197 research outputs found
The initial-final mass relationship of white dwarfs in common proper motion pairs
A promising approach to decrease the uncertainties in the initial-final mass relationship, which is still poorly constrained, is to study white dwarfs for which external constraints are available, for instance, white dwarfs in common proper motion pairs (CPMPs). Important information of the white dwarf can be inferred from the study of the companion, since they were born at the same time and with the same initial chemical composition. In this contribution, we report new results obtained from spectroscopic observations of both members of several CPMPs composed of a F, G or K type star and a DA white dwarf
The white dwarf cooling sequence of NGC 6791: a unique tool for stellar evolution
NGC 6791 is a well-studied, metal-rich open cluster that is so close to us
that can be imaged down to luminosities fainter than that of the termination of
its white dwarf cooling sequence, thus allowing for an in-depth study of its
white dwarf population. We use a Monte Carlo simulator that employs up-to-date
evolutionary cooling sequences for white dwarfs with hydrogen-rich and
hydrogen-deficient atmospheres, with carbon-oxygen and helium cores. The
cooling sequences for carbon-oxygen cores account for the delays introduced by
both Ne^22 sedimentation in the liquid phase and by carbon-oxygen phase
separation upon crystallization. We do not find evidence for a substantial
fraction of helium-core white dwarfs, and hence our results support the
suggestion that the origin of the bright peak of the white dwarf luminosity
function can only be attributed to a population of unresolved binary white
dwarfs. Moreover, our results indicate that the number distribution of
secondary masses of the population of unresolved binaries has to increase with
increasing mass ratio between the secondary and primary components of the
progenitor system. We also find that the observed cooling sequence appears to
be able to constrain the presence of progenitor sub-populations with different
chemical compositions and the fraction of non-DA white dwarfs. Our simulations
place interesting constraints on important characteristics of the stellar
populations of NGC 6791. In particular, we find that the fraction of single
helium-core white dwarfs must be smaller than 5%, that a sub-population of
stars with zero metallicity must be <12%, while if the adopted metallicity of
the sub-population is solar the upper limit is ~8%. Finally, we also find that
the fraction of non-DA white dwarfs in this particular cluster is surprinsingly
small <6%.Comment: 9 pages, 14 figures, accepted for publication in Astronomy &
Astrophysic
New chemical profiles for the asteroseismology of ZZ Ceti stars
We compute new chemical profiles for the core and envelope of white dwarfs
appropriate for pulsational studies of ZZ Ceti stars. These profiles are
extracted from the complete evolution of progenitor stars, evolved through the
main sequence and the thermally-pulsing asymptotic giant branch (AGB) stages,
and from time-dependent element diffusion during white dwarf evolution. We
discuss the importance of the initial-final mass relationship for the white
dwarf carbon-oxygen composition. In particular, we find that the central oxygen
abundance may be underestimated by about 15% if the white dwarf mass is assumed
to be the hydrogen-free core mass before the first thermal pulse. We also
discuss the importance for the chemical profiles expected in the outermost
layers of ZZ Ceti stars of the computation of the thermally-pulsing AGB phase
and of the phase in which element diffusion is relevant. We find a strong
dependence of the outer layer chemical stratification on the stellar mass. In
particular, in the less massive models, the double-layered structure in the
helium layer built up during the thermally-pulsing AGB phase is not removed by
diffusion by the time the ZZ Ceti stage is reached. Finally, we perform
adiabatic pulsation calculations and discuss the implications of our new
chemical profiles for the pulsational properties of ZZ Ceti stars. We find that
the whole mode period spectrum and the mode-trapping properties of these
pulsating white dwarfs as derived from our new chemical profiles are
substantially different from those based on chemical profiles widely used in
existing asteroseismological studies. Thus, we expect the asteroseismological
models derived from our chemical profiles to be significantly different from
those found thus far.Comment: 12 pages, 11 figures, 1 table. To be published in Ap
Evolution of white dwarf stars with high-metallicity progenitors: the role of 22Ne diffusion
Motivated by the strong discrepancy between the main sequence turn-off age
and the white dwarf cooling age in the metal-rich open cluster NGC 6791, we
compute a grid of white dwarf evolutionary sequences that incorporates for the
first time the energy released by the processes of 22Ne sedimentation and of
carbon/oxygen phase separation upon crystallization. The grid covers the mass
range from 0.52 to 1.0 Msun, and it is appropriate for the study of white
dwarfs in metal-rich clusters. The evolutionary calculations are based on a
detailed and self-consistent treatment of the energy released from these two
processes, as well as on the employment of realistic carbon/oxygen profiles, of
relevance for an accurate evaluation of the energy released by carbon/oxygen
phase separation. We find that 22Ne sedimentation strongly delays the cooling
rate of white dwarfs stemming from progenitors with high metallicities at
moderate luminosities, whilst carbon/oxygen phase separation adds considerable
delays at low luminosities. Cooling times are sensitive to possible
uncertainties in the actual value of the diffusion coefficient of 22Ne.
