24 research outputs found
Pulsations powered by hydrogen shell burning in white dwarfs
In the absence of a third dredge-up episode during the asymptotic giant
branch phase, white dwarf models evolved from low-metallicity progenitors have
a thick hydrogen envelope, which makes hydrogen shell burning be the most
important energy source. We investigate the pulsational stability of white
dwarf models with thick envelopes to see whether nonradial -mode pulsations
are triggered by hydrogen burning, with the aim of placing constraints on
hydrogen shell burning in cool white dwarfs and on a third dredge-up during the
asymptotic giant branch evolution of their progenitor stars. We construct
white-dwarf sequences from low-metallicity progenitors by means of full
evolutionary calculations, and analyze their pulsation stability for the models
in the range of effective temperatures
K. We demonstrate that, for white dwarf models with masses M_{\star} \lesssim
0.71\,\rm M_{\sun} and effective temperatures K that evolved from low-metallicity progenitors (,
, and ) the dipole () and quadrupole ()
modes are excited mostly due to the hydrogen-burning shell through the
-mechanism, in addition to other modes driven by either the
or the convective driving mechanism. However, the
mechanism is insufficient to drive these modes in white dwarfs evolved from
solar-metallicity progenitors. We suggest that efforts should be made to
observe the dipole mode in white dwarfs associated with low-metallicity
environments, such as globular clusters and/or the galactic halo, to place
constraints on hydrogen shell burning in cool white dwarfs and the third
dredge-up episode during the preceding asymptotic giant branch phase.Comment: 6 pages, 4 figures, 1 table. To be published in Astronomy and
Astrophysic
White dwarf evolutionary sequences for low-metallicity progenitors: The impact of third dredge-up
We present new white dwarf evolutionary sequences for low-metallicity
progenitors. White dwarf sequences have been derived from full evolutionary
calculations that take into account the entire history of progenitor stars,
including the thermally-pulsing and the post-asymptotic giant branch phases. We
show that for progenitor metallicities in the range 0.00003--0.001, and in the
absence of carbon enrichment due to the occurrence of a third dredge-up
episode, the resulting H envelope of the low-mass white dwarfs is thick enough
to make stable H burning the most important energy source even at low
luminosities. This has a significant impact on white dwarf cooling times. This
result is independent of the adopted mass-loss rate during the
thermally-pulsing and post-AGB phases, and the planetary nebulae stage. We
conclude that in the absence of third dredge-up episodes, a significant part of
the evolution of low-mass white dwarfs resulting from low-metallicity
progenitors is dominated by stable H burning. Our study opens the possibility
of using the observed white dwarf luminosity function of low-metallicity
globular clusters to constrain the efficiency of third dredge up episodes
during the thermally-pulsing AGB phase of low-metallicity progenitors.Comment: To be published in Astronomy and Astrophysics. 12 pages, 11 figure
The Gaia DR2 halo white dwarf population: the luminosity function, mass distribution and its star formation history
We analyze the volume-limited nearly complete 100 pc sample of 95 halo white
dwarf candidates identified by the second data release of Gaia. Based on a
detailed population synthesis model, we apply a method that relies on Gaia
astrometry and photometry to accurately derive the individual white dwarf
parameters (mass, radius, effective temperature, bolometric luminosity and
age). This method is tested with 25 white dwarfs of our sample for which we
took optical spectra and performed spectroscopic analysis. We build and analyse
the halo white dwarf luminosity function, for which we find for the first time
possible evidences of the cut-off at its faintest end, leading to an age
estimate of Gyr. The mass distribution of the sample peaks at
, with of the white dwarf masses below
and just two massive white dwarfs of more than
. From the age distribution we find three white dwarfs with
total ages above 12 Gyr, of which J1312-4728 is the oldest white dwarf known
with an age of Gyr. We prove that the star formation history is
mainly characterised by a burst of star formation that occurred from 10 to 12
Gyr in the past, but extended up to 8 Gyr. We also find that the peak of the
star formation history is centered at around 11 Gyr, which is compatible with
the current age of the Gaia-Enceladus encounter. Finally, of our halo
sample is contaminated by high-speed young objects (total age<7 Gyr). The
origin of these white dwarfs is unclear but their age distribution may be
compatible with the encounter with the Sagittarius galaxy.Comment: 15 pages, 9 figures, 2 tables; accepted for publication in MNRA
The effect of Ne diffusion in the evolution and pulsational properties of white dwarfs with solar metallicity progenitors
Because of the large neutron excess of Ne, this isotope rapidly
sediments in the interior of the white dwarfs. This process releases an
additional amount of energy, thus delaying the cooling times of the white
dwarf. This influences the ages of different stellar populations derived using
white dwarf cosmochronology. Furthermore, the overabundance of Ne in the
inner regions of the star, modifies the Brunt-V\"ais\"al\"a frequency, thus
altering the pulsational properties of these stars. In this work, we discuss
the impact of Ne sedimentation in white dwarfs resulting from Solar
metallicity progenitors (). We performed evolutionary calculations of
white dwarfs of masses , , and M_{\sun},
derived from full evolutionary computations of their progenitor stars, starting
at the Zero Age Main Sequence all the way through central hydrogen and helium
burning, thermally-pulsing AGB and post-AGB phases. Our computations show that
at low luminosities (\log(L/L_{\sun})\la -4.25), Ne sedimentation
delays the cooling of white dwarfs with Solar metallicity progenitors by about
1~Gyr. Additionally, we studied the consequences of Ne sedimentation on
the pulsational properties of ZZ~Ceti white dwarfs. We find that Ne
sedimentation induces differences in the periods of these stars larger than the
present observational uncertainties, particularly in more massive white dwarfs.Comment: Accepted for publication in ApJ. 8 pages, 6 figure
White dwarf evolutionary sequences for low-metallicity progenitors: The impact of third dredge-up
Context. White dwarfs are nowadays routinely used as reliable cosmochronometers, allowing several stellar populations to be dated.
