Formation, fractionation and excitation of carbon monoxide in diffuse clouds

Aims: Our aims are threefold: a) To compare the $uv$ and mm-wave results; b) to interpret 13CO and 12CO abundances in terms of the physical processes which separately and jointly determine them; c) to interpret observed J=1-0 rotational excitation and line brightness in terms of ambient gas properties. Methods: A simple phenomenological model of CO formation as the immediate descendant of quiescently-recombining HCO+ is used to study the accumulation, fractionation and rotational excitation of CO in more explicit and detailed models of H2-bearing diffuse/H I clouds Results: The variation of N(CO) with N(H2) is explained by quiescent recombination of a steady fraction n(HCO+)/n(H2) = 2 x 10^{-9}. Observed N(12CO))/N(13CO) ratios generally do not require a special chemistry but result from competing processes and do not provide much insight into the local gas properties, especially the temperature. J=1-0 CO line brightnesses directly represent N(CO), not N(H2), so the CO-H2 conversion factor varies widely; it attains typical values at N(12CO) \la 10^{16}cm^{-2}. Models of CO rotational excitation account for the line brightnesses and CO-H2 conversion factors but readily reproduce the observed excitation temperatures and optical depths of the rotational transitions only if excitation by H-atoms is weak -- as seems to be the case for the very most recent calculations of these excitation rates.Comment: 11 pages, 6 figures, A&A 2007 or 2008 (in press

Rotational Excitation of polar molecules by H2 and electrons in diffuse clouds

[This is truncated to suit the whims of the archivers ...] Parameter studies in LVG models are used to show how the low-lying rotational transitions of common polar molecules HCO+, HCN and CS vary with number density, column density and electron fraction; with molecular properties such as the charge state and permanent dipole moment; and with observational details such as the transition that is observed. Physically-based models are used to check the parameter studies and provide a basis for relating the few extant observations. The parameter studies of LVG radiative transfer models show that lines of polar molecules are uniformly brighter for ions, for lower J-values and for higher dipole moments. Excitation by electrons is more important for J=1-0 lines and contributes rather less to the brightness of CS J=2-1 lines. If abundances are like those seen in absorption, the HCO+ J=1-0 line will be the brightest line after CO, followed by HCN (1-0) and CS (2-1). Because of the very weak rotational excitation in diffuse clouds, emission brightnesses and molecular column densities retain a nearly-linear proportionality under fixed physical conditions, even when transitions are quite optically thick; this implies that changes in relative intensities among different species can be used to infer changes in their relative abundances.Comment: To appear in A&

Imaging galactic diffuse gas: Bright, turbulent CO surrounding the line of sight to NRAO150

To understand the environment and extended structure of the host galactic gas whose molecular absorption line chemistry, we previously observed along the microscopic line of sight to the blazar/radiocontinuum source NRAO150 (aka B0355+508), we used the IRAM 30m Telescope and Plateau de Bure Interferometer to make two series of images of the host gas: i) 22.5 arcsec resolution single-dish maps of 12CO J=1-0 and 2-1 emission over a 220 arcsec by 220 arcsec field; ii) a hybrid (interferometer+singledish) aperture synthesis mosaic of 12CO J=1-0 emission at 5.8 arcsec resolution over a 90 arcsec-diameter region. CO components that are observed in absorption at a moderate optical depth (0.5) and are undetected in emission at 1 arcmin resolution toward NRAO 150 remain undetected at 6 arcsec resolution. This implies that they are not a previously-hidden large-scale molecular component revealed in absorption, but they do highlight the robustness of the chemistry into regions where the density and column density are too low to produce much rotational excitation, even in CO. Bright CO lines around NRAO150 most probably reflect the variation of a chemical process, i.e. the C+-CO conversion. However, the ultimate cause of the variations of this chemical process in such a limited field of view remains uncertain.Comment: 18 pages, 22 PostScript files giving 14 figures. Accepted for publication in Astronomy & Astrophysics in the letter section. Uses aa LaTeX macro

Mm-wave HCO+, HCN and CO absorption toward NGC1052

We used the Plateau de Bure Interferometer to observe $\lambda$3mm J=1-0 absorption lines of \hcop, HCN and CO toward the core of the nearby elliptical, megamaser-host galaxy NGC1052. The lines are relatively weak, with peak optical depths 0.03 for \hcop and HCN and 0.1 for CO. Nonetheless the inferred column density of molecular gas 2N(\HH) \simeq 5\times10^{21}~\pcc is consistent with the degree of reddening inferred toward the nucleus from observations of the Balmer series of hydrogen. Mm-wave absorption line profiles are somewhat broader than those of H I and OH, perhaps because lower free-free opacity at mm-wavelengths exposes higher-velocity material nearer the nucleus. Overall, the OH/\hcop ratio in NGC1052 is as expected from the strong relationship established in local diffuse clouds but the optical depth ratio varies strongly over the line profiles. Similar variations are also seen toward Cen A, which has very different line ratios among H I, OH and \hcop for very nearly the same amount of OH absorption.Comment: Accepted, A&A August 26, 200

Time-dependent H2 formation and protonation

Methods: The microscopic equations of H2-formation and protonation are integrated numerically over time in such a manner that the overall structures evolve self-consistently under benign conditions. Results: The equilibrium H2 formation timescale in an H I cloud with N(H) ~ 4x10^{20}/cm^2 is 1-3 x 10^7 yr, nearly independent of the assumed density or H2 formation rate constant on grains, etc. Attempts to speed up the evolution of the H2-fraction would require densities well beyond the range usually considered typical of diffuse gas. The calculations suggest that, under benign, quiescent conditions, formation of H2 is favored in larger regions having moderate density, consistent with the rather high mean kinetic temperatures measured in H2, 70-80 K. Formation of H3+ is essentially complete when H2-formation equilibrates but the final abundance of H3+ appears more nearly at the very last instant. Chemistry in a weakly-molecular gas has particular properties so that the abundance patterns change appreciably as gas becomes more fully molecular, either in model sequences or with time in a single model. One manifestation of this is that the predicted abundance of H3+ is much more weakly dependent on the cosmic-ray ionization rate when n(H2)/n(H) < 0.05. In general, high abundances of H3+ do not enhance the abundances of other species (e.g. HCO+) but late-time OH formation proceeds most vigourously in more diffuse regions having modest density, extinction and H2 fraction and somewhat higher fractional ionization, suggesting that atypically high OH/H2 abundance ratios might be found optically in diffuse clouds having modest extinction

Imaging diffuse clouds: Bright and dark gas mapped in CO

We wish to relate the degree scale structure of galactic diffuse clouds to sub-arcsecond atomic and molecular absorption spectra obtained against extragalactic continuum background sources. To do this, we used the ARO 12m telescope to map J=1-0 CO emission at 1' resolution over 30' fields around the positions of 11 background sources occulted by 20 molecular absorption line components, of which 11 had CO emission counterparts. We compare maps of CO emission to sub-arcsec atomic and molecular absorption spectra and to the large-scale distribution of interstellar reddening. The main results are: 1) Typical covering factors of individual features at the 1 K.km/s level were 20%. 2) CO-H2 conversion factors as much as 4-5 times below the mean value N(H2)/Wco = 2e20 H2 cm^-2 /(K.km/s) are required to explain the luminosity of CO emission at/above the level of 1 K.km/s. Small conversion factors and sharp variability of the conversion factor on arcminute scales are due primarily to CO chemistry and need not represent unresolved variations in reddening or total column density. Hence, like FERMI and PLANCK we see some gas that is dark in CO and other gas in which CO is overluminous per H2. A standard CO-H2 conversion factor applies overall owing to balance between the luminosities per H2 and surface covering factors of bright and dark CO., but with wide variations.Comment: 23 pages, 22 PostScript figures. Accepted for publication in Astronomy \& Astrophysics. Uses aa LaTeX macro

Imaging galactic diffuse clouds: CO emission, reddening and turbulent flow in the gas around Zeta Oph

Methods: 12CO emission is imaged in position and position-velocity space analyzed statistically, and then compared with maps of total reddening and with models of the C+ - CO transition in H2-bearing diffuse clouds. Results: Around Zeta Oph, 12CO emission appears in two distinct intervals of reddening centered near EBV = 0.4 and 0.65 mag, of which < 0.2 mag is background material. Within either interval, the integrated 12CO intensity varies up to 6-12 K-km/s compared to 1.5 K-km/s toward Zeta Oph. Nearly 80% of the individual profiles have velocity dispersions < 0.6 km/s, which are subsonic at the kinetic temperature derived from H2 toward Zeta Oph, 55 K. Partly as a result, 12CO emission exposes the internal, turbulent, supersonic (1-3 km/s) gas flows with especial clarity in the cores of strong lines. The flows are manifested as resolved velocity gradients in narrow, subsonically-broadened line cores. Conclusions: The scatter between N(CO) and EBV in global, CO absorption line surveys toward bright stars is present in the gas seen around Zeta Oph, reflecting the extreme sensitivity of N(12CO) to ambient conditions. The two-component nature of the optical absorption toward Zeta Oph is coincidental and the star is occulted by a single body of gas with a complex internal structure, not by two distinct clouds. The very bright 12CO lines in diffuse gas arise at N(H2) ~ 10^21/cm^2 in regions of modest density n(H) ~ 200-500/cc and somewhat more complete C+-CO conversion. Given the variety of structure in the foreground gas, it is apparent that only large surveys of absorption sightlines can hope to capture the intrinsic behavior of diffuse gas.Comment: 2009 A&A, in pres

Comparative chemistry of diffuse clouds III: sulfur-bearing molecules

Using data from IRAM's Plateau de Bure Interferometer and 30 m Telescope, we discuss the mm-wave absorption lines of CS, SO, H2S and HCS+ which arise in diffuse clouds occulting several extragalactic continuum sources. Typical relative abundances are X(CS)/X(HCO+) ~ 2, X(CS)/X(SO) ~ 2, X(CS)/X(H2S) ~ 6 and X(CS)/X(HCS+) ~ 13.Comment: Accepted by A&A 2002-Jan-1