Astronomy and Astrophysics – Astronomy
Scientific paper
Nov 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999afz....42..501g&link_type=abstract
Astrofizika, Tom 42, Vyp. 4, p. 501-518
Astronomy and Astrophysics
Astronomy
Line Formation: Radiative Transfer, Line Formation: Numerical Methods
Scientific paper
The author considers spectral line formation in moving media of spherical geometry. There are some special functions which define the line force and line source function when Sobolev approximation is used. Of major importance is the case of small velocity gradient (i.e. large dimensionless Sobolev length τ) and small ratio β of continuum to line opacity. There is no detailed analytical information on the asymptotic behavior of those functions. For the case of the Doppler profile the author elucidates the nontrivial structure of their asymptotic expansions for τ ≫ 1, β ≪ 1 and arbitrary βτ. He gives an algorithm to obtain all the coefficients of these expansions and presents explicit expressions for the first several of them. A comparison with the available numerical calculations is also made. The author also considers briefly the case of the power law opacity in the line wings (and more specifically - the case of the Lorentz wings of the Voigt profile).
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