Aims. We present new white dwarf evolutionary sequences for low-metallicity progenitors. This is motivated by the recent finding that residual H burning in low-mass white dwarfs resulting from Z = 0.0001 progenitors is the main energy source over a significant part of their evolution.
Methods. White dwarf sequences have been derived from full evolutionary calculations that take the entire history of progenitor stars into account, including the thermally pulsing and the post-asymptotic giant branch (AGB) phases.
Results. We show that for progenitor metallicities in the range 0.00003 ≲ Z ≲ 0.001, and in the absence of carbon enrichment from the occurrence of a third dredge-up episode, the resulting H envelope of the low-mass white dwarfs is thick enough to make stable H burning the most important energy source even at low luminosities. This has a significant impact on white dwarf cooling times. This result is independent of the adopted mass-loss rate during the thermally-pulsing and post-AGB phases and in the planetary nebulae stage.
Conclusions. We conclude that in the absence of third dredge-up episodes, a significant part of the evolution of low-mass white dwarfs resulting from low-metallicity progenitors is dominated by stable H burning. Our study opens the possibility of using the observed white dwarf luminosity function of low-metallicity globular clusters to constrain the efficiency of third dredge up episodes during the thermally-pulsing AGB phase of low-metallicity progenitors.Facultad de Ciencias Astronómicas y GeofÃsicasInstituto de AstrofÃsica de La Plat
Carbon-oxygen ultra-massive white dwarfs in general relativity
We employ the La Plata stellar evolution code, LPCODE, to compute the first
set of constant rest-mass carbon-oxygen ultra-massive white dwarf evolutionary
sequences for masses higher than 1.29 Msun that fully take into account the
effects of general relativity on their structural and evolutionary properties.
In addition, we employ the LP-PUL pulsation code to compute adiabatic g-mode
Newtonian pulsations on our fully relativistic equilibrium white dwarf models.
We find that carbon-oxygen white dwarfs more massive than 1.382 Msun become
gravitationally unstable with respect to general relativity effects, being this
limit higher than the 1.369 Msun we found for oxygen-neon white dwarfs. As the
stellar mass approaches the limiting mass value, the stellar radius becomes
substantially smaller compared with the Newtonian models. Also, the
thermo-mechanical and evolutionary properties of the most massive white dwarfs
are strongly affected by general relativity effects. We also provide magnitudes
for our cooling sequences in different passbands. Finally, we explore for the
first time the pulsational properties of relativistic ultra-massive white
dwarfs and find that the period spacings and oscillation kinetic energies are
strongly affected in the case of most massive white dwarfs. We conclude that
the general relativity effects should be taken into account for an accurate
assessment of the structural, evolutionary, and pulsational properties of white
dwarfs with masses above 1.30 Msun.Comment: 12 pages, 12 figures, accepted for publication in MNRAS. arXiv admin
note: text overlap with arXiv:2208.1414
General relativistic pulsations of ultra-massive ZZ Ceti stars
Ultra-massive white dwarf stars are currently being discovered at a
considerable rate, thanks to surveys such as the {\it Gaia} space mission.
These dense and compact stellar remnants likely play a major role in type Ia
supernova explosions. It is possible to probe the interiors of ultra-massive
white dwarfs through asteroseismology. In the case of the most massive white
dwarfs, General Relativity could affect their structure and pulsations
substantially. In this work, we present results of relativistic pulsation
calculations employing relativistic ultra-massive ONe-core white dwarf models
with hydrogen-rich atmospheres and masses ranging from to with the aim of assessing the impact of General Relativity on the
adiabatic gravity ()-mode period spectrum of very-high mass ZZ Ceti stars.