Changing the diffusion coefficient by a factor of 2, leads to maximum age
differences of approx. 8-20% depending on the stellar mass. We find that the
magnitude of the delays resulting from chemical changes in the core is
consistent with the slow down in the white dwarf cooling rate that is required
to solve the age discrepancy in NGC 6791.Comment: 10 pages, 6 figures, to be published in The Astrophysical Journa
An upper limit to the secular variation of the gravitational constant from white dwarf stars
A variation of the gravitational constant over cosmological ages modifies the
main sequence lifetimes and white dwarf cooling ages. Using an state-of-the-art
stellar evolutionary code we compute the effects of a secularly varying G on
the main sequence ages and, employing white dwarf cooling ages computed taking
into account the effects of a running G, we place constraints on the rate of
variation of Newton's constant. This is done using the white dwarf luminosity
function and the distance of the well studied open Galactic cluster NGC 6791.
We derive an upper bound G'/G ~ -1.8 10^{-12} 1/yr. This upper limit for the
secular variation of the gravitational constant compares favorably with those
obtained using other stellar evolutionary properties, and can be easily
improved if deep images of the cluster allow to obtain an improved white dwarf
luminosity function.Comment: 15 pages, 4 figures, accepted for publication in JCA
The impact of chemical differentiation of white dwarfs on thermonuclear supernovae
Gravitational settling of 22Ne in cooling white dwarfs can affect the outcome
of thermonuclear supernovae. We investigate how the supernova energetics and
nucleosynthesis are in turn influenced by this process. We use realistic
chemical profiles derived from state-of-the-art white dwarf cooling sequences.
The cooling sequences provide a link between the white dwarf chemical structure
and the age of the supernova progenitor system. The cooling sequence of a 1
M_sun white dwarf was computed until freezing using an up-to-date stellar
evolutionary code. We computed explosions of both Chandrasekhar mass and
sub-Chandrasekhar mass white dwarfs, assuming spherical symmetry and neglecting
convective mixing during the pre-supernova carbon simmering phase to maximize
the effects of chemical separation. Neither gravitational settling of 22Ne nor
chemical differentiation of 12C and 16O have an appreciable impact on the
properties of Type Ia supernovae, unless there is a direct dependence of the
flame properties (density of transition from deflagration to detonation) on the
chemical composition. At a fixed transition density, the maximum variation in
the supernova magnitude obtained from progenitors of different ages is ~0.06
magnitudes, and even assuming an unrealistically large diffusion coefficient of
22Ne it would be less than ~0.09 mag. However, if the transition density
depends on the chemical composition (all other things being equal) the oldest
SNIa can be as much as 0.4 magnitudes brighter than the youngest ones (in our
models the age difference is 7.4 Gyr). In addition, our results show that 22Ne
sedimentation cannot be invoked to account for the formation of a central core
of stable neutron-rich Fe-group nuclei in the ejecta of sub-Chandrasekhar
models, as required by observations of Type Ia supernovae.Comment: 8 pages, 8 figures, 3 tables, accepted for Astronomy and
Astrophysics. Revised version with corrected typo
Optimization of time data codification and transmission schemes: Application to Gaia
Gaia is an ambitious space observatory devoted to obtain the largest and most precise astrometric catalogue of astronomical objects from our Galaxy and beyond. On-board processing and transmission of the huge amount of data generated by the instruments is one of its several technological challenges. The measurement time tags are critical for the scientific results of the mission, so they must be measured and transmitted with the highest precision - leading to an important telemetry channel occupation. In this paper we present the optimisation of time data, which has resulted in a useful software tool. We also present how time data is adapted to the Packet Telemetry standard. The several communication layers are illustrated and a method for coding and transmitting the relevant data is described as well. Although our work is focused on Gaia, the timing scheme and the corresponding tools can be applied to any other instrument or mission with similar operational principles
The cooling of CO white dwarfs: influence of the internal chemical distribution
In this paper we compute detailed evolutionary models providing chemical
profiles for white dwarfs having progenitors in the mass range from 1.0 to 7
M_{\sun} and we examine the influence of such profiles in the cooling process.
The influence of the process of separation of carbon and oxygen during
crystallization is decreased as a consequence of the initial stratification,
but it is still important and cannot be neglected. As an example, the best fit
to the luminosity functions of Liebert et al. (1988) and Oswalt et al. (1996)
gives and age of the disk of 9.3 and 11.0 Gyr, respectively, when this effect
is taken into account, and only 8.3 and 10.0 Gyrs when it is neglected.Comment: Accepted for publication in ApJ (26 pages, 7 figures, aasms4
Interpolation of the magnetic field at the test masses in eLISA
A feasible design for a magnetic diagnostics subsystem for eLISA will be based on that of its precursor mission, LISA Pathfinder. Previous experience indicates that magnetic field estimation at the positions of the test masses has certain complications. This is due to two reasons. The first is that magnetometers usually back-act due to their measurement principles (i.e., they also create their own magnetic fields), while the second is that the sensors selected for LISA Pathfinder have a large size, which conflicts with space resolution and with the possibility of having a sufficient number of them to properly map the magnetic field around the test masses. However, high-sensitivity and small-sized sensors that significantly mitigate the two aforementioned limitations exist, and have been proposed to overcome these problems. Thus, these sensors will be likely selected for the magnetic diagnostics subsystem of eLISA. Here we perform a quantitative analysis of the new magnetic subsystem, as it is currently conceived, and assess the feasibility of selecting these sensors in the final configuration of the magnetic diagnostic subsystem.Peer ReviewedPostprint (author's final draft
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