Time-dependent H₂ formation and protonation in diffuse clouds

Abstract

Aims. To demonstrate the time-approach to equilibrium of H 2 -formation and protonation in models of diffuse or H I interstellar gas clouds previously published by the author. Methods. The microscopic equations of H 2 -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 H 2 formation timescale in an H I cloud with N(H) ≈ 4 × 10 20 cm −2 is 1−3 × 10 7 yr, nearly independent of the assumed density or H 2 formation rate on grains, etc. Attempts to speed up the evolution of the H 2 -fraction would require densities well beyond the range usually considered typical of diffuse gas. The calculations suggest that, under benign, quiescent conditions, H 2 in the diffuse ISM formation of H 2 is favored in larger regions having moderate density, consistent with the rather high mean kinetic temperatures measured in H 2 , 70−80 K. Formation of H 3 + is essentially complete when H 2 -formation equilibrates but the final abundance of H 3 + 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 H 3 + is much more weakly dependent on the cosmic-ray ionization rate when n(H 2 )/n(H) < ∼ 0.05. In general, high abundances of H 3 + 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 H 2 fraction and somewhat higher fractional ionization, suggesting that atypically high OH/H 2 abundance ratios might be found optically in diffuse clouds having modest extinction.

Excited OH+, H2O+, and H3O+in NGC 4418 and Arp 220

“…The distribution is roughly described with two values of T rot , ∼180 K up to the 5 + 5 level, and ∼550 K for higher-lying levels. This is strikingly similar to that found by Lis et al (2012) towards Sgr B2. The column of the 8 + 8 level (E l ≈ 660 K) appears to be higher than expected, suggesting some contamination by HCN at 169.4 μm (Fig.…”

“…This resembles the double distribution found by Lis et al (2012) toward Sgr B2. The triangles in Fig.…”

“…They therefore will be "thermally" populated at a rotational temperature, T rot , which reflects either T gas or, if the formation-destruction rates are faster than the rate for collisional relaxation, the formation process characteristics (e.g. Bruderer et al 2010a;Lis et al 2012, see also Appendix B). Spectroscopic data for H 3 O + were taken from the JPL catalogue (Pickett et al 1998).…”

“…Then, the molecule will "freeze-out" in that metastable level (ignoring here radiative excitation for simplicity) until destruction through recombination or until a collisional thermalization event takes place. If the destruction rate dominates over collisional relaxation, the observed population distribution in the metastable levels (i.e., T rot ) will be governed by the formation process rather than by T gas (Lis et al 2012). Writing the rate of H 3 O + formation per unit volume as Γ H 3 O + = H 3 O + n H ζ, where H 3 O + is the efficiency with which ionizations are transfered to the production of H 3 O + (see Neufeld et al 2010, for OH + ), the condition for formation pumping to dominate the observed excitation over collisions can be written as 6…”

“…The more stable H 3 O + was first detected in the Sgr B2 molecular cloud complex near the Galactic center (Wootten et al 1991), and further mapped with APEX in the same region (van der Tak et al 2006) through the non-metastable 3 + 2 −2 − 2 submillimeter line at 365 GHz. Recently, high-lying lines of H 3 O + from metastable levels have been detected in Sgr B2 by Lis et al (2012), who suggested that the excitation could be due to formation pumping in X-ray irradiated gas.…”