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

Abstract

Context. Many pulsating low-mass white dwarf stars have been detected in the past years in the field of our Galaxy. Some of them exhibit multiperiodic brightness variation, therefore it is possible to probe their interiors through asteroseismology. Aims. We present a detailed asteroseismological study of all the known low-mass variable white dwarf stars based on a complete set of fully evolutionary models that are representative of low-mass He-core white dwarf stars. Methods. We employed adiabatic radial and nonradial pulsation periods for low-mass white dwarf models with stellar masses ranging from 0.1554 to 0.4352 M that were derived by simulating the nonconservative evolution of a binary system consisting of an initially 1 M zero-age main-sequence (ZAMS) star and a 1.4 M neutron star companion. We estimated the mean period spacing for the stars under study (where this was possible), and then we constrained the stellar mass by comparing the observed period spacing with the average of the computed period spacings for our grid of models. We also employed the individual observed periods of every known pulsating low-mass white dwarf star to search for a representative seismological model. Results. We found that even though the stars under analysis exhibit few periods and the period fits show multiplicity of solutions, it is possible to find seismological models whose mass and effective temperature are in agreement with the values given by spectroscopy for most of the cases. Unfortunately, we were not able to constrain the stellar masses by employing the observed period spacing because, in general, only few periods are exhibited by these stars. In the two cases where we were able to extract the period spacing from the set of observed periods, this method led to stellar mass values that were substantially higher than expected for this type of stars.Conclusions. The results presented in this work show the need for further photometric searches, on the one hand, and that some improvements of the theoretical models are required on the other hand in order to place the asteroseismological results on a firmer ground.

The Physical Nature of Subdwarf A Stars: White Dwarf Impostors

“…Hence, there may be only four likely pulsating ELM WDs: the three pulsators presented in Hermes et al (2013b) and the WD companion to PSR J1738+0333 . Measuring the rate of pulsation period change could clarify their stellar nature, because different classes of pulsators have different rates of period change (Calcaferro et al 2017). …”

Pulsating white dwarfs: new insights

“…We close this section by briefly describing the first attempts of asteroseismology of ELMV stars. A detailed asteroseismological study of all the known and suspected ELMV stars based on the low-mass He-core WD models of Althaus et al (2013) was presented by Calcaferro et al (2017b). Despite ELMVs exhibiting very few periods and the period-to-period fits showing multiple solutions, they found that it is still possible to find asteroseismological models with M ⋆ and T eff compatible with the values derived by spectroscopy for most cases.…”

“…The second asteroseismological approach for WDs was developed at La Plata Observatory and is based on non-static WD evolutionary models that result from the complete evolution of the progenitor stars. This method has been already applied to GW Virginis stars (see, e.g., Córsico et al, 2007aCórsico et al, ,b, 2008Córsico et al, , 2009aCalcaferro et al, 2016, and references therein), to DBV WDs (see, e.g., Córsico et al, 2012aCórsico et al, , 2014Bognár et al, 2014), and recently to ELMV stars (Calcaferro et al, 2017b(Calcaferro et al, , 2018b. Regarding ZZ Ceti stars, this approach has been employed by Romero et al (2012Romero et al ( , 2013Romero et al ( , 2017.…”

Gravitational-wave Radiation of Double Degenerates with Extremely Low-mass White Dwarf Companions

“…And then we assumed that the envelope structure of these ELM WDs is similar to that of ELM WDs with the same mass from RL channel. If we overestimate the envelope thickness of ELM WDs from the CE channel, then some systems would not be detected by optical telescope, due to the large surface gravity and low luminosity (Calcaferro et al 2017). The combination of the EM and GW observations are expect to give a constraint of the envelope structure of ELM WDs from the CE channel.…”