Skip to main content

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

Asteroseismic signatures of the helium core flash

Abstract

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.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.

from$8.99

All prices are NET prices.

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.

References

  1. 1.

    Schwarzschild, M. & Härm, R. Red giants of population II. II. Astrophys. J. 136, 158–165 (1962).

    ADS  Article  Google Scholar 

  2. 2.

    Thomas, H.-C. Sternentwicklung VIII. Der helium-flash bei einem stern von 1.3 sonnenmassen. Z. Astrophys. 67, 420–455 (1967).

    ADS  Google Scholar 

  3. 3.

    Sullivan, P. W. et al. The Transiting Exoplanet Survey Satellite: simulations of planet detections and astrophysical false positives. Astrophys. J. 809, 77 (2015).

    ADS  Article  Google Scholar 

  4. 4.

    Kippenhahn, R., Weigert, A. & Weiss, A. Stellar Structure and Evolution (Springer, 2012).

  5. 5.

    Cassisi, S. & Salaris, M. Old Stellar Populations: How to Study the Fossil Record of Galaxy Formation (Wiley, 2013).

  6. 6.

    Chaplin, W. J. et al. Ensemble asteroseismology of solar-type stars with the NASA Kepler mission. Science 332, 213–216 (2011).

    ADS  Article  Google Scholar 

  7. 7.

    Mosser, B. et al. Red-giant seismic properties analyzed with CoRoT. Astron. Astrophys. 517, A22 (2010).

    Article  Google Scholar 

  8. 8.

    Beck, P. G. et al. Kepler detected gravity-mode period spacings in a red giant star. Science 332, 205 (2011).

    ADS  Article  Google Scholar 

  9. 9.

    Bildsten, L., Paxton, B., Moore, K. & Macias, P. J. Acoustic signatures of the helium core flash. Astrophys. J. Lett. 744, L6 (2012).

    ADS  Article  Google Scholar 

  10. 10.

    Deheuvels, S. & Belkacem, K. Seismic characterization of red giants going through the helium-core flash. Astron. Astrophys. 620, A43 (2018).

    ADS  Article  Google Scholar 

  11. 11.

    Goldreich, P. & Kumar, P. Wave generation by turbulent convection. Astrophys. J. 363, 694–704 (1990).

    ADS  Article  Google Scholar 

  12. 12.

    Shiode, J. H., Quataert, E., Cantiello, M. & Bildsten, L. The observational signatures of convectively excited gravity modes in main-sequence stars. Mon. Not. R. Astron. Soc. 430, 1736–1745 (2013).

    ADS  Article  Google Scholar 

  13. 13.

    Heber, U. Hot subluminous stars. Publ. Astron. Soc. Pac. 128, 082001 (2016).

    ADS  Article  Google Scholar 

  14. 14.

    Castellani, M. & Castellani, V. Mass loss in globular cluster red giants: an evolutionary investigation. Astrophys. J. 407, 649–656 (1993).

    ADS  Article  Google Scholar 

  15. 15.

    Brown, T. M., Sweigart, A. V., Lanz, T., Landsman, W. B. & Hubeny, I. Flash mixing on the white dwarf cooling curve: understanding hot horizontal branch anomalies in NGC 2808. Astrophys. J. 562, 368–393 (2001).

    ADS  Article  Google Scholar 

  16. 16.

    Cassisi, S., Schlattl, H., Salaris, M. & Weiss, A. First full evolutionary computation of the helium flash-induced mixing in population II stars. Astrophys. J. Lett. 582, L43–L46 (2003).

    ADS  Article  Google Scholar 

  17. 17.

    Lanz, T., Brown, T. M., Sweigart, A. V., Hubeny, I. & Landsman, W. B. Flash mixing on the white dwarf cooling curve: Far Ultraviolet Spectroscopic Explorer observations of three He-rich sdB stars. Astrophys. J. 602, 342–355 (2004).

    ADS  Article  Google Scholar 

  18. 18.

    Miller Bertolami, M. M., Althaus, L. G., Unglaub, K. & Weiss, A. Modeling He-rich subdwarfs through the hot-flasher scenario. Astron. Astrophys. 491, 253–265 (2008).

    ADS  Article  Google Scholar 

  19. 19.

    Tailo, M. et al. Rapidly rotating second-generation progenitors for the ‘blue hook’ stars of ω Centauri. Nature 523, 318–321 (2015).

    ADS  Article  Google Scholar 

  20. 20.

    Battich, T., Miller Bertolami, M. M., Córsico, A. H. & Althaus, L. G. Pulsational instabilities driven by the ϵ mechanism in hot pre-horizontal branch stars. I. The hot-flasher scenario. Astron. Astrophys. 614, A136 (2018).

    ADS  Article  Google Scholar 

  21. 21.

    Córsico, A. H., Althaus, L. G. & Miller Bertolami, M. M. New nonadiabatic pulsation computations on full PG 1159 evolutionary models: the theoretical GW Virginis instability strip revisited. Astron. Astrophys. 458, 259–267 (2006).

