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Abstract
Abstract The ε mechanism is a self-excitation mechanism of stellar pulsations which acts in regions where nuclear burning takes place. It has been shown that the ε mechanism can excite pulsations in hot pre-horizontal branch stars before they settle into the stable helium core-burning phase and that the shortest periods of LS IV-14º116 could be explained that way.We aim to study the ε mechanism in stellar models appropriate for hot pre-horizontal branch stars to predict their pulsational properties.We perform detailed computations of non-adiabatic non-radial pulsations on such stellar models.We predict a new instability domain of long-period gravity modes in the log g − log Teff plane at roughly 22000 K ≲ Teff ≲ 50000 K and 4.67 ≲ log g ≲ 6.15, with a period range from ~ 200 to ~ 2000 s. Comparison with the three known pulsating He-rich subdwarfs shows that the ε mechanism can excite pulsations in models with similar surface properties except for modes with the shortest observed periods. Based on simple estimates we expect at least 3 stars in the current samples of hot-subdwarf stars to be pulsating by the ε mechanism. Our results could constitute a theoretical basis for future searches of pulsators in the Galactic field.
Highlights
IntroductionHot horizontal branch stars are hot (effective temperature, Te 7000 K) and evolved low-mass (∼ 0.5 M⊙) stars burning helium in their core (Heber 2016), that lost almost all of their H-rich envelope at (or near) the tip of the Red Giant Branch (RGB)
Hot horizontal branch stars are hot and evolved low-mass (∼ 0.5 M⊙) stars burning helium in their core (Heber 2016), that lost almost all of their H-rich envelope at the tip of the Red Giant Branch (RGB)
It has been shown that the ε mechanism can excite pulsations in hot pre-horizontal branch stars before they settle into the stable helium core-burning phase and that the shortest periods of LS IV-14∘116 could be explained that way
Summary
Hot horizontal branch stars are hot (effective temperature, Te 7000 K) and evolved low-mass (∼ 0.5 M⊙) stars burning helium in their core (Heber 2016), that lost almost all of their H-rich envelope at (or near) the tip of the Red Giant Branch (RGB). The position of stars in the BHB or the EHB depends mainly on their hydrogen-rich envelope mass, with stars harbouring less massive envelopes being located at higher effective temperatures. Before settling on the stable core He-burning stage, low-mass stars undergo a helium-core flash and subsequent subflashes. There are some sdBs with mixed H and He compositions These stars could be at the preEHB phase where the ongoing diffusion has had no time to turn the envelope hydrogen pure (Naslim et al 2010)
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