Paul F. Goldsmith

Population Diagram Analysis of Molecular Line Emission

We develop the use of the population diagram method to analyze molecular emission in order to derive physical properties of interstellar clouds. We focus particular attention on how the optical depth affects the derived total column density and the temperature. We derive an optical depth correction factor that can be evaluated based on observations and that incorporates the effect of saturation on derived upper level populations. We present analytic results for linear molecules in local thermodynamic equilibrium (LTE). We investigate numerically how subthermal excitation influences the population diagram technique, studying how the determination of kinetic temperature is affected when the local density is insufficient to achieve LTE. We present results for HC3N and CH3OH, representative of linear and nonlinear molecules, respectively. In some cases, alternative interpretations to the standard optically thin and thermalized picture yield significantly different results for column density and kinetic temperature, and we discuss this behavior. The population diagram method can be a very powerful tool for determining physical conditions in dense clouds if proper recognition is given to effects of saturation and subthermal excitation. We argue that the population diagram technique is, in fact, superior to fitting intensities of different transitions directly, and we indicate how it can be effectively employed.

Molecular Depletion and Thermal Balance in Dark Cloud Cores

We analyze the effects of molecular depletion on the thermal balance of well-shielded, quiescent dark cloud cores. Recent observations of the significant depletion of molecules from the gas phase onto grain surfaces in dark clouds suggest the possibility that the gas-phase cooling in these regions is greatly reduced and consequently that gas kinetic temperatures might be increased. We reexamine cooling and heating processes in light of possible molecular depletion, including the effect of coupling between the gas and the grains. At densities ≤103.5 cm-3, the gas temperature can be significantly increased by the depletion of coolant species without significantly affecting the dust temperature because of the relatively weak gas-dust coupling. At higher densities, this coupling becomes sufficiently rapid to overwhelm the effect of the reduced gas-phase cooling, and depletion has little effect on the gas temperature while raising the dust temperature 1 K. The result is that depletion at densities ≥104.5 cm-3 can proceed without being evident as an enhanced gas temperature or without self-limiting due to an increase in the dust temperature increasing the desorption rate. This is consistent with observations of depletion in cold, dense regions of quiescent molecular clouds. It also suggests that depletion in moderate density regions can increase the thermal gas pressure, effectively enhancing the confinement of denser portions of molecular clouds and possibly accelerating the collapse of cloud cores.

Focal plane imaging systems for millimeter wavelengths

The authors discuss critical aspects of imaging system design and describe several different imaging systems employing focal plane array receivers operating in the 3-mm-2-mm wavelength range. Recent progress in millimeter-wavelength optics, antennas, receivers and other components permits greatly enhanced system performance in a wide range of applications. A radiometric camera for all-weather autonomous aircraft landing capability and a high sensitivity cryogenically cooled array for use in radio astronomical spectroscopy are presented. A near-focus system for identification of plastic materials concealed underneath clothing employs a two element lens, and has been demonstrated in active (transmitting) and passive (radiometric) modes. A dual-mode imaging system for plasma diagnostics utilizes both active and passive modes at its approximately=140-GHz operating frequency to study small-scale structure. The radiometric imaging systems employ between 15 and 256 Schottky barrier diode mixers, while the imaging receivers for the active systems include 64-element video detector arrays. >

Gas-phase chemistry in dense interstellar clouds including grain surface molecular depletion and desorption

