Comparison of Sobolev Approximation with the Exact ALI in P Cygni type Profile
Abstract
Sobolev approximation can be adopted to a macroscopic supersonic motion comparatively larger than a random(thermal) one. It has recently been applied not only to the winds of hot early type stars, but also to envelopes of late type giants and/or supergiants. However, since the ratio of wind velocity to stochastic one is comparatively small in the winds of these stars, the condition for applying the Sobolev approximation is not fulfilled any more. Therefore, the validity of Sobolev approximation must be checked. We have calculated exact P Cygni profiles with various velocity ratios, Vinf/Vsto, using the accelerated lambda iteration method, comparing with those obtained by the Sobolev approximation. While the velocity ratio decrease, serious deviations have been occured over the whole line profile. When the gradual increase in the velocity structure happens near the surface of star, the amount of deviations become more serious even at the high velocity ratios. The investigations have been applied to observed UV line profile of CIV in the Copernicus spectrums of Zeta Puppis and NV of Tau Sco. In case of Tau Sco which has an expanding envelope with the gradual velocity increase in the inner region, the Sobolev approximation has given the serious deviations in the line profiles.
 Publication:

Journal of Korean Astronomical Society
 Pub Date:
 April 1997
 Bibcode:
 1997JKAS...30...13C
 Keywords:

 P CYGNI PROFILE;
 RADIATIVE TRANSFER;
 SOBOLEV APPROXIMATION;
 ACCELERATED LAMBDA ITERATION