Skip to main content
SpringerLink
Log in

Pulsating white dwarfs: new insights

  • Review Article
  • Published:
The Astronomy and Astrophysics Review Aims and scope

Abstract

Stars are extremely important astronomical objects that constitute the pillars on which the Universe is built, and as such, their study has gained increasing interest over the years. White dwarf stars are not the exception. Indeed, these stars constitute the final evolutionary stage for more than 95% of all stars. The Galactic population of white dwarfs conveys a wealth of information about several fundamental issues and are of vital importance to study the structure, evolution and chemical enrichment of our Galaxy and its components—including the star formation history of the Milky Way. Several important studies have emphasized the advantage of using white dwarfs as reliable clocks to date a variety of stellar populations in the solar neighborhood and in the nearest stellar clusters, including the thin and thick disks, the Galactic spheroid and the system of globular and open clusters. In addition, white dwarfs are tracers of the evolution of planetary systems along several phases of stellar evolution. Not less relevant than these applications, the study of matter at high densities has benefited from our detailed knowledge about evolutionary and observational properties of white dwarfs. In this sense, white dwarfs are used as laboratories for astro-particle physics, being their interest focused on physics beyond the standard model, that is, neutrino physics, axion physics and also radiation from “extra dimensions”, and even crystallization. The last decade has witnessed a great progress in the study of white dwarfs. In particular, a wealth of information of these stars from different surveys has allowed us to make meaningful comparison of evolutionary models with observations. While some information like surface chemical composition, temperature and gravity of isolated white dwarfs can be inferred from spectroscopy, and the total mass and radius can be derived as well when they are in binaries, the internal structure of these compact stars can be unveiled only by means of asteroseismology, an approach based on the comparison between the observed pulsation periods of variable stars and the periods predicted by appropriate theoretical models. The asteroseismological techniques allow us to infer details of the internal chemical stratification, the total mass, and even the stellar rotation profile. In this review, we first revise the evolutionary channels currently accepted that lead to the formation of white-dwarf stars, and then, we give a detailed account of the different sub-types of pulsating white dwarfs known so far, emphasizing the recent observational and theoretical advancements in the study of these fascinating variable stars.

This is a preview of subscription content, log in via an institution to check access.

Access this article

Log in via an institution

Subscribe and save

Springer+ Basic
$34.99 /Month
  • Get 10 units per month
  • Download Article/Chapter or eBook
  • 1 Unit = 1 Article or 1 Chapter
  • Cancel anytime
Subscribe now

Buy Now

Price excludes VAT (USA)
Tax calculation will be finalised during checkout.

Instant access to the full article PDF.

Institutional subscriptions

Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6
Fig. 7
Fig. 8
Fig. 9
Fig. 10
Fig. 11
Fig. 12
Fig. 13
Fig. 14
Fig. 15
Fig. 16
Fig. 17
Fig. 18
Fig. 19
Fig. 20
Fig. 21
Fig. 22
Fig. 23
Fig. 24
Fig. 25

Similar content being viewed by others

Notes

  1. Catelan (2018) describes other methods that use WDs to infer ages of stellar populations.

  2. An exception are the high-field magnetic WDs that represent the \(\sim 20\%\) of the local population of WDs, and for which there is no observational evidence of variability due to pulsations.

  3. Spheroidal modes are characterized by \(({\mathbf {\nabla }} \times \varvec{\xi })_r= 0\) and \(\sigma \ne 0\), where \(\varvec{\xi }\) is the Lagrangian displacement and \(\sigma \) the pulsation frequency (Unno et al. 1989).

  4. This, at variance with the forced pulsations such as stochastic excitation by turbulent convection, in which the modes, that are intrinsically stable, are actually excited by convective motions.

  5. Although see Quirion et al. (2012) for the case of GW Vir stars and Luan and Goldreich (2018) for the case of ZZ Ceti stars.

