787 research outputs found
Pulsating white dwarf stars and asteroseismology
At present, a large number of pulsating white dwarf (WD) stars is being
discovered either from Earth-based surveys such as the Sloan Digital Sky
Survey, or through observations from space (e.g., the Kepler mission). The
asteroseismological techniques allow us to infer details of internal chemical
stratification, the total mass, and even the stellar rotation profile. In this
paper, we first describe the basic properties of WD stars and their pulsations,
as well as the different sub-types of these variables known so far.
Subsequently, we describe some recent findings about pulsating low-mass WDs.Comment: 10 pages, 4 figures. To be published in the proceedings of the "THIRD
CONFERENCE ON STELLAR ASTROPHYSICS" to honor Prof. Dr. Juan J. Clari\'a, June
21 st to 24rd, 2016, C\'ordoba, Argentina, AAA Workshop Series, Vol. 9, 201
On the systematics of asteroseismological mass determinations of PG1159 stars
We analyze systematics in the asteroseismological mass determination methods
in pulsating PG 1159 stars. We compare the seismic masses resulting from the
comparison of the observed mean period spacings with the usually adopted
asymptotic period spacings, and the average of the computed period spacings.
Computations are based on full PG1159 evolutionary models with stellar masses
ranging from 0.530 to 0.741 Mo that take into account the complete evolution of
progenitor stars. We conclude that asteroseismology is a precise and powerful
technique that determines the masses to a high internal accuracy, but it
depends on the adopted mass determination method. In particular, we find that
in the case of pulsating PG 1159 stars characterized by short pulsation
periods, like PG 2131+066 and PG 0122+200, the employment of the asymptotic
period spacings overestimates the stellar mass by about 0.06 Mo as compared
with inferences from the average of the period spacings. In this case, the
discrepancy between asteroseismological and spectroscopical masses is markedly
reduced when use is made of the mean period spacing instead of the asymptotic
period spacing.Comment: 7 pages, 4 figures, 1 table. To be published in Astronomy and
Astrophysic
Asteroseismology of the Kepler V777 Her variable white dwarf with fully evolutionary models
DBV stars are pulsating white dwarfs with atmospheres rich in He.
Asteroseismology of DBV stars can provide valuable clues about the origin,
structure and evolution of hydrogen-deficient white dwarfs, and may allow to
study neutrino and axion physics. Recently, a new DBV star, KIC 8626021, has
been discovered in the field of the \emph{Kepler} spacecraft. It is expected
that further monitoring of this star in the next years will enable astronomers
to determine its detailed asteroseismic profile. We perform an
asteroseismological analysis of KIC 8626021 on the basis of fully evolutionary
DB white-dwarf models. We employ a complete set of evolutionary DB white-dwarf
structures covering a wide range of effective temperatures and stellar masses.
They have been obtained on the basis of a complete treatment of the
evolutionary history of progenitors stars. We compute g-mode adiabatic
pulsation periods for this set of models and compare them with the pulsation
properties exhibited by KIC 8626021. On the basis of the mean period spacing of
the star, we found that the stellar mass should be substantially larger than
spectroscopy indicates. From period-to-period fits we found an
asteroseismological model characterized by an effective temperature much higher
than the spectroscopic estimate. In agreement with a recent asteroseismological
analysis of this star by other authors, we conclude that KIC 8626021 is located
near the blue edge of the DBV instability strip, contrarily to spectroscopic
predictions. We also conclude that the mass of KIC 8626021 should be
substantially larger than thought.Comment: 7 pages, 5 figures, 3 tables. To be published in Astronomy and
Astrophysic
Probing the internal rotation of pre-white dwarf stars with asteroseismology: the case of PG 122+200
We put asteroseismological constraints on the internal rotation profile of
the GW Vir (PG1159-type) star PG 0122+200. To this end we employ a
state-of-the-art asteroseismological model for this star and we assess the
expected frequency splittings induced by rotation adopting a forward approach
in which we compare the theoretical frequency separations with the observed
ones assuming different types of plausible internal rotation profiles. We also
employ two asteroseismological inversion methods for the inversion of the
rotation profile of PG 0122+200. We find evidence for differential rotation in
this star. We demonstrate that the frequency splittings of the rotational
multiplets exhibited by PG 0122+200 are compatible with a rotation profile in
which the central regions are spinning about 2.4 times faster than the stellar
surface.Comment: 8 pages, 6 figures, 2 tables. To be published in MNRA
Asteroseismological constraints on the coolest GW Vir variable star (PG 1159-type)PG 0122+200
We present an asteroseismological study on PG 0122+200, the coolest known
pulsating PG1159 (GW Vir) star. Our results are based on an augmented set of
the full PG1159 evolutionary models recently presented by Miller Bertolami &
Althaus (2006). We perform extensive computations of adiabatic g-mode pulsation
periods on PG1159 evolutionary models with stellar masses ranging from 0.530 to
0.741 Msun. We derive a stellar mass of 0.626 Msun from a comparison between
the observed period spacing and the computed asymptotic period spacing, and a
stellar mass of 0.567 Msun by comparing the observed period spacing with the
average of the computed period spacing. We also find, on the basis of a
period-fit procedure, an asteroseismological model representative of PG
0122+200 which is able to reproduce the observed period pattern with an average
of the period differences of 0.88 s. The model has an effective temperature of
81500 K, a stellar mass of 0.556 Msun, a surface gravity log g= 7.65, a stellar
luminosity and radius of log(L/Lsun)= 1.14 and log(R/Rsun)= -1.73,
respectively, and a He-rich envelope thickness of Menv= 0.019 Msun. We derive a
seismic distance of about 614 pc and a parallax of about 1.6 mas. The results
of the period-fit analysis carried out in this work suggest that the
asteroseismological mass of PG 0122+200 could be 6-20 % lower than thought
hitherto and in closer agreement (to within 5 %) with the spectroscopic mass.
