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 to 7 solar mass 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 pulsation instability extends over Hertzsprung-Russel diagram from the red giants branch to the region of effective temperatures from about 30,000 K to about 50,000 K. Variations of stellar luminosity should be observed mostly in the ultraviolet. The amplitudes of pulsations of the studied models reach 0.8 stellar magnitude and increase, as the stellar radii decrease. Pulsation periods of stars with effective temperatures exceeding 10,000 K range from 0.17 to 8.5 day and decrease with decreasing radii. The stars have substantially larger effective temperatures than their companions, which could be Be-stars. They are 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 about 1000.