  6. The sound of the bells (their eigenfrequencies) does not depend on how the bells are rung (Baade 1992).

  7. Note, however, that in many cases, the studies of solar-type pulsators seem to be limited to using the frequency separations and frequency maximum to derive the astrophysical parameters of the stars, using the so-called “scaling relations” (Lund et al. 2017).

  8. However, the dependence of the period spacing on the thickness of the outer envelope of DA and DB WDs is generally weaker than its dependence upon the effective temperature and the stellar mass (Tassoul et al. 1990).

  9. Gänsicke et al. (2010) discovered two WDs exposing dredged-up, O-rich core material that could have been produced in the interior of a Super-AGB star. Recently, Kepler et al. (2016a) identified a WD having an O-dominated atmosphere with traces of Ne and Mg, that could be the bare core of a Super-AGB star. Finally, another O- and Ne-rich WD but with a very low mass was discovered by Vennes et al. (2017).

  10. Previous efforts to constrain the \(^{12}\hbox {C}(\alpha ,\gamma )^{16}\hbox {O}\) reaction rate using WD asteroseismology have been done using DBVs (see, e.g., Metcalfe 2003, and references therein).

  11. Working Group 8: Evolved compact stars with TESS (https://tasoc.dk/wg8/).

  12. Actually, Dziembowski and Koester (1981) found g-mode instability due to the partial ionization of He.

  13. Pulsational excitation of g modes due to the \(\epsilon \) mechanism in H-deficient pre-WD models was investigated by Kawaler et al. (1986) and Córsico et al. (2009b) (see, e.g., Althaus et al. 2010b; Catelan and Smith 2015, for a review of this topic).

  14. A phenomenon reminiscent of these outburst-like events was the sforzando event detected in 1996 for the DBV GD358, in which the star dramatically altered its pulsation characteristics on a timescale of hours (Provencal et al. 2009).

  15. The definition of an ELM WD is still under debate. In the context of the ELM Survey (Brown et al. 2010), an ELM WD is defined as an object with surface gravity of \(5 \lesssim \log g \lesssim 7\) and effective temperature in the range of \(8000 \lesssim T_{\mathrm{eff}} \lesssim 22\,000\,\mathrm {K}\) (see, e.g., Brown et al. 2016). Here (see also Córsico and Althaus 2014a), we propose to define an ELM WD as a WD that does not undergo H shell flashes, because in this way, the pulsational properties are quite different as compared with the systems that experience flashes, although this mass limit depends on the metallicity of the progenitor stars (Serenelli et al. 2002; Istrate et al. 2016b).

  16. According to Kilic et al. (2018), there are only four confirmed pulsating ELM WDs in short-period binaries (which are the four that show RV variations), that occupy a similar parameter space and there is no question about their nature as WDs. These are: SDSS J1112+1117, SDSS J1518+0658, SDSS J1840+6423, and PSR J1738+0333. We have to add SDSSJ1618+3854 to that list, based on Bell et al. (2018).

  17. Hermes et al. (2013d) reported the discovery of short-period pulsations compatible with p modes or radial modes in an ELMV WD (SDSS J111215.82+111745.0), but this needs to be confirmed with further observations.

  18. They have been called “binary evolution pulsators” by Smolec et al. (2013).

  19. The CP symmetry establishes that the laws of physics should be the same if particles were replaced with their antiparticles (C symmetry) and their spatial coordinates were inverted (P symmetry) (Luders 1954; Pauli et al. 1995).

  20. The name of axion comes from the Axion laundry detergent, and was introduced by Frank Wilczek “to clean QCD from the CP problem”.

  21. Exceptions to this assertion are WDs coming from low-metallicity progenitors (Miller Bertolami et al. 2013; Althaus et al. 2015) and ELM WDs (Althaus et al. 2013).

  22. This is at variance with solar neutrinos, which are the result of nuclear fusion. For the Sun, thermal neutrino emission is negligible (Raffelt 1996).

References