Asteroseismological Study of Massive Zz Ceti Stars With Fully Evolutionary Models

Abstract

We present the first asteroseismological study for 42 massive ZZ Ceti stars based on a large set of fully evolutionary carbon−oxygen core DA white dwarf models characterized by a detailed and consistent chemical inner profile for the core and the envelope. Our sample comprise all the ZZ Ceti stars with spectroscopic stellar masses between 0.72 and 1.05M ⊙ known to date. The asteroseismological analysis of a set of 42 stars gives the possibility to study the ensemble properties of the massive pulsating white dwarf stars with carbon−oxygen cores, in particular the thickness of the hydrogen envelope and the stellar mass. A significant fraction of stars in our sample have stellar mass high enough as to crystallize at the effective temperatures of the ZZ Ceti instability strip, which enables us to study the effects of crystallization on the pulsation properties of these stars. Our results show that the phase diagram presented in Horowitz et al. (2010) seems to be a good representation of the crystallization process inside white dwarf stars, in agreement with the results from white dwarf luminosity function in globular clusters.

Four new massive pulsating white dwarfs including an ultramassive DAV

“…For example, several DAVs presented in Castanheira et al (2013) and Romero et al (2013) have dominant periods of pulsation with amplitudes lower than 5 mma. In addition, J0840, a newly discovered DAV in our sample, is another example of a ZZ Ceti WD with two low amplitude pulsations with A < 7 mma which is below the computed detection threshold for both J0116 and J0446.…”

“…The DA WD models used in this work were generated using LPCODE evolutionary code (see Renedo et al (2010) oxygen core. The sequences with stellar mass larger than 1.08 M were obtained by artificially scaling the stellar mass from the ∼ 1M sequence at high effective temperatures, and the remaining ones are those presented in Romero et al (2013).…”

“…Crystallization processes and the additional energy sources are included following the Horowitz, Schnieder, & Berry (2010) phase diagram. Note that the effective temperature where crystallization begins increases with stellar mass (Romero et al 2013). For instance, crystallization starts at 14 500 K for 1.05 M and at 17 250 K for 1.14 M .…”

Triennial Report (2012-2015): The Legacy Issue

“…For compact pulsators, the Sloan Digital Sky Survey has been the tool to find and characterize new white dwarf and subdwarf stars, and led to the discovery of more than 200 new pulsators, mainly DAVs, also called ZZ Cetis (Mukadam et al 2004;Mullally et al 2005;Kepler et al 2005a;Castanheira et al 2006Castanheira et al , 2007, and their modelling (Castanheira & Kepler 2008, 2009Romero et al 2012Romero et al , 2013, but also DBVs Nitta et al (2009) and a few DOVs, also called variable PG 1159 or GW Vir stars (Quirion 2009a;Quirion, Fontaine & Brassard 2009b;Woudt, Warner,& Zietsman 2012;Kepler et al 2014). Two new classes of white dwarf pulsators were found, the DQVs (Montgomery et al 2008;Fontaine et al 2009;Williams et al 2013) and the ELMVs Hermes et al (2013a,b); Van Grootel et al (2012, 2013a; Córsico & Althaus (2014).…”

Revealing the pulsational properties of the V777 Herculis star KUV 05134+2605 by its long-term monitoring

“…The DB white dwarf model that shows the lowest value of χ 2 is adopted as the bestfit model. Our asteroseismological approach combines (i) a significant exploration of the parameter space; and (ii) detailed and updated input physics, in particular, regarding the internal structure, which is a crucial aspect for correctly separating the information encoded in the pulsation patterns of variable white dwarfs (see Córsico et al 2007aCórsico et al ,b, 2008Córsico et al , 2009aCórsico et al , 2012Romero et al 2012Romero et al , 2013. We evaluated the function χ 2 (M * , T eff ) for stellar masses of 0.515, 0.530, 0.542, 0.565, 0.584, 0.609, 0.664, 0.741, and 0.870 M .…”