Elusive Helium Stars in the Gap between Subdwarfs and Wolf–Rayet Stars II. Nonlinear Pulsations of Stripped Helium Stars

Abstract

It is shown for the first time that the stripped helium stars with masses (2–7) \(M_{\odot}\) which are formed in close binary systems in the so-called case B of mass-exchange and retained low-mass hydrogen-helium envelopes, experience nonlinear radial pulsations. Pulsations are excited by the \(\kappa\)-mechanism due to helium ionization. The region of pulsational instability extends over the Hertzsprung–Russell diagram from the red giant branch to the region of effective temperatures \(4.5\lesssim\log T_{\textrm{eff}}\lesssim 4.7\). Stellar luminosity variations must be observed mostly in the ultraviolet. The amplitudes of pulsations of the studied models reach \(\Delta(M_{\textrm{bol}})=0.8\) and increase as the stellar radii \(R\) decrease. The pulsation periods of stars with \(\log T_{\textrm{eff}}>4\) range from 0.17 to 3.9 days and decrease with decreasing \(R\). The stars have substantially larger \(T_{\textrm{eff}}\) than their companions, which could be Be stars. They are the components of relatively wide binaries with orbital periods up to several years. The number of pulsating moderate-mass stripped helium stars in the Galaxy is \({\simeq}1000\).