Hydrodynamical Models of Radially Pulsating Hot Extreme Helium Stars

Abstract

AbstractThe blue edge of the instability region of radially pulsating helium stars is located on the H-R diagram nearly vertically at luminosities less than 104L⊙ and corresponds to Teff = 7400K and 1.1 · 104K for stars with mass of 1M⊙ and 0.7M⊙, respectively. The pulsation instability of the stars located along the vertical part of the instability edge is characterised by the increasing order of the pulsation mode with decreasing luminosity. For example, for stars with M = 0.7M⊙ and Tteff = 104K the principal pulsation mode is nearly second overtone for Mbol = –5 mag and is nearly fourth overtone for Mbol = –4 mag. For stars brighter than Mbol = –5 mag the blue edge turns blueward so that the models with Teff as high as 3 · 104K are pulsationally unstable. The sequences of the hydrodynamic models characterized by constant luminosity Mbol < – 5 mag) reveal the decrease of the light amplitude with increasing Teff, whereas the amplitude of the radial velocities of the outer layers is almost independent of Teff. For example, for stars with Teff > 2·104K the light amplitude is less than 0.01 mag, whereas the radial velocity amplitude is in the range from 20 to 50 km/s. The pulsation instability of these stars is driven mainly due to the γ-mechanism. Fourier analysis of the hydrodynamic solution shows that the pulsation motions of the hot helium stars can be represented as a superposition of the running waves (in contrast to Classical cepheids where the pulsation motions are described in the terms of superposition of standing waves). The pulsation constant gradually decreases with increasing Teff, down to Q ≈ 0.012 day at Teff ≈ 3 · 104K.