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Horizontal-Branch Stars

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Atomic Diffusion in Stars

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Abstract

Most stars with \(M\stackrel{<}{\sim }2.0\) M ⊙ move up the red giant branch till the He core reaches a mass of \(\sim 0.5\) M ⊙. Helium then ignites, some 4 % by mass is transformed into C, degeneracy is lifted in the core and the star moves to the Horizontal Branch (HB in what follows). HB stars are thought to burn He in their center and to have a H envelope. The central region of HB stars is like a He star . The surface T eff depends on the thickness of the remaining H envelope above the He core. When the H envelope is thick enough, there is a H burning shell . After exhaustion of the He core, these stars evolve to the asymptotic giant branch (AGB) . Above 20,000 K, the field equivalent to hot globular cluster stars are usually called sdOs and sdBs while below 20,000 K, they are called HBBs and HBAs. While it is understood that the spread in T eff on the HB is caused by the range in thickness of the H shell, the origin of this range is not established. Even if they started with the same mass on the main-sequence, RGB stars loose a different fraction of their mass on their way to the HB. It is possibly related to the rotation rate. At least some of the sdBs above 20,000 K are currently believed to come from binary star evolution. See § 1.1.7 for a brief introduction to their abundance anomalies , and to the link with abundance anomalies in Pop I stars.

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Notes

  1. 1.

    For reviews see Sweigart (1994), Moehler (2001) and Salaris et al. (2002).

  2. 2.

    Except if they lose too much mass on the RGB, Castellani and Castellani (1993).

  3. 3.

    For a review see Heber (2009).

  4. 4.

    This was suggested by Greenstein and Sargent (1974) depending on the presence or absence of H and He lines, in analogy with the main-sequence. They have a similar dependence on T eff. There have been a few attempts to modernize the nomenclature to sdO, sdB and sdA but this has not found wide spread acceptance.

  5. 5.

    See § 13.3.1 and Fig. 6 of Michaud et al. 2007.

  6. 6.

    See § 3 and 4 of Michaud et al. 2007.

  7. 7.

    See § 3 of Michaud et al. (2007).

  8. 8.

    Cassisi et al. (1997) first obtained that luminosity changes of the red giant bump and the ZAHB partly canceled and that the final effect is smaller than can be distinguished given current observational uncertainties. Castellani et al. (1997) obtained ZAHB models including atomic diffusion from the ZAMS for the settling of both He and metals. Since their calculations were done without radiative accelerations they obtain underabundaces for all metals rather than overabundances for most metals in the exterior regions.

  9. 9.

    Because of the small concentration of both H and He.

  10. 10.

    RR Lyræ are pulsating stars of \(\sim 7000\) K on the HB.

  11. 11.

    See Fig. 6 of Michaud et al. (2008) where it may be noted that the He abundance hardly decreases in the 0. 73 M ⊙ model.

  12. 12.

    See the end of § II b of Michaud et al. (1983b).

  13. 13.

    The M bcz may be found in Fig. 3 of Michaud et al. (2011a).

  14. 14.

    Organized magnetic fields of one kG or more are not generally present in sdAs and sdBs according to Mathys et al. (2012) and Landstreet et al. (2012).

  15. 15.

    As may be seen in Figs. 11 and 12 of Michaud et al. (2008).

  16. 16.

    Stratification of S had been tentatively observed (Khalack et al. 2007) in one field star suggested to be a BHB star. However given its high He concentration, its relation to BHB stars is dubious.

  17. 17.

    See § IV of Michaud et al. (1983b).

  18. 18.

    See § IV of Glaspey et al. (1989).

  19. 19.

    See § 7 of Michaud (1991b).

  20. 20.

    By Unglaub (2008) who however neglected the shadowing of lines, leading to an overestimate of the calculated mass loss rate. See §7.5.2.

  21. 21.

    For a review see Heber (2009).

  22. 22.

    See § 11 of Heber (2009).

  23. 23.

    See also Charpinet et al. (1997) and for a review Charpinet et al. (2001).

  24. 24.

    For a review, see Fontaine et al. (2008).

  25. 25.

    See § 7 of Fontaine et al. (2008).

  26. 26.

    See § 9 of Heber (2009).

  27. 27.

    See § 5.4 of Michaud et al. (2011a).

  28. 28.

    See the analysis in § 9.2.1 and in particular after Eq. (9.8).

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Authors and Affiliations

  1. Département de physique, Université de Montréal, Montréal, Québec, Canada

    Georges Michaud

  2. Laboratoire Univers et Théorie, CNRS-Observatoire de Paris, Meudon, France

    Georges Alecian & Jacques Richer

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  1. Georges Michaud
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Michaud, G., Alecian, G., Richer, J. (2015). Horizontal-Branch Stars. In: Atomic Diffusion in Stars. Astronomy and Astrophysics Library. Springer, Cham. https://doi.org/10.1007/978-3-319-19854-5_12

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