Model Atmosphere Studies of Early Type Stars

Citation

Mihalas, Dimitri Manuel (1964) Model Atmosphere Studies of Early Type Stars. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/7TH1-H154. https://resolver.caltech.edu/CaltechETD:etd-10102002-094425

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. Model atmospheres have been constructed over a wide range of temperature and gravities. The condition of radiative equilibrium is enforced using a variation of the temperature correction scheme devised by M. Krook, thus extending the work of Gingerich to the realm of higher temperatures; flux constancy is attained to within [plus or minus]1% to a depth of [tau][subscript STD] = 10 or more, the standard wavelength being either [lamda]4000 or [lamda]5050. The source function is determined from the Milne-Eddington equation allowing for coherent scattering in the continuum. The properties of the atmospheres are described and given in detailed tabular form, and data of rather general use is presented. For each model we have computed H[beta] and H[alpha] profiles and equivalent widths, and for the hotter models, profiles and equivalent widths of [lamda]4686, [lamda]4200, [lamda]4542 [lamda]5412 of He II and [lamda]5876, [lamda]4713, [lamda]4438, and [lamda]4121 of He I. Interpolating formulae are devised to describe the temperature and gravity variations of the line strengths. A fit is carried out between theory and observations made with the 100" coude scanner of six O-type stars, yielding the effective temperature, gravity, and an estimate of the helium abundance for these stars. It is found that the computed hydrogen and ionized helium lines appear consistent with observation whereas the computed neutral helium lines seem to be incorrect for these spectral types. The results indicate that a typical O9 star has the following properties: [theta][subscript e] = 0.146, log g = 4.2, N(He)/N(H) ~ 0.15 or 0.2, and from an assumed mass-luminosity law we find: R/R[...] = 10, [...] = 60, and L/L [...] = 1.4 x 10 [superscript 5]. The helium abundance derived here is in substantial agreement with that found by the Kiel group but in disagreement with nebular studies. The masses and radii are probably quite doubtful because of the possible incorrectness of the assumed mass-luminosity law.

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