The H₂–H infrared absorption bands at temperatures from 1000 K to 2500 K

Abstract

Abstract. H 2 -H collision-induced absorption spectra are computed for temperatures from 1000 K to 2500 K and frequencies from 100 cm −1 to 10 000 cm −1 . The calculations are quantum mechanical and the isotropic potential approximation has been applied. The computed absorption in the fundamental band agrees roughly with the one determined by Patch (1974). However, the absorption in the translational band, which has not been obtained before, is significantly stronger than in the fundamental band.Key words. molecular processes -infrared: stars -line: profiles -radiation mechanisms: thermal -scatteringstars: atmospheres Colliding atoms or molecules acquire a transient electric dipole moment even if the non-interacting (i.e., well separated) individual atoms or molecules are nonpolar. As a consequence, at high enough gas densities, collision-induced infrared absorption is observed, even if at lower densities the corresponding gases and gas mixtures are infrared-inactive (Welsh 1972;Frommhold 1993). Collision-induced absorption of compressed, nonpolar gases and gas mixtures, such as hydrogen, were discovered by Welsh et al. (1949) and their significance for astrophysics was almost instantly recognized (Welsh 1972;Herzberg 1952a;Herzberg 1952b;Trafton 1964). Since then, extensive investigations of collision-induced absorption in planetary atmospheres and, more recently, in the atmospheres of certain cool stars (Borysow et al. 1997;Burrows et al. 1997;Jørgensen et al. 2000) have been carried out.Whereas for systems such as H 2 -H 2 and H 2 -He a number of excellent laboratory measurements of the collision-induced spectra exist that permit stringent tests of the quantum chemical computations (Frommhold 1993), no such measurements are known for the H 2 -H system and its spectra are obtained from theory. This is similar to the H-He pair, which also has been investigated theoretically (Gustafsson & Frommhold 2001). The absorption due to H 2 -H pairs could be important for the dynamics of the atmospheres of stars, when temperatures are such that atomic and molecular hydrogen coexist. Opacities of zero-metallicity gas were thoroughly investigated by Lenzuni et al. (1991). Collision-induced absorption is in general most important at the lowest of the temperatures considered there, namely 3000 K. At higher temperatures there are other strong Send offprint requests to: M. Gustafsson, e-mail: magnus@physics.utexas.edu continuum absorbers, such as ionic hydrogen. It is at temperatures below 3000 K and at relatively low pressures (for significant atomic hydrogen abundance) that we anticipate the H 2 -H collision-induced absorption to be important.We have employed quantum calculations (Frommhold 1993) of the absorption spectra of H 2 -H collisional pairs at temperatures from 1000 K to 2500 K. This is a range of temperatures where atomic hydrogen is abundant under pressures which could be of interest. For the modeling of stellar atmospheres, the maximum of the Planck distribution of black body radiation should be considered, or rat...

NEW H2COLLISION-INDUCED ABSORPTION AND NH3OPACITY AND THE SPECTRA OF THE COOLEST BROWN DWARFS

“…Collisional broadening of the individual lines is comparable to the line spacing and thus H 2 CIA is effectively a continuum source of opacity. Borysow and her collaborators calculated the H 2 -H 2 and H 2 -He CIA coefficients in the temperature regime of interest for brown dwarf and white dwarf atmospheres Borysow et al 1989;Jorgensen et al 2000;Borysow et al 2001;Borysow 2002;Gustafsson & Frommhold 2003) 1 , and those are universally used by brown dwarf modelers (Freedman et al 2008;Sharp & Burrows 2007;Allard et al 2001). The H 2 -H 2 and H 2 -He CIA coefficients are shown in Figures 1 and 2, respectively.…”

Atmospheric circulation of tidally locked exoplanets: II. Dual-band radiative transfer and convective adjustment

“…A detailed modelling of the opacity sources present in hot Jovian atmospheres (e.g. Gustafsson & Frommhold 2003) will inform us about the actual value of ε, but for now we are content with being able to account for the effect of collision‐induced absorption via a single parameter. Readers interested in broader generalizations of Guillot (2010) are referred to Heng et al (2011b).…”

“…Note that in deriving the results in this sub-section, we have retained the values of the first and second (longwave) Eddington coefficients (1/3 and 1/2, respectively) as adopted by Guillot (2010), as well as the closure relation that the ratio of the second to the zeroth moments of the shortwave intensity is 1/3. Detailed modelling of the opacity sources present in hot Jovian atmospheres (e.g., Gustafsson & Frommhold 2003) will inform us about the actual value of ǫ, but for now we are content with being able to account for the effect of collision-induced absorption via a single parameter. Readers interested in broader generalizations of Guillot (2010) are referred to Heng et al (2011b).…”

Lyman α wing absorption in cool white dwarf stars

“…The Lyman α wing opacity analysed in the preceding sections represents a partial contribution to the total gas opacity in atmospheres of cool DA WDs. For pure hydrogen models, the opacity sources in the numerical code also include bound–free (H, H − , H + 2 ) and free–free (H, H 2 , H 3 , H − , H − 2 , H + 2 ) transitions, electronic and Rayleigh (H, H 2 ) scattering, CIAs of H–H (Doyle 1968), H–H 2 (Gustafsson & Frommhold 2003) and H 2 –H 2 (Borysow, Jorgensen & Fu 2001), and the most significant H line series. Details are described at length in Rohrmann et al (2002).…”