Temperature and density dependent cooling function for H2 with updated H2/H collisional rates

Abstract

The energy transfer among the components in a gas determines its fate. Especially at low temperatures, inelastic collisions drive the cooling and the heating mechanisms. In the early Universe as well as in zero-or low-metallicity environments the major contribution comes from the collisions among atomic and molecular hydrogen, also in its deuterated version. The present work shows some updated calculations of the H 2 cooling function based on novel collisional data which explicitely take into account the reactive pathway at low temperatures. Deviations from previous calculations are discussed and a multivariate data analysis is performed to provide a fit depending on both the gas temperature and the density of the gas.

Collisional cooling of primordial and interstellar media by H2

“…the new H + rate coefficients are apparent at low T , in both atomic and molecular gas, and differences arising from the new H rate coefficients are evident at high T , in atomic gas. Coppola et al (2019) computed the dependence of the cooling function on the temperature (100 T 4000 K) and density (10 −4 nH 10 8 cm −3 ) of gas in which x(H + ) = 2 × 10 −4 and x(He) = 0.10. They neglected the excitation of H2 by H2, and hence the appropriate comparison with our results is for the case of an atomic gas.…”

Rate constants for the H+ + H2 reaction from 5 K to 3000 K with a statistical quantum method

“…Finally, it is useful to fit the H 2 cooling rate with simple functions of density and temperature, for use in hydrodynamical studies, as in the recent work on H 2 -H collisions. 32 This will be addressed in a future work where the contribution of all colliders (H, He, H 2 , H + , and electrons) will be considered.…”

Turbulent dissipation, CH$^+$ abundance, H$_2$ line luminosities, and polarization in the cold neutral medium

“…4b). Coppola et al (2019) (hereafter CLM19) provide a fitting function for H 2 cooling over the 10 2 K -4000 K temperature range. Figure 4 compares the CLM19 fitting function to our calculated H 2 cooling rates in predominantly atomic gas: the CLM19 fitting function tends to underestimate our computed cooling rates by factors of ∼ 2 for T 500 K.…”

“…Both CLM19 and the present study use H-H 2 collision cross sections from Lique (2015). The difference in the T 500 K cooling appears to be due to differences in adopted rates for collisional excitation by He: Coppola et al (2019) used quasi-classical trajectory cross sections from Celiberto et al (2017) whereas we use quantum-mechanical results from Le Bourlot et al (1999), which are believed to be more accurate at low energies.…”