20 research outputs found
Uncertainties in the 12C+12C reaction rate and their impact on the composition of ultra-massive WDs
Stars with initial masses 7 Msun . MZAMS . 9 Msun reach temperatures high
enough to ignite C under degenerate conditions after the end of He-core burning
(Garcia-Berro & Iben 1994). These isolated stars are expected to evolve into
the so-called super AGB (SAGB) phase and may end their lives as ultra-massive
ONe WDs (see Siess 2006,2007, 2010; Camisassa et al. 2019, and references
therein). The exact proportions of O and Ne found in the core at the end of the
SAGB phase will determine the cooling times and pulsational properties of these
WDs. Uncertainties affecting the rates of nuclear reactions occurring during
the C burning phase should have a measurable impact on the distribution of 16O,
20Ne, 23Na and 24Mg and, consequently, on the evolution of the WD. Here we
present a study of the impact of uncertainties in the 12C(12C, {\alpha})20Ne
and 12C(12C, p)23Na nuclear reaction rates (and their branching ratios) on the
chemical structure of intermediate- to high-mass progenitors at the end of the
C-burning phase. Using the stellar evolution code Modules for Experiments in
Stellar Astrophysics (MESA) we computed evolutionary sequences for stars with
initial masses 7.25<= MZAMS /Msun <=8.25, from the ZAMS to the SAGB phase,
adopting different prescriptions for the 12C+12C burning rates. We found that
adopting lower reaction rates for the 12C+12C burning delays C-ignition by at
most 2700 yrs, and the ignition takes place in a position further from the
center. Our results shows that differences in the 20Ne central abundances
remain modest, below 14%.Comment: Poster presented at the 22nd European Workshop on White Dwarfs. No
proceedings were published at the conferenc
New fully evolutionary models for asteroseismology of ultra-massive white dwarf stars
Ultra-massive hydrogen-rich (DA spectral type) white dwarf (WD) stars
() coming from single-star evolution are expected to
harbor cores made of O and Ne, resulting from semi-degenerate
carbon burning when the progenitor star evolves through the super asymptotic
giant branch (S-AGB) phase. These stars are expected to be crystallized by the
time they reach the ZZ Ceti instability strip ( K).
Theoretical models predict that crystallization leads to a separation of
O and Ne in the core of ultra-massive WDs, which impacts their
pulsational properties. This property offers a unique opportunity to study the
processes of crystallization. Here, we present the first results of a detailed
asteroseismic analysis of the best-studied ultra-massive ZZ Ceti star
BPM~37093. As a second step, we plan to repeat this analysis using
ultra-massive DA WD models with C/O cores in order to study the possibility of
elucidating the core chemical composition of BPM~37093 and shed some light on
its possible evolutionary origin. We also plan to extend this kind of analyses
to other stars observed from the ground and also from space missions like
Kepler and TESS.Comment: 4 pages, 2 tables, 2 figures, poster contribution at the conference
"Stars and their variability observed from space - Celebrating the 5th
anniversary of BRITE-Constellation", Vienna, Austria, August 19 - 23, 2019.