    ADS  Article  Google Scholar 

  22. 22.

    Lecoanet, D. & Quataert, E. Internal gravity wave excitation by turbulent convection. Mon. Not. R. Astron. Soc. 430, 2363–2376 (2013).

    ADS  Article  Google Scholar 

  23. 23.

    Couston, L.-A., Lecoanet, D., Favier, B. & Le Bars, M. The energy flux spectrum of internal waves generated by turbulent convection. J. Fluid Mech. 854, R3 (2018).

    MathSciNet  Article  Google Scholar 

  24. 24.

    Rogers, T. M., Lin, D. N. C., McElwaine, J. N. & Lau, H. H. B. Internal gravity waves in massive stars: angular momentum transport. Astrophys. J. 772, 21 (2013).

    ADS  Article  Google Scholar 

  25. 25.

    Miller Bertolami, M. M. New models for the evolution of post-asymptotic giant branch stars and central stars of planetary nebulae. Astron. Astrophys. 588, A25 (2016).

    ADS  Article  Google Scholar 

  26. 26.

    Geier, S. et al. The catalogue of radial velocity variable hot subluminous stars from the MUCHFUSS project (corrigendum). Astron. Astrophys. 602, C2 (2017).

    Article  Google Scholar 

  27. 27.

    Naslim, N., Jeffery, C. S., Ahmad, A., Behara, N. T. & Şahìn, T. Abundance analyses of helium-rich subluminous B stars. Mon. Not. R. Astron. Soc. 409, 582–590 (2010).

    ADS  Article  Google Scholar 

  28. 28.

    Østensen, R. H. et al. KIC 1718290: a helium-rich V1093-Her-like pulsator on the blue horizontal branch. Astrophys. J. Lett. 753, L17 (2012).

    ADS  Article  Google Scholar 

  29. 29.

    Jeffery, C. S. et al. Discovery of a variable lead-rich hot subdwarf: UVO 0825+15. Mon. Not. R. Astron. Soc. 465, 3101–3124 (2017).

    ADS  Article  Google Scholar 

  30. 30.

    Latour, M., Green, E. M. & Fontaine, G. Discovery of a second pulsating intermediate helium-enriched sdOB star. Astron. Astrophys. 623, L12 (2019).

    ADS  Article  Google Scholar 

  31. 31.

    Ahmad, A. & Jeffery, C. S. Discovery of pulsation in a helium-rich subdwarf B star. Astron. Astrophys. 437, L51–L54 (2005).

    ADS  Article  Google Scholar 

  32. 32.

    Saio, H. & Jeffery, C. S. The excitation of g-mode pulsations in hot helium-rich subdwarfs. Mon. Not. R. Astron. Soc. 482, 758–761 (2019).

    ADS  Article  Google Scholar 

  33. 33.

    Gesicki, K., Zijlstra, A. A. & Miller Bertolami, M. M. The mysterious age invariance of the planetary nebula luminosity function bright cut-off. Nat. Astron. 2, 580–584 (2018).

    ADS  Article  Google Scholar 

  34. 34.

    Guerrero, M. A. et al. The inside-out planetary nebula around a born-again star. Nat. Astron. 2, 784–789 (2018).

    ADS  Article  Google Scholar 

  35. 35.

    Unno, W., Osaki, Y., Ando, H., Saio, H. & Shibahashi, H. Nonradial Oscillations of Stars (Univ. of Tokyo Press, 1989).

  36. 36.

    Vitense, E. Die wasserstoffkonvektionszone der sonne. Mit 11 textabbildungen. Z. Astrophys. 32, 135–164 (1953).

    ADS  Google Scholar 

  37. 37.

    Dziembowski, W. Light and radial velocity variations in a nonradially oscillating star. Acta Astron. 27, 203–211 (1977).

    ADS  Google Scholar 

  38. 38.

    Randall, S. K., Bagnulo, S., Ziegerer, E., Geier, S. & Fontaine, G. The enigmatic He-sdB pulsator LS IV-14°116: new insights from the VLT. Astron. Astrophys. 576, A65 (2015).

    ADS  Article  Google Scholar 

  39. 39.

    Dorman, B., Rood, R. T. & O’Connell, R. W. Ultraviolet radiation from evolved stellar populations. I. Models. Astrophys. J. 419, 596–614 (1993).

    ADS  Article  Google Scholar 

Download references

Acknowledgements

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.

Author information

Affiliations

Authors

Contributions

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.

Corresponding author

Correspondence to M. M. Miller Bertolami.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

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

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary information

Supplementary Information

Supplementary text, Figs. 1–10 and references.

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Miller Bertolami, M.M., Battich, T., Córsico, A.H. et al. Asteroseismic signatures of the helium core flash. Nat Astron 4, 67–71 (2020). https://doi.org/10.1038/s41550-019-0890-0

Download citation

Further reading

Search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing