Pulsating White Dwarfs: New Insights
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The Astronomy and Astrophysics Review (2019) 27:7 https://doi.org/10.1007/s00159-019-0118-4 REVIEW ARTICLE Pulsating white dwarfs: new insights Alejandro H. Córsico1,2 · Leandro G. Althaus1,2 · Marcelo M. Miller Bertolami1,2 · S. O. Kepler3 Received: 11 March 2019 / Published online: 3 September 2019 © Springer-Verlag GmbH Germany, part of Springer Nature 2019 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 B Alejandro H. Córsico [email protected] Leandro G. Althaus [email protected] Marcelo M. Miller Bertolami [email protected] S. O. Kepler [email protected] 1 Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque s/n, (1900), La Plata, Argentina 2 Instituto de Astrofísica La Plata, IALP, CONICET-UNLP, La Plata, Argentina 3 Departamento de Astronomia, Universidade Federal do Rio Grande do Sul, Av. Bento Goncalves 9500, Porto Alegre, RS 91501-970, Brazil 123 7 Page 2 of 92 A. H. Córsico et al. 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. Keywords Stellar evolution · White dwarf stars · Stellar interiors · Stellar oscillations · Asteroseismology Contents 1 Introduction ............................................. 2 2 Evolutionary channels and uncertainties in progenitor evolution .................. 8 2.1 Single evolution ......................................... 9 2.2 Binary evolution ......................................... 13 3 Asteroseismology of pulsating WDs and pre-WDs ......................... 14 3.1 Recent observational achievements ............................... 14 3.2 Asteroseismic approaches .................................... 29 3.3 Massive and ultra-massive WDs ................................ 37 3.4 Excitation mechanisms ..................................... 41 3.5 Outbursting pulsating DAV stars ................................ 45 3.6 ELMV and pre-ELMV stars .................................. 46 3.7 Blue large-amplitude pulsators (BLAPs) ............................ 58 3.8 The sdA problem ........................................ 63 3.9 WD pulsators as cosmic laboratories for fundamental physics ................. 63 3.9.1 Upper bounds on the axions mass ............................ 64 3.9.2 Constraints on the neutrino magnetic dipole moment .................. 68 3.9.3 Limits on the secular rate of change of the gravitational constant ............ 71 4 Summary ............................................... 72 References ................................................ 72 1 Introduction White-dwarf (WD) stars represent the final evolutionary stage of the majority of stars. Indeed, all stars with stellar masses lower than ∼ 10 to 11 M, depending on their ini- tial metallicity (e.g., Woosley and Heger 2015), will end their lives as WDs, earth-sized electron-degenerate stellar configurations. As such, they play a unique and fundamen- tal role for our understanding of the formation and evolution of stars, evolution of 123 Pulsating white dwarfs: new insights Page 3 of 92 7 planetary systems, and the history of our Galaxy itself. The study of WDs thus results thus of central relevance for a vast variety of topics of modern astrophysics, ranging from the final fate of planetary systems to the characterization of dark matter (Far- ihi 2016; Salaris and Cassisi 2018). The present population of WDs keeps a detailed record of the early star formation in the Galaxy. Therefore, accurate WD luminosity functions can be used to infer the age, structure and evolution of the Galactic disk and the nearest open and globular clusters (Fontaine et al. 2001; Bedin et al. 2009; García-Berro et al. 2010; Bedin et al. 2015; Campos et al. 2013, 2016; García-Berro and Oswalt 2016; Kilic et al. 2017).1 In a different context, the host stars of most planetary systems, including our Sun, will evolve into WDs, and nowadays obser- vational evidence convincingly demonstrates that numerous WDs foster remnants of planetary systems—even planetary matter, shedding light on the chemical composi- tion of extra-solar planets (Gänsicke et al. 2012; Hollands et al. 2018). Also, WDs are found in binary systems, thus offering a test bed to explore complex stellar interactions amongst stars, including WDs exploding as type Ia supernovae (Maoz et al. 2014). In addition, WDs can be used as cosmic laboratories of extreme physics, ranging from atomic and molecular physics in strong magnetic fields, and high-density plasmas and even solid-state physics (through crystallization; Winget et al. 2009; Tremblay et al. 2019), to exotic physics, like constraining the axion mass and the possible variation of the gravitational constant (Isern et al. 1992; Córsico et al. 2012b, 2013), and also vari- ations of the fine-structure constant (Hu et al. 2019). Last but not least, fundamental properties of WDs, either individually or collectively, like the mass distribution, core chemical composition, and cooling times are key to place constraints on the stellar evolution theory, including third dredge up and mass loss on the Asymptotic Giant Branch (AGB), the efficiency of extra-mixing during core helium burning, and nuclear reaction rates (Kunz et al. 2002; Salaris et al. 2009; Fields et al. 2016). Excellent review papers describing the evolutionary properties of WDs are those of D’Antona and Mazz- itelli (1990), Koester and Chanmugam (1990), Fontaine et al. (2001), Koester (2002), Hansen and Liebert (2003), Hansen (2004), Fontaine and Brassard (2008), Winget and Kepler (2008), and Althaus et al. (2010b). Like many stars, when relevant layers are required to transport energy through high opacity, WDs exhibit periodic brightness variations which are due to global pulsations associated with their normal modes (Ledoux and Walraven 1958;Cox1980; Unno et al. 1989). The existence of these intrinsic luminosity variations implies that, in principle, we have available a unique window to “look” inside these stars, otherwise inaccessible by other means. The analysis of pulsations of a variety of stars has led, in the last decades, to the development of novel techniques which, taken together, are known today as asteroseismology (Aerts et al. 2010; Balona 2010; Catelan and Smith 2015). In principle, a key factor for a successful asteroseismological analysis is the number of periods visible in the star, i.e., the more periods a pulsating star exhibits, the stronger the constraints that asteroseismology could place. It must be emphasized, however, that the crucial point for a successful asteroseismological exercise is not the absolute number of modes itself, but the diversity in the eigenfunctions of these modes. Putting it in other terms: the information provided by a few periods corresponding to 1 Catelan (2018) describes other methods that use WDs to infer ages of stellar populations. 123 7 Page 4 of 92 A. H. Córsico et al. low-order modes is generally richer than the information that provides a larger set of modes with periods in the asymptotic regime (high-order modes).