Employing the relativistic Cowling approximation for the pulsation analysis, we
find that the critical buoyancy (Brunt-V\"ais\"al\"a) and acoustic (Lamb)
frequencies are larger for the relativistic case, compared to the Newtonian
case, due to the relativistic white dwarf models having smaller radii and
higher gravities for a fixed stellar mass. In addition, the -mode periods
are shorter in the relativistic case than in the Newtonian computations, with
relative differences of up to \% for the highest-mass models () and for effective temperatures typical of the ZZ Ceti instability
strip. Hence, the effects of General Relativity on the structure, evolution,
and pulsations of white dwarfs with masses larger than
cannot be ignored in the asteroseismological analysis of ultra-massive ZZ Ceti
stars.Comment: 15 pages, 21 figures, 2 tables. Accepted for publication in MNRA
The unusual planetary nebula nucleus in the Galactic open cluster M37 and six further hot white dwarf candidates
Planetary nebulae in Galactic open star clusters are rare objects; only three
are known to date. They are of particular interest because their distance can
be determined with high accuracy, allowing one to characterize the physical
properties of the planetary nebula and its ionizing central star with high
confidence. Here we present the first quantitative spectroscopic analysis of a
central star in an open cluster, namely the faint nucleus of IPHASX
J055226.2323724 in M37. This cluster contains 14 confirmed white dwarf
members, which were previously used to study the initial-to-final-mass relation
of white dwarfs, and six additional white dwarf candidates. We performed an
atmosphere modeling of spectra taken with the 10m Gran Telescopio Canarias. The
central star is a hot hydrogen-deficient white dwarf with an effective
temperature of 90,000 K and spectral type PG1159 (helium- and carbon-rich). We
know it is about to transform into a helium-rich DO white dwarf because the
relatively low atmospheric carbon abundance indicates ongoing gravitational
settling of heavy elements. The star belongs to a group of hot white dwarfs
that exhibit ultrahigh-excitation spectral lines possibly emerging from
shock-heated material in a magnetosphere. We find a relatively high stellar
mass of M. This young white dwarf is
important for the semi-empirical initial-final mass relation because any
uncertainty related to white-dwarf cooling theory is insignificant with respect
to the pre-white-dwarf timescale. Its post-asymptotic-giant-branch age of
yr suggests that the extended planetary nebula is
extraordinarily old. We also performed a spectroscopic analysis of the six
other white dwarf candidates of M37, confirming one as a cluster member.Comment: Accepted for publication in A&
White dwarf evolutionary sequences for low-metallicity progenitors: The impact of third dredge-up
Context. White dwarfs are nowadays routinely used as reliable cosmochronometers, allowing several stellar populations to be dated.
Aims. We present new white dwarf evolutionary sequences for low-metallicity progenitors. This is motivated by the recent finding that residual H burning in low-mass white dwarfs resulting from Z = 0.0001 progenitors is the main energy source over a significant part of their evolution.
Methods. White dwarf sequences have been derived from full evolutionary calculations that take the entire history of progenitor stars into account, including the thermally pulsing and the post-asymptotic giant branch (AGB) phases.
Results. We show that for progenitor metallicities in the range 0.00003 ≲ Z ≲ 0.001, and in the absence of carbon enrichment from the occurrence of a third dredge-up episode, the resulting H envelope of the low-mass white dwarfs is thick enough to make stable H burning the most important energy source even at low luminosities. This has a significant impact on white dwarf cooling times. This result is independent of the adopted mass-loss rate during the thermally-pulsing and post-AGB phases and in the planetary nebulae stage.
Conclusions. We conclude that in the absence of third dredge-up episodes, a significant part of the evolution of low-mass white dwarfs resulting from low-metallicity progenitors is dominated by stable H burning. Our study opens the possibility of using the observed white dwarf luminosity function of low-metallicity globular clusters to constrain the efficiency of third dredge up episodes during the thermally-pulsing AGB phase of low-metallicity progenitors.Facultad de Ciencias Astronómicas y GeofÃsicasInstituto de AstrofÃsica de La Plat
The white dwarf population of NGC 6397
Context. NGC 6397 is one of the most interesting, well-observed, and most thoroughly theoretically studied globular clusters. The existing wealth of observations allows us to study the reliability of the theoretical white dwarf cooling sequences of low-metallicity progenitors, to determine the age of NGC 6397 and the percentage of unresolved binaries. We also assess other important characteristics of the cluster, such as the slope of the initial mass function or the fraction of white dwarfs with hydrogen-deficient atmospheres.
Aims. We present a population synthesis study of the white dwarf population of NGC 6397. In particular, we study the shape of the color–magnitude diagram and the corresponding magnitude and color distributions.
Methods. To do this, we used an advanced Monte Carlo code that incorporates the most recent and reliable cooling sequences and an accurate modeling of the observational biases.
Results. Our theoretical models and the observed data agree well. In particular, we find that this agreement is best for those cooling sequences that take into account residual hydrogen burning. This result has important consequences for the evolution of progenitor stars during the thermally pulsing asymptotic giant branch phase, since it implies that appreciable third dredge-up in low-mass, low-metallicity progenitors is not expected to occur. Using a standard burst duration of 1.0 Gyr, we obtain that the age of the cluster is 12.8+0.50-0.75 Gyr. Greater ages are also compatible with the observed data, but then unrealistic longer durations of the initial burst of star formation are needed to fit the luminosity function.
Conclusions. We conclude that a correct modeling of the white dwarf population of globular clusters, used in combination with the number counts of main-sequence stars, provides a unique tool for modeling the properties of globular clusters