Eds: C. Neiner, W. Weiss, D. Baade, E. Griffin, C. Lovekin, A. Moffa
Probing the Structure of Kepler ZZ Ceti Stars with Full Evolutionary Models-based Asteroseismology
We present an asteroseismological analysis of four ZZ Ceti stars observed with the Kepler spacecraft: GD 1212, SDSS J113655.17+040952.6, KIC 11911480, and KIC 4552982, based on a grid of full evolutionary models of DA white dwarf (WD) stars. We employ a grid of carbon-oxygen core models, characterized by a detailed and consistent chemical inner profile for the core and the envelope. In addition to the observed periods, we take into account other information from the observational data, such as amplitudes, rotational splittings, and period spacing, as well as photometry and spectroscopy. For each star, we present an asteroseismological model that closely reproduces their observed properties. The asteroseismological stellar mass and effective temperature of the target stars are (0.632 ± 0.027 M, 10737 ± 73 K) for GD 1212, (0.745 ± 0.007 M, 11110 ± 69 K) for KIC 4552982, (05480 ± 0.01 M, 12,721 ± 228 K) for KIC11911480, and (0.570 ± 0.01 M, 12,060 ± 300 K) for SDSS J113655.17+040952.6. In general, the asteroseismological values are in good agreement with the spectroscopy. For KIC 11911480 and SDSS J113655.17+040952.6 we derive a similar seismological mass, but the hydrogen envelope is an order of magnitude thinner for SDSS J113655.17+040952.6, which is part of a binary system and went through a common envelope phase
Probing the Structure of Kepler ZZ Ceti Stars with Full Evolutionary Models-based Asteroseismology
We present an asteroseismological analysis of four ZZ Ceti stars observed with the Kepler spacecraft: GD 1212, SDSS J113655.17+040952.6, KIC 11911480, and KIC 4552982, based on a grid of full evolutionary models of DA white dwarf (WD) stars. We employ a grid of carbon–oxygen core models, characterized by a detailed and consistent chemical inner profile for the core and the envelope. In addition to the observed periods, we take into account other information from the observational data, such as amplitudes, rotational splittings, and period spacing, as well as photometry and spectroscopy. For each star, we present an asteroseismological model that closely reproduces their observed properties. The asteroseismological stellar mass and effective temperature of the target stars are (0.632 0.027 ± M☉, 10737 ± 73 K) for GD 1212, (0.745 0.007 ± M☉, 11110 ± 69 K) for KIC 4552982, (0.5480 0.01 ± M☉, 12,721 ± 228 K) for KIC11911480, and (0.570 0.01 ± M☉, 12,060 ± 300 K) for SDSS J113655.17+040952.6. In general, the asteroseismological values are in good agreement with the spectroscopy. For KIC 11911480 and SDSS J113655.17+040952.6 we derive a similar seismological mass, but the hydrogen envelope is an order of magnitude thinner for SDSS J113655.17+040952.6, which is part of a binary system and went through a common envelope phase
Asteroseismology of ZZ Ceti stars with full evolutionary white dwarf models
Context. The thermally pulsing phase on the asymptotic giant branch (TP-AGB) is the last nuclear burning phase experienced by most low- and intermediate-mass stars. During this phase, the outer chemical stratification above the C/O core of the emerging white dwarf (WD) is built up. The chemical structure resulting from progenitor evolution strongly impacts the whole pulsation spectrum exhibited by ZZ Ceti stars, which are pulsating C/O core white dwarfs located on a narrow instability strip at Teff ~ 12 000 K. Several physical processes occurring during progenitor evolution strongly affect the chemical structure of these stars; those found during the TP-AGB phase are the most relevant for the pulsational properties of ZZ Ceti stars.
Aims. We present a study of the impact of the chemical structure built up during the TP-AGB evolution on the stellar parameters inferred from asteroseismological fits of ZZ Ceti stars.
Methods. Our analysis is based on a set of carbon–oxygen core white dwarf models with masses from 0.534 to 0.6463 M⊙ derived from full evolutionary computations from the ZAMS to the ZZ Ceti domain. We computed evolutionary sequences that experience different number of thermal pulses (TP).
Results. We find that the occurrence or not of thermal pulses during AGB evolution implies an average deviation in the asteroseimological effective temperature of ZZ Ceti stars of at most 8% and on the order of ≲5% in the stellar mass. For the mass of the hydrogen envelope, however, we find deviations up to 2 orders of magnitude in the case of cool ZZ Ceti stars. Hot and intermediate temperature ZZ Ceti stars show no differences in the hydrogen envelope mass in most cases.
Conclusions. Our results show that, in general, the impact of the occurrence or not of thermal pulses in the progenitor stars is not negligible and must be taken into account in asteroseismological studies of ZZ Ceti stars