H2O in stellar atmospheres

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Abstract
We have performed a detailed ab initio computation of the dipole moment surface, the vibrational transition moments, and the spectral lines for the water molecule. A total of 412 vibrational eigenstates were identified below 30 000 cm-1, corresponding to ≈85 000 vibrational transitions. In principle there are many billions of allowed vibration-rotation transitions between these eigenstates. In our most complete test calculations we constructed a list of 3 billion () lines. At room temperature, the computed monochromatic absorption coefficient is in good overall agreement with the hitran data base, while at higher temperatures its value exceeds the hitran-based absorption coefficient by more than an order of magnitude, due to the lack of high excitation lines in hitran. The agreement with the hitemp version of hitran is considerably better than with the standard hitran. By comparing stellar model atmospheric structures and synthetic spectra based on our most extensive line list with results from calculations excluding the huge number of ultra-weak lines, we conclude that when the lines are well chosen, a few times 10 million lines are more than sufficient for all astrophysical purposes. We therefore offer to the community a line list of 100 million lines, easily accessible by anonymous ftp. Finally, we have compared results of synthetic stellar spectra based on this line list with observed ISO spectra and have found good agreement. In particular in the 2-4 μm region around the strong fundamental bands, the agreement with observations is considerably better than that obtained with other available water line lists.