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Asteroseismic signatures of the helium core flash


All evolved stars of up to 2 solar masses undergo a helium core flash at the end of their first stage as a giant star. Although theoretically predicted more than 50 years ago1,2, this core flash phase has yet to be observationally probed. We show here that gravity modes stochastically excited by helium-flash-driven convection are able to reach the stellar surface and induce periodic photometric variabilities in hot subdwarf stars with amplitudes of the order of a few thousandths of a magnitude. As such, they can now be detected by space-based photometry with the Transiting Exoplanet Survey Satellite in relatively bright stars (for example, Johnson–Cousins magnitudes of IC 13 mag)3. The range of predicted periods spans from a few thousand seconds to tens of thousands of seconds, depending on the details of the excitation region. In addition, we find that stochastically excited pulsations reproduce the pulsations observed in a few helium-rich hot subdwarf stars. These stars, particularly the future Transiting Exoplanet Survey Satellite target Feige 46, are the most promising candidates to probe the helium core flash for the first time.

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Fig. 1: Locus of hot subdwarfs in a logTeff–logg diagram.
Fig. 2: Propagation diagram and pulsation eigenfunctions for \(\ell = 1\) g modes in a pre-EHB stellar model during a He subflash.
Fig. 3: Evolution of Teff, g and LHe in our stellar models and development of pulsations compared with known He-rich subdwarf pulsators.
Fig. 4: Observed and predicted pulsation amplitudes of \(\ell = 1\) g modes.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

Code availability

The LPCODE and LP-PUL codes used in this paper are available upon request from M.M.M.B. and A.H.C., respectively. Note that LPCODE and LP-PUL are not suitable for public distribution.


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This work was partially supported by ANPCyT through grant PICT 2016-0053, and by the MinCyT-DAAD bilateral cooperation programme through grant DA/16/07. Funding for the Stellar Astrophysics Centre is provided by the Danish National Research Foundation (grant DNRF106). This research was supported in part by the National Science Foundation under grant NSF PHY-1748958. M.M.M.B. acknowledges the financial support by the Stellar Astrophysics Centre (Denmark) that allowed him to participate in several Aarhus red-giants challenge workshops where the central ideas of this paper were conceived.

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Authors and Affiliations



M.M.M.B. developed the idea, derived the theoretical expressions and performed the pulsation computations. T.B. derived the theoretical expressions and computed the stellar models with LPCODE. A.H.C. programmed LP-PUL and discussed the modelling of stochastic excitation. J.C.-D. provided insight into the nature of stochastic oscillations and the modelling of stochastic excitation. All authors participated in discussions of the results, their presentations in figures and descriptions in the manuscript and in pinpointing the conclusions.

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Correspondence to M. M. Miller Bertolami.

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Peer review information Nature Astronomy thanks Santi Cassisi and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Miller Bertolami, M.M., Battich, T., Córsico, A.H. et al. Asteroseismic signatures of the helium core flash. Nat Astron 4, 67–71 (2020).

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