Nonlinear Radial Pulsation Models for Extreme Helium Stars: Application to V65A HER (BD+ 13(deg)3224)

Details Nonlinear Radial Pulsation Models for Extreme Helium Stars: Application to V65A HER (BD+ 13(deg)3224) Nonlinear Radial Pulsation Models for Extreme Helium Stars: Application to V65A HER (BD+ 13(deg)3224)

Jan 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995oxun.rept.....f&link_type=abstract

Technical Report, OUAST/95/33 Dept. of Physics

Computer Science

Numerical Analysis

Nonlinearity, Stellar Models, Helium, Stars, Numerical Analysis, B Stars, Dynamic Models, Astronomical Models, Gravitation, Hot Stars, Luminescence, Luminosity, Mass Ratios, Opacity, Radial Velocity, Variable Stars

Scientific paper

Opacity Project (OP) and OPAL opacities were used to calculate nonlinear hydro-dynamic models of 0.7 solar mass radially pulsating extreme helium stars, having mean (averaged over a pulsation cycle) effective temperatures and luminosities in the ranges 104K less than or equal to (Teff) less than or equal to 5 x 104K and 1282 less than or equal to (L)/(solar luminosity) less than or equal to 8091 respectively. Separated helium and metal ionising layers cause local maxima (or bumps) in the depth-dependent Rosseland mean opacity, which were found to be responsible for two distinct regions of pulsation instability; they will be referred to as the Helium Instability Region (HEIR) and Z-Bump Instability Region (ZBIR). At (L) less than 3000 solar luminosity the cooler HEIR and hotter ZBIR are separated by a region of stability where radial pulsations are either not excited or characterised by a very small radial displacement amplitude ((delta)R/R approx. 10-3). At (L) = 1282 solar luminosity for example, the stability region is bounded by (Teffred approx. equals 12000 K and (Teffblue 17500 K; it becomes narrower with increasing luminosity, so that both instability regions merge at (L) approx. equals 3000 solar luminosity where (Teffred approx. equals (Teffblue approx. equals 15000 K. All ZBIR models represent fundamental mode radial pulsators; for these cases an approximate formula was derived to express the pulsation constant (Q) in terms of (L) and the mean mass-radius ratio. BD +13 deg 3224 (VG52 Her) remains the only hot extreme helium star known to be a radial pulsator; published photometric and spectroscopic data were used to test nonlinear radial pulsation models and, indirectly, new opacities upon which they were based. The best, and remarkably good, agreement between observed and theoretical (radial velocity and luminosity) curves was obtained with M = 0.72 solar mass, (Teff = 23500K and (L) = 1062 solar luminosity when OP opacities (with mixture X = 0.0015, Y = 0.98287 and Z = 0.01563) were adopted. As a consequence, BD +13(deg)3224 was identified as a ZBIR fundamental mode radial pulsator having a mean surface gravity of log g = 3.7.

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