual extinctions up to the maximum value in our data � 23 mag. We have used these data to study a sample of dense cores in Taurus. Fitting an analytical column density profile to these cores we derive an average volume density of about 1.4 × 10 4 cm −3 and a CO depletion age of about 4.2 × 10 5 yr. At visual extinctions smaller than ∼3 mag, we find that the CO fractional abundance is reduced by up to two orders of magnitude. The data show a large scatter suggesting a range of physical conditions of the gas. We estimate the H2 mass of Taurus to be about 1.5 × 10 4 M� , independently derived from the AV and N (CO) maps. We derive a CO integrated intensity to H2 conversion factor of about 2.1 × 10 20 cm −2 (K km s −1 ) −1 , which applies even in the region where the [CO]/[H2] ratio is reduced by up to two orders of magnitude. The distribution of column densities in our Taurus maps resembles a log-normal function but shows tails at large and low column densities. The length scale at which the high column density tail starts to be noticeable is about 0.4 pc.

Compact radio sources associated with molecular outflows

The results of a search for radio emission associated with molecular outflows in regions with a compact object are reported. The data were collected with the VLA in 1981 and 1983 and covered 12 fields and 15 outflows. The frequencies used were 1.4 GHz, 5 GHz and 15 GHz aimed at S187, NGC 1333, L1551, NGC 2024, M78, NGC 2071, S255, CFK 961, S68, AS 353A, S106, and NGC 7129. The positions, flux densities, spectra and sizes of the compact radio sources scanned are provided for each region and discussed. Radio continuum emissions were recorded whenever the bolometric luminosity exceeded 100 solar luminosities, and from three sources with luminosities below that magnitude. The sources S106, CRL 961 and LkH-alpha 234 all had spectra and angular breadths typical of stars undergoing spherical mass outflow. Smooth surface brightness distributions for S254, S255 and S257 support an interpretation as uniform density spherical Stromgren spheres. High resolution radio maps of NGC 2071 and S255 reveal that both have three compact objects distributed as the objects in Trapezium. The NGC 2071 objects are confined to a space the size of the solar system, with electron densities greater than 10,000/cu cm. 85 references

MOLECULAR OUTFLOWS ASSOCIATED WITH BRIGHT FAR-INFRARED SOURCES

A systematic search was carried out for high-velocity CO emission associated with bright 100-micron sources from the IRAS Point Source Catalog, in an effort to increase the understanding of the nature and evolutionary status of the objects producing molecular outflows. Eighteen sources with 100-micron flux densities greater than 500 Jy were selected and maps were made in the J = 1-0 (C-12) line around each source. Almost every source observed was found to lie toward, or in the immediate vicinity of, strong CO emission and in most cases close to the maximum of this emission. Thus, all of the far-infrared sources can be associated with molecular clouds. Five of the sources show clear evidence of high-velocity molecular emission and at least three have bipolar morphologies. The newly detected outflow sources are all intrinsically luminous objects, and their outflows energetic. The infrared characteristics of all the sources in the present survey suggest that they are young stellar objects still embedded in their parent molecular clouds. Statistics on the occurrence of outflows from bright far-infrared sources are used to set an upper limit of 400,000 yr for the dispersal time scale of material around young, luminous stellar objects. 31 references.

The Chemical Composition and Evolution of Giant Molecular Cloud Cores: A Comparison of Observation and Theory

We present the results of an observational and theoretical study of the chemical composition and evolution of three giant cloud cores in Orion A, M17, and Cepheus A. This study is the culmination of a chemical survey of 32 transitions of 20 different molecules and isotopic variants in these cloud cores. Using these data, combined with observationally derived physical conditions, chemical abundances were calculated for several positions in each cloud. A global analysis of the molecular abundances shows that, although abundance differences exist, the chemical composition of giant cloud cores is remarkably homogeneous. This agreement suggests that the chemical evolution of the individual giant cloud cores is not unique. The molecular abundances of giant cloud cores are also systematically lower than those observed in the more quiescent dark cloud core TMC-1. A one-dimensional chemical model is presented that examines internal chemical structure induced by a radiation field enhanced by a factor of 103-105 above the normal interstellar radiation field. This model integrates the abundances of the various species as a function of depth, producing column densities that can be compared with observations. The one-dimensional model is unable to reproduce the abundances of many molecules for any single time. Two assumptions have been investigated to improve the agreement between theory and observations. These are adding clumps and raising the initial C/O ratio. We find that the inclusion of clumps in the chemical model can reproduce the abundance of C and C+. However, because of the greater weight placed on the photon-dominated region within smaller clumps, clumps have a detrimental effect on reproducing the abundances of other species. Models with a range of C/O ratios are also compared with the measured abundances. Good agreement between this model and the observations at two positions with disparate physical properties is found for early times (t ~ 105 yr) and for C/O increased to ~0.8. We suggest that one possible interpretation of these results is that the cores are dynamically evolving objects. Either giant cloud cores are intrinsically young objects or the dense material is effectively young by virtue of a complex interchange of material between the clumps and the interclump medium. We suggest that the CS/SO ratio can be used to probe the evolutionary state of and the initial C/O ratio in dense molecular clouds.

Dense cores in dark clouds. V - CO outflow

Sixteen dense cores in nearby clouds having low-mass stars have been surveyed in the J = 1-0 line of C-12O at 2.6 mm with the 14-m Five College Radio Astronomy Observatory telescope in Massachusetts. Results suggest that CO outflows occur in more than half of the low-mass star-core systems and that the typical outflow lasts most of the life of the star-core system. The outflows are shown to be characterized by frequent incidence, long duration, and significantly great momentum and kinetic energy. These results indicate that outflows traced by CO may be the main agent of dispersal of low-mass cores. Outflows would thus play an important role in determining the main-sequence mass of low-mass stars. 53 references.

EMBEDDED STAR-CLUSTERS ASSOCIATED WITH LUMINOUS IRAS POINT SOURCES

A 16 arcmin 2 region toward each of 20 bright IRAS sources in the second and third quadrants of the Galaxy have been imaged at J, H, and K bands and mapped in the CS(J=2-1) transition. The presence of one or two OB stars associated with each of these IRAS sources had been previously inferred from radio continuum observations, and the near-infrared images have revealed that a cluster of stars has formed in at least 19 of the 20 regions. These results strongly suggest that high-mass stars almost always form in clusters and not in isolation. The number of observed cluster members brighter than m H =15.5 mag vary between 15 and 91 stars

Molecular Clouds in the Milky Way and External Galaxies

The volume consists of up-to-date reviews and a selection of contributed papers on subjects including the structure and physical properties of molecular clouds, their role in the star formation process, their dust and chemical properties, molecular cloud surveys of the Milky Way, cloud evolution, problems in cloud mass determinations (a panel discussion and review), the CO properties of external galaxies, nuclei of galaxies as revealed by molecular observations, and galactic spiral structure as reflected by molecular cloud distributions. The abstracts of poster papers on these topics presented at the conference are also included. This book is both a valuable reference and a compendium of current knowledge in this field. It should be of special interest to all students and researchers who work on the physics of star formation, the interstellar medium, molecular clouds and galactic structure.