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

By Di Li, Paul F. Goldsmith. 2002

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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 Å. 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.

Subfields: Astronomy and Astrophysics, Atmospheric Science

View Paper: https://www.semanticscholar.org/paper/1b896a75613e94ebe119ae1092bd2cebab12b55b

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Citations in the corpus

ENVIRONMENTAL DEPENDENCE OF THE KENNICUTT–SCHMIDT RELATION IN GALAXIES (2010, background): “…surface densities in nearby spirals from Wong & Blitz (2002b) and measurements of gas fractions along the lines of sight to individual stars for atomic (Goldsmith & Li 2005) and molecular gas in the Milky Way and Magellanic Clouds (Tumlinson et al. 2002; Gillmon et al. 2006; Wolfire et al. 2008).”
MODELING MOLECULAR HYDROGEN AND STAR FORMATION IN COSMOLOGICAL SIMULATIONS (2008, background): “While the measurements of atomic hydrogen fraction in the mostly molecular gas are not numerous (Goldsmith & Li 2005; Leroy et al. 2007), they provide a useful constraint for our model (see also the Appendix) in the regime where the gas is mostly molecular (fH I ≪ 1).”
Microscopic simulation of methanol and formaldehyde ice formation in cold dense cores (2009, methodology): “Goldsmith & Li (2005) showed observationally that the H-atom abundance is higher than 1 cm−3 and we therefore use a higher H abundance of 10 cm−3 for our high density case which will be discussed below.”
The big problems in star formation: The star formation rate, stellar clustering, and the initial mass function (2014, background): “Moreover, the results appear on their surface to contradict observations showing that clouds on such small scales do have close-to-equilibrium H i to H2 ratios (Goldsmith & Li 2005; Lee et al. 2012).”
A WIDE-FIELD HIGH-RESOLUTION H i MOSAIC OF MESSIER 31. I. OPAQUE ATOMIC GAS AND STAR FORMATION RATE DENSITY (2009, background): “Goldsmith & Li (2005)), while the majority have no apparent CO counterparts (Gibson et al. 2000).”
Theory of Star Formation (2007, background): “…of molecule formation in a turbulent (and therefore clumpy) medium, Glover & Mac Low (2007) find that H2 formation is indeed accelerated when compared with the non-turbulent case, although the atomic fractions they found are substantially greater than those observed by Goldsmith & Li (2005).”
Ultraviolet Survey of CO and H2 in Diffuse Molecular Clouds: The Reflection of Two Photochemistry Regimes in Abundance Relationships (2008, background): “This connection is also related to OH emission that has been detected from intermediate regions around denser molecular CO-emitting clouds (Wannier et al. 1993) and the lack of a correlation between N(13CO) and N(OH) in another sample of molecular clouds (Goldsmith & Li 2005).”
Clump morphology and evolution in MHD simulations of molecular cloud formation (2008, background): “This supports the notion that giant molecular clouds may consist of molecular clumps immersed in an atomic substrate (e.g., Goldsmith & Li 2005; Hennebelle & Inutsuka 2006).”
Experimental evidence for glycolaldehyde and ethylene glycol formation by surface hydrogenation of CO molecules under dense molecular cloud conditions (2015, methodology): “In these two runs, a high value for the hydrogen atom density n(H) = 10 cm−3 (Goldsmith & Li 2005) is used as described in Cuppen et al. (2009).”
KINEMATIC DISTANCES TO MOLECULAR CLOUDS IDENTIFIED IN THE GALACTIC RING SURVEY (2009, background): “H i SELF-ABSORPTION AND 21 cm CONTINUUM ABSORPTION AS A MEAN TO RESOLVE THE KDA The structure and composition of molecular clouds have been discussed by Goldsmith & Li (2005), Goldsmith et al. (2007), and other authors.”
Molecular Clouds in the Milky Way (2015, background): “Goldsmith & Li (2005) examined the H2 formation timescale but only considered cosmic-ray ionization of molecular hydrogen that occurs in the strongly self-shielded regime of molecular clouds.”
Cold Dark Clouds: The Initial Conditions for Star Formation (2007, background): “Studies of cold H I in dark clouds, on the other hand, suggest longer timescales > 3×106 yrs (Goldsmith & Li 2005).”