605 research outputs found
Hydrogen model atmospheres for white dwarf stars
We present a detailed calculation of model atmospheres for DA white dwarfs.
Our atmosphere code solves the atmosphere structure in local thermodynamic
equilibrium with a standard partial linearization technique, which takes into
account the energy transfer by radiation and convection. This code incorporates
recent improved and extended data base of collision induced absorption by
molecular hydrogen. We analyse the thermodynamic structure and emergent flux of
atmospheres in a range 2500 < Teff < 60000$ K and 6.5 < log g < 9.0. Bolometric
correction and colour indices are provided for a subsample of the model grid.
Comparison of the colours is made with published observational material and
results of other recent model calculations. Motivated by the increasing
interest on helium core white dwarfs, we analyse the photometric
characteristics of these stars during their cooling, using evolutionary models
recently available. Effective temperatures, surface gravities, masses and ages
have been determined for some helium core white dwarf candidates, and their
possible binary nature is briefly discussed.Comment: 12 pages, 13 figures. Accepted for publication in MNRA
Lyman-alpha wing absorption in cool white dwarf stars
Kowalski & Saumon (2006) identified the missing absorption mechanism in the
observed spectra of cool white dwarf stars as the Ly-alpha red wing formed by
the collisions between atomic and molecular hydrogen and successfully explained
entire spectra of many cool DA-type white dwarfs. Owing to the important
astrophysical implications of this issue, we present here an independent
assessment of the process. For this purpose, we compute free-free
quasi-molecular absorption in Lyman-alpha due to collisions with H and H2
within the one-perturber, quasi-static approximation. Line cross-sections are
obtained using theoretical molecular potentials to describe the interaction
between the radiating atom and the perturber. The variation of the
electric-dipole transition moment with the interparticle distance is also
considered. Six and two allowed electric dipole transitions due to H-H and H-H2
collisions, respectively, are taken into account. The new theoretical
Lyman-alpha line profiles are then incorporated in our stellar atmosphere
program for the computation of synthetic spectra and colours of DA-type white
dwarfs. Illustrative model atmospheres and spectral energy distributions are
computed, which show that Ly-alpha broadening by atoms and molecules has a
significant effect on the white dwarf atmosphere models. The inclusion of this
collision-induced opacity significantly reddens spectral energy distributions
and affects the broadband colour indices for model atmospheres with Teff<5000
K. These results confirm those previously obtained by Kowalski & Saumon (2006).
Our study points out the need for reliable evaluations of H3 potential energy
surfaces covering a large region of nuclear configurations, in order to obtain
a better description of H-H2 collisions and a more accurate evaluation of their
influence on the spectrum of cool white dwarfs.Comment: 11 pages, 12 figures, 1 table, to be published in MNRA
The ages and colours of cool helium-core white dwarf stars
The purpose of this work is to explore the evolution of helium-core white
dwarf stars in a self-consistent way with the predictions of detailed non-gray
model atmospheres and element diffusion. To this end, we consider helium-core
white dwarf models with stellar masses of 0.406, 0.360, 0.327, 0.292, 0.242,
0.196 and 0.169 solar masses and follow their evolution from the end of mass
loss episodes during their pre-white dwarf evolution down to very low surface
luminosities. We find that when the effective temperature decreases below
4000K, the emergent spectrum of these stars becomes bluer within time-scales of
astrophysical interest. In particular, we analyse the evolution of our models
in the colour-colour and colour-magnitude diagrams and we find that helium-core
white dwarfs with masses ranging from approx. 0.18 to 0.3 solar masses can
reach the turn-off in their colours and become blue again within cooling times
much less than 15 Gyr and then remain brighter than M_V approx. 16.5. In view
of these results, many low-mass helium white dwarfs could have had time enough
to evolve to the domain of collision-induced absorption from molecular
hydrogen, showing blue colours.Comment: 11 pages, 9 figures. Accepted for publication in MNRA
DQ white-dwarf stars with low C abundance: Possible progenitors
The present paper focuses on the evolution of hydrogen-deficient white dwarfs
with the aim of exploring the consequences of different initial envelope
structures on the carbon abundances expected in helium-rich,
carbon-contaminated DQ white dwarfs. In particular, the evolutionary link
between the DQs with low detected carbon abundances and the PG1159, extreme
horizontal branch, and helium-rich R Coronae Borealis (RCrB) stars is explored.
