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Register a new userThe Effects of Molecular Anions on the Chemistry of Dark Clouds
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Catherine Walsh
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We have investigated the role of molecular anion chemistry in pseudo-time dependent chemical models of dark clouds. With oxygen-rich elemental abundances, the addition of anions results in a slight improvement in the overall agreement between model results and observations of molecular abundances in TMC-1 (CP). More importantly, with the inclusion of anions, we see an enhanced production efficiency of unsaturated carbon-chain neutral molecules, especially in the longer members of the families CnH, CnH2, and HCnN. The use of carbon-rich elemental abundances in models of TMC-1 (CP) with anion chemistry worsens the agreement with observations obtained in the absence of anions.
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We show that the inter-cloud Larson scaling relation between mean volume density and size $rhopropto R^{-1}$, which in turn implies that mass $Mpropto R^2$, or that the column density $N$ is constant, is an artifact of the observational methods used. Specifically, setting the column density threshold near or above the peak of the column density probability distribution function Npdf ($Nsim 10^{21}$ cmalamenos 2) produces the Larson scaling as long as the Npdf decreases rapidly at higher column densities. We argue that the physical reasons behind local clouds to have this behavior are that (1) this peak column density is near the value required to shield CO from photodissociation in the solar neighborhood, and (2) gas at higher column densities is rare because it is susceptible to gravitational collapse into much smaller structures in specific small regions of the cloud. Similarly, we also use previous results to show that if instead a threshold is set for the volume density, the density will appear to be constant, implying thus that $M propto R^3$. Thus, the Larson scaling relation does not provide much information on the structure of molecular clouds, and does not imply either that clouds are in Virial equilibrium, or have a universal structure. We also show that the slope of the $M-R$ curve for a single cloud, which transitions from near-to-flat values for large radii to $alpha=2$ as a limiting case for small radii, depends on the properties of the Npdf.
We study Giant Molecular Cloud (GMC) environments surrounding 10 Infrared Dark Clouds (IRDCs), using $^{13}$CO(1-0) emission from the Galactic Ring Survey. We measure physical properties of these IRDCs/GMCs on a range of scales extending to radii, R, of 30 pc. By comparing different methods for defining cloud boundaries and for deriving mass surface densities and velocity dispersions, we settle on a preferred CE,$tau$,G method of Connected Extraction in position-velocity space plus Gaussian fitting to opacity-corrected line profiles for velocity dispersion and mass estimation. We examine how cloud definition affects measurements of the magnitude and direction of line-of-sight velocity gradients and velocity dispersions, including associated dependencies on size scale. CE,$tau$,G-defined GMCs show velocity dispersion versus size relations $sigmapropto{s}^{1/2}$, which are consistent with the large-scale gradients being caused by turbulence. However, IRDCs have velocity dispersions that are moderately enhanced above those predicted by this scaling relation. We examine the dynamical state of the clouds finding mean virial parameters $bar{alpha}_{rm{vir}}simeq 1.0$ for GMCs and 1.6 for IRDCs, broadly consistent with models of magnetized virialized pressure-confined polytropic clouds, but potentially indicating that IRDCs have more disturbed kinematics. CE,$tau$,G-defined clouds exhibit a tight correlation of $sigma/R^{1/2}proptoSigma^n$, with $nsimeq0.7$ for GMCs and 1.3 for IRDCs (c.f., a value of 0.5 expected for a population of virialized clouds). We conclude that while GMCs show evidence for virialization over a range of scales, IRDCs may be moderately super virial. Alternatively, IRDCs could be virialized but have systematically different $^{13}$CO gas phase abundances, i.e., due to freeze-out, affecting mass estimations.
We present a general parameter study, in which the abundance of interstellar argonium (ArH$^+$) is predicted using a model for the physics and chemistry of diffuse interstellar gas clouds. Results have been obtained as a function of UV radiation field, cosmic-ray ionization rate, and cloud extinction. No single set of cloud parameters provides an acceptable fit to the typical ArH$^+$, OH$^+$ and $rm H_2O^+$ abundances observed in diffuse clouds within the Galactic disk. Instead, the observed abundances suggest that ArH$^+$ resides primarily in a separate population of small clouds of total visual extinction of at most 0.02 mag per cloud, within which the column-averaged molecular fraction is in the range $10^{-5} - 10^{-2}$, while OH$^+$ and $rm H_2O^+$ reside primarily in somewhat larger clouds with a column-averaged molecular fraction $sim 0.2$. This analysis confirms our previous suggestion that the argonium molecular ion is a unique tracer of almost purely atomic gas.
The 1-50 GHz GBT PRIMOS data contains ~50 molecular absorption lines observed in diffuse and translucent clouds located in the Galactic Center, Bar, and spiral arms in the line-of-sight to Sgr B2(N). We measure the column densities and estimate abundances, relative to H2, of 11 molecules and additional isotopologues. We use absorption by optically thin transitions of c-C3H2 to estimate the N(H2), and argue that this method is preferable to more commonly used methods. We discuss the kinematic structure and abundance patterns of small molecules including the sulfur-bearing species CS, SO, CCS, H2CS, and HCS+; oxygen-bearing molecules OH, SiO, and H2CO; and simple hydrocarbon molecules c-C3H2, l-C3H, and l-C3H+. We discuss the implications of the observed chemistry for the structure of the gas and dust in the ISM. Highlighted results include the following. First, whereas gas in the disk has a molecular hydrogen fraction of 0.65, clouds on the outer edge of the Galactic Bar and in or near the Galactic Center have molecular fractions of 0.85 and >0.9, respectively. Second, we observe trends in isotope ratios with Galactocentric distance; while carbon and silicon show enhancement of the rare isotopes at low Galactocentric distances, sulfur exhibits no trend with Galactocentric distance; the ratio of c-C3H2/c-H13CCCH provides a good estimate of the 12C:13C ratio, whereas H2CO/H2^13CO exhibits fractionation. Third, we report the presence of l-C3H+ in diffuse clouds for the first time. Finally, we suggest that CS has an enhanced abundance within higher density clumps of material in the disk, and therefore may be diagnostic of cloud conditions. If this holds, the diffuse clouds in the Galactic disk contain multiple embedded hyperdensities in a clumpy structure, and the density profile is not a simple function of A_V.
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