We have computed the absorption strength at frequencies corresponding to a number of collision-induced transitions in the H2 -H2 complex, which are of relevance at the physical conditions that prevail in stellar atmospheres and for which quantum mechanical results did not exist previously. We have also updated the existing data for H2 -H2 and H2 -He with estimates of the intensity in the far spectral wings, and with data for additional overtones and hot bands, so that the whole set of data is more complete for stellar atmosphere computations.
We have applied the new data set in computations of a grid of cool, low-metallicity stellar atmospheres, in order to investigate the impact of CIA processes on such models. We found that CIA is the dominant opacity source in the coolest, low-metallicity main sequence stars. The effect on the flux distribution is pronounced for dwarfs of low metallicity and 4000 K, for all dwarfs with Z 0.1 as long as is low enough ( 3000 K), and for dwarfs and sub-giants if both the metallicity and the effective temperature are low enough.
This group of stars includes cool white dwarfs, metal-poor brown dwarfs, M dwarfs in the halo and in globular clusters, sub-giants in the most metal-poor globulars and in metal-poor external galaxies such as the SMC and the Fornax dSph, and metal-poor carbon dwarfs. For analysis of the structure, colours and spectra of such stars inclusion of CIA is essential.
For several of the collision-induced transitions much more accurate modelling than we have presented here is, however, still required, and there exists no laboratory work for comparison with the the quantum mechanical results at elevated temperatures. For the H2 -H CIA transitions, which might likewise be of importance for stellar atmospheres, neither computations nor laboratory measurements exist. Although our results represent the most detailed study of the CIA phenomena in stars, we point out that they are still very limited and preliminary compared to what is known about atoms and dipole transitions in single polar molecules and their role for the radiative transfer in stars.
© European Southern Observatory (ESO) 1997
Online publication: May 26, 1998