We present time-dependent models of the chemical evolution of molecular clouds which include depletion of atoms and molecules onto grain surfaces and desorption, as well as gas-phase interactions. We have included three mechanisms to remove species from the grain mantles: thermal evaporation, cosmic-ray-induced heating, and photodesorption. A wide range of parameter space has been explored to examine the abundance of species present both on the grain mantles and in the gas phase as a function of both position in the cloud (visual extinction) and of evolutionary state (time). The dominant mechanism that removes molecules from the grain mantles is cosmic-ray desorption. At times greater than the depletion timescale, the abundances of some simple species agree with abundances observed in the cold dark cloud TMC-1. Even though cosmic-ray desorption preserves the gas-phase chemistry at late times, molecules do show significant depletions from the gas phase. Examination of the dependence of depletion as a function of density shows that when the density increases from 10(exp 3)/cc to 10(exp 5)/cc several species including HCO(+), HCN, and CN show gas-phase abundance reductions of over an order of magnitude. The CO: H2O ratio in the grain mantles for our standard model is on the order of 10:1, in reasonable agreement with observations of nonpolar CO ice features in rho Ophiuchus and Serpens. We have also examined the interdependence of CO depletion with the space density of molecular hydrogen and binding energy to the grain surface. We find that the observed depletion of CO in Taurus in inconsistent with CO bonding in an H2O rich mantle, in agreement with observations. We suggest that if interstellar grains consist of an outer layer of CO ice, then the binding energies for many species to the grain mantle may be lower than commonly used, and a significant portion of molecular material may be maintained in the gas phase.

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.

Implications of Submillimeter Wave Astronomy Satellite Observations for Interstellar Chemistry and Star Formation

A long-standing prediction of steady state gas-phase chemical theory is that H2O and O2 are important reservoirs of elemental oxygen and major coolants of the interstellar medium. Analysis of Submillimeter Wave Astronomy Satellite (SWAS) observations has set sensitive upper limits on the abundance of O2 and has provided H2O abundances toward a variety of star-forming regions. Based on these results, we show that gaseous H2O and O2 are not dominant carriers of elemental oxygen in molecular clouds. Instead, the available oxygen is presumably frozen on dust grains in the form of molecular ices, with a significant portion potentially remaining in atomic form, along with CO, in the gas phase. H2O and O2 are also not significant coolants for quiescent molecular gas. In the case of H2O, a number of known chemical processes can locally elevate its abundance in regions with enhanced temperatures, such as warm regions surrounding young stars or in hot shocked gas. Thus, water can be a locally important coolant. The new information provided by SWAS, when combined with recent results from the Infrared Space Observatory, also provides several hard observational constraints for theoretical models of the chemistry in molecular clouds, and we discuss various models that satisfy these conditions.

Carbon Monoxide and Dust Column Densities: The Dust-to-Gas Ratio and Structure of Three Giant Molecular Cloud Cores

We have observed emission in the three lowest rotational transitions of the optically thin species C18O and the dust continuum emission at three millimeter/submillimeter wavelengths. By employing the proper combination of the intensities of the three lowest rotational transitions of C18O, we can obtain the total molecular column density, with relatively little sensitivity to density and temperature variations along the line of sight. We use the line and continuum data to determine column densities of the dust and gas across three giant molecular cloud cores. We find that two of the three sources, M17 and Cepheus A, have the same gas column density-to-dust optical depth ratio, given by log [N(C18O)/τ(790 μm)] = 18.8. In the third source, the Orion molecular cloud, the gas-to-dust ratio is typically a factor of 3 lower than in the other two sources. The gas-to-dust ratio shows only a small (factor ≤ 3) variation across the region of M17 that we have mapped and a comparable reduction at the center of Cepheus A relative to the cloud edge. We have good evidence for the correlation of the continuum emission in different bands for the Orion molecular cloud and find the frequency dependence of the optical depth in the densest regions near the embedded sources to be given by τ ∝ ν1.9. For positions away from the embedded sources, there is a larger scatter in the data points, with a suggestion that the frequency dependence is steeper, such that τ ∝ ν2.4. This may be an indication of a change in the grain properties between less dense and very dense regions and is consistent with the results of grain growth. Using standard values for the fractional abundance of C18O relative to H2, the mean densities of the cloud cores are 3-5 × 104 cm-3 . These regions appear to be close to virial equilibrium. The dense gas [revealed by multiple transition studies of tracers such as CS and HC3N to have n(H2) 106 cm-3] has a volume filling factor of a few percent. Assuming a fractional abundance of C18O equal to 1.7 × 10-7, we find that the 790 μm dust optical depth to mass column density ratio for M17 and Cepheus A is 0.0062 cm2 g-1, while the average value for the Orion molecular cloud is a factor of 3 larger.

