The Formation of Resonance Lines in Extended and Expanding Atmospheres
Abstract
Approximate expressions are derived for the profiles of lines formed by coherent scattering in extended and expanding atmospheres with spherical symmetry. For an extended, static atmosphere, the derived expression gives a greater residual intensity at every point in the line than does the corresponding result for a plane-parallel atmosphere. For an expanding atmosphere, an expression for the resulting P Cygni profile is derived on the assumptions that the expansion velocity v(r) is a strictly increasing function of radius r, and that v is large compared with the thermal velocity of the scattering ions but small compared with the velocity of light. With these assumptions, the calculation of the residual intensity reduces to a quadrature through the expanding atmosphere. This theory is applied to simple models for the expanding atmospheres of hot stars to obtain profiles for ultraviolet resonance lines. It is found that the rates of mass loss predicted by Lucy and Solomon give profiles in qualitative agreement with those observed in the far-ultraviolet spectra of early-type supergiants and of the Of star Puppis. The theory is also applied to a model for the quasistellar object PHL 5200 to obtain a profile for the N 1548 resonance line of C iv. A comparison of this profile with the spectra published by Lynds suggests that the expanding envelope of PHL 5200 is spherically symmetric.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- January 1971
- DOI:
- 10.1086/150748
- Bibcode:
- 1971ApJ...163...95L