![]() ![]() These find that the transition from atomic hydrogen to molecular hydrogen occurs at a point closer to the cloud surface than the transition from ionized carbon, via neutral atomic carbon, to CO.ġ where W CO is the velocity-integrated intensity of the CO J = 1 → 0 emission line, averaged over the projected area of the GMC, and is the mean H 2 column density of the GMC, averaged over the same area. ![]() For these reasons, we would expect the CO/H 2 ratio to vary through a cloud, and this expectation is confirmed by detailed models of slab-like or spherical clouds (e.g. 1996), requiring a higher column density, and the low abundance of carbon relative to hydrogen in the ISM means that the required column density typically corresponds to a situation in which a large fraction of the available carbon is already locked up in the form of CO. The corresponding process for CO is less effective ( Lee et al. Moreover, H 2 can protect itself from UV radiation via self-shielding, which becomes effective for relatively low H 2 column densities ( Draine & Bertoldi 1996). However, the two molecules form rather differently: H 2 forms predominantly on the surface of dust grains ( Gould & Salpeter 1963), while CO forms almost exclusively in the gas phase, via any one of a number of chains of ion–neutral or neutral–neutral reactions. Both are readily dissociated by the absorption of ultraviolet (UV) photons with energies below the Lyman limit of atomic hydrogen, so in low density, low extinction gas such as the warm neutral component of the interstellar medium (ISM), the abundances of both molecules will be small. In order to use CO as a proxy for H 2, however, it is necessary to understand the relationship between the distributions of these two molecules. For this reason it is common to use emission from carbon monoxide (CO), the second-most abundant molecule in GMCs, as a proxy for the H 2. Moreover, the lowest lying rotational energy levels of H 2 are widely spaced, and so are very rarely excited in gas with the temperatures typical of GMCs, T∼ 10–20 K. Radiative transitions in H 2 are weak, owing to the molecule's lack of a permanent dipole moment. However, it is extremely difficult to directly observe this molecular hydrogen in situ. The main chemical constituent of any GMC is molecular hydrogen (H 2). Observed star formation takes place within giant molecular clouds (GMCs), so understanding how these clouds form and evolve is a key step towards understanding star formation on galactic scales. Molecular processes, ISM: clouds, ISM: molecules, galaxies: ISM 1 INTRODUCTION Our results further explain the narrow range of observed molecular cloud column densities as a threshold effect, without requiring the assumption of virial equilibrium. Our work predicts that CO-bright clouds in low metallicity systems should be systematically larger or denser than Milky Way clouds, or both. This explains the discrepancy observed in low metallicity systems between cloud masses derived from CO observations and other techniques such as infrared emission. At lower values of A V, we find that the ratio of H 2 column density to CO emissivity X CO∝ A −3.5 V. As a result, there is a sharp cut-off in CO abundance at mean visual extinctions A V≲ 3. On the other hand, CO forms quickly, within a dynamical time, but its abundance depends primarily on photodissociation, with only a weak secondary dependence on H 2 abundance. Photodissociation only becomes important at extinctions under a few tenths of a visual magnitude, in agreement with both observational and prior theoretical work. We find that the abundance of H 2 is primarily determined by the amount of time available for its formation, which is proportional to the product of the density and the metallicity, but insensitive to photodissociation. We study the relationship between H 2 and CO abundances using a fully dynamical model of magnetized turbulence coupled to a chemical network simplified to follow only the dominant pathways for H 2 and CO formation and destruction, and including photodissociation using a six-ray approximation. However, the reliability of this tracer has long been questioned in environments different from the Milky Way. The most usual tracer of molecular gas is line emission from CO. ![]()
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