This result suggests that a reasonable consistency between the stellar mass
values obtained from spectroscopy and asteroseismology can be expected when
detailed PG1159 evolutionary models are considered.Comment: 10 pages, 6 figures. To be published in Astronomy & Astrophysic
Asteroseismological constraints on the pulsating planetary nebula nucleus (PG1159-type) RX J2117.1+3412
We present asteroseismological inferences on RX J2117.1+3412, the hottest
known pulsating PG1159 star. Our results are based on full PG1159 evolutionary
models recently presented by Miller Bertolami & Althaus (2006). We performed
extensive computations of adiabatic g-mode pulsation periods on PG1159
evolutionary models with stellar masses ranging from 0.530 to 0.741 Mo. PG1159
stellar models are extracted from the complete evolution of progenitor stars
started from the ZAMS, through the thermally pulsing AGB and born-again phases
to the domain of the PG 1159 stars. We constrained the stellar mass of RX
J2117.1+3412 by comparing the observed period spacing with the asymptotic
period spacing and with the average of the computed period spacings. We also
employed the individual observed periods to find a representative seismological
model. We derive a stellar mass of 0.56-0.57 Mo from the period spacing data
alone. In addition, we found a best-fit model representative for RX
J2117.1+3412 with an effective temperature of 163,400 K, a stellar mass of
0.565 Mo, and a surface gravity log g= 6.61. The derived stellar luminosity and
radius are log(L/Lo)= 3.36 and log(R/Ro)= -1.23, respectively, and the He-rich
envelope thickness is Menv= 0.02 Mo. We derive a seismic distance of 452 pc and
a linear size of the planetary nebula of 1.72 pc. These inferences seem to
solve the discrepancy between the RX J2117.1+3412 evolutionary timescale and
the size of the nebula. All of the seismological tools we use concur to the
conclusion that RX J2117.1+3412 must have a stellar mass of 0.565 Mo much in
agreement with recent asteroseismology studies and in clear conflict with the
predictions of spectroscopy plus evolutionary tracks.Comment: 10 pages, 6 figures, 2 tables. Accepted for publication in Astronomy
and Astrophysics. Erratum available as a separate fil
Asteroseismology of the GW Virginis stars SDSS J0349-0059 and VV 47
We present an asteroseismological study of SDSS J0349-0059 and VV 47 aimed
mainly at deriving their total mass on the basis of state-of-the-art PG 1159
evolutionary models. We compute adiabatic nonradial -mode pulsation periods
for PG 1159 evolutionary models with stellar masses ranging from to
0.741\ M_{\sun}, that take into account the complete evolution of the
progenitor stars. We first estimate a mean period spacing for both SDSS
J0349-0059 and VV 47. By comparing the observed period spacing with the
asymptotic period spacing we obtain M_{\star}\sim 0.569\ M_{\sun} for SDSS
J0349-0059 and M_{\star}\sim 0.523\ M_{\sun} for VV 47. If we compare the
observed period spacing with the average of the computed period spacings we
found M_{\star}\sim 0.535\ M_{\sun} for SDSS J0349-0059 and M_{\star}\sim
0.528 M_{\sun} for VV 47. Searching for the best period fit we found, in the
case of SDSS J0349-0059, an asteroseismological model with $M_{\star}= 0.542\
M_{\sun}T_{\rm eff}= 91\, 255\ P_{\rm rot}= 1/\Omega \sim 0.407$
days. The results presented in this work constitute a further step in the study
of GW Vir stars through asteroseismology in the frame of fully evolutionary
models of PG 1159 stars. In particular, once again it is shown the potential of
asteroseismology to derive stellar masses of PG 1159 stars with an
unprecedented precision.Comment: 13 pages, 16 figures, 6 tables. To be published in Astronomy and
Astrophysic
New evolutionary sequences for extremely low mass white dwarfs: Homogeneous mass and age determinations, and asteroseismic prospects
We provide a fine and homogeneous grid of evolutionary sequences for He-core
white dwarfs with masses 0.15-0.45 Msun, including the mass range for ELM white
dwarfs (<0.20Msun). The grid is appropriate for mass and age determination, and
to study their pulsational properties. White dwarf sequences have been computed
by performing full evolutionary calculations that consider the main energy
sources and processes of chemical abundance changes during white dwarf
evolution. Initial models for the evolving white dwarfs have been obtained by
computing the non-conservative evolution of a binary system consisting of a
Msun ZAMS star and a 1.4 Msun neutron star for various initial orbital periods.
To derive cooling ages and masses for He-core white dwarf we perform a least
square fitting of the M(Teff, g) and Age(Teff, g) relations provided by our
sequences by using a scheme that takes into account the time spent by models in
different regions of the Teff-g plane. This is useful when multiple solutions
for cooling age and mass determinations are possible in the case of
CNO-flashing sequences. We also explore the adiabatic pulsational properties of
models near the critical mass for the development of CNO flashes (~0.2 Msun).
This is motivated by the discovery of pulsating white dwarfs with stellar
masses near this threshold value. We obtain reliable and homogeneous mass and
cooling age determinations for 58 very low-mass white dwarfs, including 3
pulsating stars. Also, we find substantial differences in the period spacing
distributions of g-modes for models with stellar masses ~ 0.2 Msun, which could
be used as a seismic tool to distinguish stars that have undergone CNO flashes
in their early cooling phase from those that have not. Finally, for an easy
application of our results, we provide a reduced grid of values useful to
obtain masses and ages of He-core white dwarf.Comment: 12 pages, 9 figures, to be published in Astronomy and Astrophysic
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