The cooling of astrophysical media by H2

Abstract

We have computed the rate of radiative energy loss from a gas containing H 2 which is collisionally excited by H, He and H 2 . For this purpose, we have used the results of recent quantum mechanical calculations of the cross-sections for rovibrational transitions between all the energy levels up to approximately 20 000 K above the ground state. The temperature dependence of the rate coef®cients for collisional de-excitation is found to be well represented by a simple functional form. The cooling function has been evaluated in steady state on a grid covering a wide range of values of the gas density and temperature, the atomic to molecular hydrogen density ratio, and the ortho-to para-H 2 ratio. A fortran program is provided for the purpose of rapid numerical interpolation to any desired set of values of these parameters. The properties of the cooling function are discussed, as are the time-scales required for the attainment of steady state.

Rotational energy transfer in H2+H2

“…The model has been applied in few previous works, see for example [1,4]. For the considered range of kinetic energies of astrophysical interest the rotor model is considered to be adequate [14].…”

“…The investigation of elastic and inelastic collisions between molecules and between molecules and atoms can provide valuable information about interactions, chemical properties and energy transfer dynamics [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. The hydrogen molecule is the simplest and most abundant molecule in the universe's molecular clouds and plays an important role in many areas of astrophysics.…”

Excitation of H2in photodissociation regions as seen bySpitzer

“…For the collisional excitation and de-excitation of H 2 , we adopt here the quantum mechanical calculations given by Le Bourlot et al (1999). It must be underlined that the cross sections for collisional excitation and deexcitation of H 2 are still not completly known.…”

“…For comparison of the predicted and observed line intensities, in the model the PDRs have been tilted with respect to the line of sight with cos(θ) = 0.15. Gas abundance (Savage & Sembach 1996) N/H 7 .5 × 10 −5 Gas abundance (Meyer et al 1997) The first pure rotational lines (e.g., 0-0 S(0) and S(1)) essentially result from collisional excitation, since their upper states (v = 0, J = 2 and 3) are relatively low lying and their critical densities are low even at low temperatures (n crit < 10 4 cm −3 for T ≥ 100 K, Le Bourlot et al 1999). These line intensities depend mainly on the gas temperature at the photodissociation front.…”

Commission 14: Atomic and Molecular Data: (Donnees Atomiques et Moleculaires)

“…Rate coefficients have been computed for rovibrational excitation of H 2 by H, He, and H 2 impact (Le Bourlot et al 1999;Balakrishnan et al 1997Balakrishnan et al , 1999Forrey et al 1997); of HD by H (Roueff k Flower 1999), He (Roueff k Zeippen 1999), and H 2 impact (Flower 1999); and of CO by H impact (Green et al 1996). Also of interest are recent works describing ionization of astrophysically important molecules: He* + H 2 0 , H 2 , N 2 by Ishida (1996), Vojtik (1996), and Ishida & Horime 1996), respectively, H + N 2 (Quintana k Pollack 1996), and He+ + H 2 (Hsu et al 1996), Of significant interest to circumstellar, interstellar, primordial cloud, planetary atmosphere and other environments, in which molecules can survive, is dissociation.…”