Asteroseismic test of rotational mixing in low-mass white dwarfs

Abstract

We exploit the recent discovery of pulsations in mixed-atmosphere (He/H), extremely low-mass white dwarf precursors (ELM protoWDs) to test the proposition that rotational mixing is a fundamental process in the formation and evolution of low-mass helium core white dwarfs. Rotational mixing has been shown to be a mechanism able to compete efficiently against gravitational settling, thus accounting naturally for the presence of He, as well as traces of metals such as Mg and Ca, typically found in the atmospheres of ELM proto-WDs. Here we investigate whether rotational mixing can maintain a sufficient amount of He in the deeper driving region of the star, such that it can fuel, through Heii-Heiii ionization, the observed pulsations in this type of stars. Using state-of-the-art evolutionary models computed with MESA, we show that rotational mixing can indeed explain qualitatively the very existence and general properties of the known pulsating, mixed-atmosphere ELM proto-WDs. Moreover, such objects are very likely to pulsate again during their final WD cooling phase.Key words. asteroseismology -binaries: close -white dwarfs -stars: evolution Gianninas et al. (2016) recently reported the discovery of pulsations in three mixed-atmosphere, extremely low-mass white dwarf precursors (ELM proto-WDs). Their location in the log g− T eff 1 diagram and the detected periods are similar to those of the first discovered pulsating ELM proto-WDs WASP 0247-25B (T eff = 10 840 ± 300 K, log g = 4.576 ± 0.011; Maxted et al. 2013) and WASP 1628+10B (T eff = 9200 ± 600 K, log g = 4.49 ± 0.05; Maxted et al. 2014). It is expected that the nature of the pulsation driving is the same in both types of systems (see, e.g., Jeffery & Saio 2013;Córsico et al. 2016). While the (likely) presence of He in the atmosphere of the two WASP systems has yet to be confirmed 2 , the results of Gianninas et al. (2016) represent the first empirical evidence that pulsations in relatively hot ELM proto-WDs can only occur when a significant amount of He is present in their atmospheres. We disregard here the two cool-ELM proto-WD candidates proposed by Corti et al. (2016), and also the system discussed by Zhang et al. (2016), as their nature is currently unclear. Astrophysical contextHelium is the ingredient needed to drive pulsations in a regime of effective temperature well above the blue edge of the ZZ Ceti instability strip, as well as its extension into the 1 J0756+6704: T eff = 11 640 ± 250 K, log g = 4.90 ± 0.14, X(He) = 0.50 ± 0.20; J1141+3850: T eff = 11 290 ± 210 K, log g = 4.94 ± 0.10, X(He) = 0.54 ± 0.14; J1157+0546: T eff = 11 870 ± 260 K, log g = 4.81 ± 0.13, X(He) = 0.53 ± 0.20. 2 A difficult task as the light of the A-type companion dominates the optical spectrum.low-gravity domain (e.g., Steinfadt et al. 2010;Córsico et al. 2012;Van Grootel et al. 2013). The ZZ Ceti instability strip only contains pure H atmosphere (DA) WDs for which pulsation driving is confined to the regions of partial ionization of H. Van Grootel et al. (2015) showed ...

Pulsating low-mass white dwarfs in the frame of new evolutionary sequences

“…As described in Althaus et al (2013), models with masses higher than ∼0.18 M experience multiple CNO flashes after this pre-WD stage and before enter their final cooling tracks. The nonadiabatic properties of pulsating low-mass pre-WD stars, or pre-ELMVs, have been explored at length in Córsico et al (2016a), Gianninas et al (2016), Istrate et al (2016a). In Fig.…”

“…6 we present a T eff − log g diagram showing our low-mass He-core pre-WD evolutionary tracks (thin dotted black curves) without element diffusion. We have chosen to explore the case in which element diffusion is not allowed to operate, due to that recent nonadiabatic studies indicate that element diffusion must be weakened so that pulsations can be excited by the κ mechanism acting at the zone of the second partial ionization of He (Córsico et al 2016a;Istrate et al 2016a). Exploratory computations in which element diffusion is considered do not show substantial differences regarding thė Π values reported in this work.…”

“…Nonadiabatic stability computations for radial modes (Jeffery & Saio 2013) and nonradial p and g modes (Córsico et al 2016a;Gianninas et al 2016;Istrate et al 2016a) have revealed that the excitation of pulsations in these pre-WDs is the κ − γ mechanism acting mainly at the zone of the second partial ionization of He, with a weaker contribution from the region of the first partial ionization of He and the partial ionization of H. So, the abundance of He in the envelopes of this new class of pulsating stars (hereinafter preELMVs 4 ) is a crucial ingredient for destabilizing the pulsation modes (Córsico et al 2016a;Istrate et al 2016a).…”

A Model of the Pulsating Extremely Low-mass White Dwarf Precursor WASP 0247–25B

“…1 of Istrate et al (2016a) has been obtained by averaging the value of T eff = 11,380 ± 400 K of Table 1 of Maxted et al (2013) and that derived from their preliminary analysis of the disentangled spectrum of the star which leads to T eff = 10,300 ± 200 K, while keeping the log g value at 4.576 ± 0.011. The track going through that new location shown in their figure show pulsational instabilities in the correct range, but still corresponds to a mass of 0.189 M , which is too large compared to the more stringent requirement of the present paper.…”

Pulsating white dwarfs

“…[30] found 17 pre-ELMs, i.e., helium-core white dwarf precursors, and [31,32] report pulsations in six of them. Pulsations are an important tool to study the stellar interior, and [33] - [37] report on theoretical models and pulsations of ELMs. [38] and [39] show there are thousands of stars, photometrically classified as blue horizontal branch stars by [41][42][43], that have spectroscopic estimated surface gravities much higher than main sequence stars (log g ≥ 4.75) and therefore must have radii smaller than the Sun, classifying them as sdAs, in line with the hot subdwarfs reviewed by [44].…”