Water abundance in molecular cloud cores

We present Submillimeter Wave Astronomy Satellite (SWAS) observations of the 110 → 101 transition of ortho-H2O at 557 GHz toward 12 molecular cloud cores. The water emission was detected in NGC 7538, ρ Oph A, NGC 2024, CRL 2591, W3, W3OH, Mon R2, and W33 and was not detected in TMC-1, L134N, and B335. We also present a small map of the H2O emission in S140. Observations of the H218O line were obtained toward S140 and NGC 7538, but no emission was detected. The abundance of ortho-H2O relative to H2 in the giant molecular cloud cores was found to vary between 6 × 10-10 and 1 × 10-8. Five of the cloud cores in our sample have previous H2O detections; however, in all cases the emission is thought to arise from hot cores with small angular extents. The H2O abundance estimated for the hot core gas is at least 100 times larger than in the gas probed by SWAS. The most stringent upper limit on the ortho-H2O abundance in dark clouds is provided in TMC-1, where the 3 σ upper limit on the ortho-H2O fractional abundance is 7 × 10-8.

Interstellar deuterated ammonia: from NH3 to ND3

We use spectra and maps of NH 2 D, ND 2 H, and ND 3 , obtained with the CSO, IRAM 30 m and Arecibo telescopes, to study deuteration processes in dense cores. The data include the first detection of the hyperfine structure in ND 2 H. The emission of NH 2 D and ND 3 does not seem to peak at the positions of the embedded protostars, but instead at offset positions, where outflow interactions may occur. A constant ammonia fractionation ratio in star-forming regions is generally assumed to he consistent with an origin on dust grains. However, in the pre-stellar cores studied here, the fractionation varies significantly when going from NH 3 to ND 3 . We present a steady state model of the gas-phase chemistry for these sources, which includes passive depletion onto dust grains and multiply saturated deuterated species up to five deuterium atoms (e.g. CD + 5 ). The observed column density ratios of all four ammonia isotopologues are reproduced within a factor of 3 for a gas temperature of 10 K. We also predict that deuterium fractionation remains significant at temperatures up to about 20 K. ND and NHD, which have rotational transitions in the submillimeter domain are predicted to be abundant.

H I Narrow Self-Absorption in Dark Clouds: Correlations with Molecular Gas and Implications for Cloud Evolution and Star Formation

We present the results of a comparative study of H I narrow self-absorption (HINSA), OH, 13CO, and C18O in five dark clouds. We find that the HINSA generally follows the distribution of the emission of the carbon monoxide isotopologs and has a characteristic size close to that of 13CO. This confirms earlier work that determined that the HINSA is produced by cold H I that is well mixed with molecular gas in well-shielded regions. The OH and 13CO column densities are essentially uncorrelated for the sources other than L1544. Our observations indicate that the central number densities of H I are between 2 and 6 cm-3 and that the ratio of the hydrogen density to total proton density for these sources is (5-27) × 10-4. Using cloud temperatures and the density of atomic hydrogen, we set an upper limit to the cosmic-ray ionization rate of 10-16 s-1. We present a model for H I to H2 conversion in well-shielded regions that includes cosmic-ray destruction of H2 and formation of this species on grain surfaces. We include the effect of a distribution of grain sizes, and we find that for an MRN distribution, the rate of H2 formation is increased by a factor of 3.4 relative to that for a model with a single grain radius of 1700 A. Comparison of observed and modeled fractional H I abundances indicates ages for these clouds, defined as the time since the initiation of H → H2 conversion, to be 106.5-107 yr. Several effects may make this time a lower limit, but the low values of n that we have determined make it certain that the timescale for evolution from a possibly less dense atomic phase to an almost entirely molecular phase must be a minimum of several million years. This clearly sets a lower limit to the overall timescale for the process of star formation and the lifetime of molecular clouds.