Asteroseismic Observations of Hot Subdwarfs

Abstract

There are a number of reasons for studying hot subdwarf pulsation; the most obvious being that these stars remain a poorly understood late-stage of stellar evolution and knowledge of their interior structure, which pulsation studies reveal, constrains evolution models. Of particular interest are the red giant progenitors as in looking at a hot subdwarf we are seeing a stripped-down red giant as it would have been just before the Helium Flash. Moreover, hot subdwarfs may have formed through the merger of two helium white dwarfs and their study gives insight into how such a merger may have happened. A less obvious reason for studying pulsation in hot subdwarfs is that they provide a critical test of stellar envelope opacities and the atomic physics upon which they depend.

The OmegaWhite Survey for Short-Period Variable Stars VII: High amplitude, short period blue variables

“…The multi-periodic nature of OW J0820-3301, and its high temperature (Table 5) are consistent with the properties of V361 Hya (or EC 14026) variables (Kilkenny et al 1997;Heber 2016;Lynas-Gray 2021). Pulsation amplitudes in V361 Hya variables are typically less than 1% compared with nearly 10% in OW J0820-3301.…”

The OmegaWhite survey for short-period variable stars – VII. High amplitude short-period blue variables

“…The multiperiodic nature of OW J0820-3301, and its high temperature (Table 5 ) are consistent with the properties of V361 Hya (or EC 14026) variables (Kilkenny et al 1997 ;Heber 2016 ;Lynas-Gray 2021 ). Pulsation amplitudes in V361 Hya variables are typically less than 1 per cent compared with nearly 10 per cent in OW J0820-3301.…”

Discovery of post-mass-transfer helium-burning red giants using asteroseismology

“…A star expands to become a red giant when it has fused all the hydrogen in its core into helium. If the star is in a binary system, its envelope can overflow onto its companion or be ejected into space, leaving a hot core and potentially forming a subdwarf-B star [1][2][3] . However, most red giants that have partially transferred envelopes in this way remain cool on the surface and are almost indistinguishable from those that have not.…”

“…If the star loses its entire hydrogen-rich envelope before reaching the RGB tip, it leaves a bare non-burning helium core, forming a lowmass white dwarf [16][17][18] (M/M < 0.5; M is the stellar mass). On the other hand, a stripped CHeB red giant could form a hot subluminous star of spectral type B (sdB) on the extreme horizontal branch 2,3,19,20 . Indeed, most sdB stars are found to be in binary systems with short periods [21][22][23] .…”