We present full evolutionary calculations that take a self-consistent treatment
of element diffusion into account as well as expectations for the outer layer
chemical stratification of progenitor stars upon entering the white dwarf
regime. We find that PG1159 stars cannot be related to any DQ white dwarfs with
low C abundances. Instead, we suggest that the latter could constitute the
progeny of the giant, helium-rich RCrB stars.Comment: 10 pages, 10 figures. Accepted for publication in Astronomy and
Astrophysic
The evolutionary status of the white dwarf companion of the binary pulsar PSR J1713+0747
Splaver and coworkers have measured the masses of the white dwarf and the neutron star components of the PSR J1713+0747 binary system pair by Shapiro Delay. We attempt to find the original configuration of this system performing a set of binary evolution calculations to simultaneously account for the masses of both stars and the orbital period. We considered initial masses of 1.5 and 1.4 \msun for the normal (donor) and the neutron star, respectively. We assumed two metallicity values (Z = 0.010 and 0.020), and an initial orbital period near 3 days. We assume that the neutron star is only able to retain \lesssim 0.10 of the matter transferred by the donor star. Calculations were performed employing our binary hydro code that handles the mass transfer rate in a fully implicit way together with state-of-the-art physical ingredients, diffusion and a non-grey atmospheres. We compare the structure of the resulting white dwarfs with the characteristic age of PSR J1713+0747 finding a nice agreement with observations by Lundgren et al. especially for the case of a donor star with Z= 0.010. This result indicates that the evolution of this kind of binary system is well understood. The models predict that, due to diffusion, the atmosphere of the white dwarf is an almost hydrogen-pure one. We find that such structures are unable to account for the colours measured by Lundgren et al. within their error bars. Thus, some discrepances in the white dwarf emergent radiation remain to be explained
Outer boundary conditions for evolving cool white dwarfs
White dwarf evolution is essentially a gravothermal cooling process,
which,for cool white dwarfs, sensitively depends on the treatment of the outer
boundary conditions. We provide detailed outer boundary conditions appropriate
for computing the evolution of cool white dwarfs employing detailed non-gray
model atmospheres for pure H composition. We also explore the impact on the
white dwarf cooling times of different assumptions for energy transfer in the
atmosphere of cool white dwarfs. Detailed non-gray model atmospheres are
computed taken into account non-ideal effects in the gas equation of state and
chemical equilibrium, collision-induced absorption from molecules, and the
Lyman alpha quasi-molecular opacity. Our results show that the use of detailed
outer boundary conditions becomes relevant for effective temperatures lower
than 5800 and 6100K for sequences with 0.60 and 0.90 M_sun, respectively.
Detailed model atmospheres predict ages that are up to approx 10% shorter at
log L/L_sun=-4 when compared with the ages derived using Eddington-like
approximations at tau_Ross=2/3. We also analyze the effects of various
assumptions and physical processes of relevance in the calculation of outer
boundary conditions. In particular, we find that the Ly_alpha red wing
absorption does not affect substantially the evolution of white dwarfs. White
dwarf cooling timescales are sensitive to the surface boundary conditions for
T_eff < 6000K. Interestingly enough, non-gray effects have little consequences
on these cooling times at observable luminosities. In fact, collision-induced
absorption processes, which significantly affect the spectra and colors of old
white dwarfs with hydrogen-rich atmospheres, have not noticeable effects in
their cooling rates, except throughout the Rosseland mean opacity.Comment: 6 pages, 9 figures, to be published in Astronomy and Astrophysic
Thermo-statistical description of gas mixtures from space partitions
The new mathematical framework based on the free energy of pure classical
fluids presented in [R. D. Rohrmann, Physica A 347, 221 (2005)] is extended to
multi-component systems to determine thermodynamic and structural properties of
chemically complex fluids. Presently, the theory focuses on -dimensional
mixtures in the low-density limit (packing factor ). The formalism
combines the free-energy minimization technique with space partitions that
assign an available volume to each particle. is related to the
closeness of the nearest neighbor and provides an useful tool to evaluate the
perturbations experimented by particles in a fluid. The theory shows a close
relationship between statistical geometry and statistical mechanics. New,
unconventional thermodynamic variables and mathematical identities are derived
as a result of the space division. Thermodynamic potentials ,
conjugate variable of the populations of particles class with the
nearest neighbors of class are defined and their relationships with the
usual chemical potentials are established. Systems of hard spheres are
treated as illustrative examples and their thermodynamics functions are derived
analytically. The low-density expressions obtained agree nicely with those of
scaled-particle theory and Percus-Yevick approximation. Several pair
distribution functions are introduced and evaluated. Analytical expressions are
also presented for hard spheres with attractive forces due to K\^ac-tails and
square-well potentials. Finally, we derive general chemical equilibrium
conditions.Comment: 14 pages, 8 figures. Accepted for publication in Physical Review
Environment-wide association study to comprehensively test and validate associations between nutrition and lifestyle factors and testosterone deficiency: NHANES 1988-1994 and 1999-2004
PURPOSE
Testosterone (T) plays an important role in men's health and its deficiency is linked with poorer health. However, the role of nutritional and lifestyle factors in T regulation and production remains unclear. The objectives are to comprehensively test the cross-sectional associations of nutritional and lifestyle factors with T deficiency and to validate the associations in the NHANES survey.
METHODS
We performed weighted multivariable logistic regression analysis to examine the association of 173 nutritional and lifestyle factors with T deficiency (total testosterone ≤ 3.5 ng/mL) in NHANES III as the discovery set (mean age 41). We controlled for multiple comparisons with a false discovery rate (FDR) < 5% and replicated in NHANES 1999-2004 (mean age 44).
RESULTS
We identified seven nutritional factors as being inversely associated with T deficiency in NHANES 1999-2004, namely dietary intake of vitamin A, protein, saturated fatty acids, monounsaturated fatty acids, total fats, saturated fatty acid 16:0, and phosphorus. In a multivariable model, only vitamin A intake remained significantly associated with T deficiency (OR 0.97, 95% CI 0.94-0.99). Principal component analysis suggested that the two principal components, (1) dietary fats, protein, and phosphorous and (2) total vitamin A, may be associated with T deficiency.
CONCLUSION
Our systematic evaluation provided new insight into the modifiable factors that could play a role in the regulation of T production. This study has the potential to contribute to the current body of literature which seeks to formulate a clinical definition of T deficiency after taking into account nutritional and lifestyle factors
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