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

Abstract

Context. Many low-mass (M /M < ∼ 0.45) and extremely low-mass (ELM, M /M < ∼ 0.18−0.20) white-dwarf stars are currently being found in the field of the Milky Way. Some of these stars exhibit long-period gravity-mode (g-mode) pulsations, and constitute the class of pulsating white dwarfs called ELMV stars. In addition, two low-mass pre-white dwarfs, which could be precursors of ELM white dwarfs, have been observed to show multiperiodic photometric variations. They could constitute a new class of pulsating low-mass pre-white dwarf stars. Aims. Motivated by this finding, we present a detailed nonadiabatic pulsation study of such stars, employing full evolutionary sequences of low-mass He-core pre-white dwarf models. Methods. Our pulsation stability analysis is based on a set of low-mass He-core pre-white dwarf models with masses ranging from 0.1554 to 0.2724 M , which were derived by computing the nonconservative evolution of a binary system consisting of an initially 1 M ZAMS star and a 1.4 M neutron star companion. We have considered models in which element diffusion is accounted for and also models in which it is neglected. Results. We confirm and explore in detail a new instability strip in the domain of low gravities and low effective temperatures of the T eff − log g diagram, where low-mass pre-white dwarfs are currently found. The destabilized modes are radial and nonradial p and g modes excited by the κ − γ mechanism acting mainly at the zone of the second partial ionization of He, with non-negligible contributions from the region of the first partial ionization of He and the partial ionization of H. The computations with element diffusion are unable to explain the pulsations observed in the two known pulsating pre-white dwarfs, suggesting that element diffusion might be inhibited at these stages of the pre-white dwarf evolution. Our nonadiabatic models without diffusion, on the other hand, naturally explain the existence and range of periods of the pulsating pre-white dwarf star WASP J1628+10B, although they fail to explain the pulsations of WASP J0247−25B, the other known member of the class, indicating that the He abundance in the driving region of this star might be substantially higher than predicted by our models. Conclusions. Discoveries of additional members of this new class of pulsating stars and their analysis in the context of the theoretical background presented in this paper will shed new light on the evolutionary history of their progenitor stars.

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

“…Moreover, we also encounter difficulties in driving pulsation modes in view of the strong dependence of the blue edge on the total mass for a given metallicity, i.e., the more massive the star, the cooler the blue edge (see Figs. 2 and 3 of Istrate et al 2016a in this connection; also, Córsico et al 2016).…”

“…2015;Córsico et al 2016;Gianninas et al 2016). In this context, Gianninas et al (2016) provided the first empirical evidence that pulsations in ELM proto-WDs can only occur if a significant amount of He is present in the atmosphere and, by extension, in the driving region.…”

“…In that context, the last authors suggested that rotational mixing could be a mechanism of importance, given that it should occur naturally in close binary systems in which ELM proto-WDs are found. Moreover, Córsico et al (2016) showed that in order to explain the existence of the ELM proto-WD pulsator class, gravitational settling must be counteracted by another process. Istrate et al (2016b) were the first to investigate the combined effects of rotational mixing and diffusion processes (gravitational settling, thermal diffusion, and chemical diffusion) on the (proto-) WDs that are formed through stable Roche-lobe overflow (the low-mass Xray binary channel).…”

Models of low-mass helium white dwarfs including gravitational settling, thermal and chemical diffusion, and rotational mixing

“…By the beginning of the last flash, the surface structure of the model that only includes element diffusion is nearly identical to the structure of the model that includes both element diffusion and rotational mixing. Helium in the envelopes of ELM proto-WDs is a crucial ingredient for exciting pulsation modes through the κ-mechanism, as shown by Jeffery & Saio (2013) and Córsico et al (2016) for radial and nonradial modes. Gianninas et al (2016) recently provided the first empirical evidence that pulsations in ELM proto-WDs can only occur when a significant amount of helium is present in their atmospheres.…”

Comparing the asteroseismic properties of pulsating extremely low-mass pre-white dwarf stars and δ Scuti stars

“…δ Sct and pre-ELM WD stellar models were calculated as in [3] and [4], respectively. We choose two template models with similar surface gravity and effective temperature, one of them representative of δ Scuti stars and the other one representative of pulsating pre-ELM WDs, both with In the δ Sct model, this frequency becomes null in the convective core and in an outer convective layer.…”