845 research outputs found
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
Digestibility of Sunflower Seeds in Swine Diets
Limited research has been performed using sunflower seeds as an ingredient in swine diets. Because of the large amount of sunflowers produced in South Dakota, it would be beneficial to determine their usefulness as a feed ingredient in livestock rations. Work conducted at North Dakota has shown levels of over 10% sunflower seeds in diets of growing- finishing pigs produced oily carcasses. Previous research at this station (SWINE 80-8) has shown the maximum level of ground, whole sunflower seeds to be fed to sows during late gestation and early lactation is between 25 and 50%. In order to utilize sunflower seeds in swine diets more efficiently, the digestibility of the various nutrient fractions of the seeds must be determined. This study was performed to determine the coefficients of apparent digestibility, digestible energy and nitrogen retention for rations containing various levels of ground sunflower seeds
Mode identification of Pulsating White Dwarfs using the HST
We have obtained time-resolved ultraviolet spectroscopy for the pulsating DAV
stars G226-29 and G185-32, and for the pulsating DBV star PG1351+489 with the
Hubble Space Telescope Faint Object Spectrograph, to compare the ultraviolet to
the optical pulsation amplitude and determine the pulsation indices. We find
that for essentially all observed pulsation modes, the amplitude rises to the
ultraviolet as the theoretical models predict for l=1 non-radial g-modes. We do
not find any pulsation mode visible only in the ultraviolet, nor any modes
whose phase flips by 180 degrees; in the ultraviolet, as would be expected if
high l pulsations were excited. We find one periodicity in the light curve of
G185-32, at 141 s, which does not fit theoretical models for the change of
amplitude with wavelength of g-mode pulsations.Comment: Accepted for publication in the Astrophysical Journal, Aug 200
New full evolutionary sequences of H and He atmosphere massive white dwarf stars using MESA
We explore the evolution of hydrogen-rich and hydrogen-deficient white dwarf
stars with masses between 1.012 and 1.307 Msun, and initial metallicity of
Z=0.02. These sequences are the result of main sequence stars with masses
between 8.8 and 11.8 Msun. The simulations were performed with MESA, starting
at the zero-age main sequence, through thermally pulsing and mass-loss phases,
ending at the white dwarfs cooling sequence. We present reliable chemical
profiles for the whole mass range considered, covering the different expected
central compositions, i.e. C/O, O/Ne and Ne/O/Mg, and its dependence with the
stellar mass. In addition, we present detailed chemical profiles of hybrid
C/O-O/Ne core white dwarfs, found in the mass range between 1.024 and 1.15
Msun. We present the initial-to-final mass relation, mass-radius relation, and
cooling times considering the effects of atmosphere and core composition.Comment: 17 pages, 16 figures, 3 tables, accepted for publication in MNRAS.
Cooling tracks available at
ftp://cdsarc.u-strasbg.fr/pub/cats/J/MNRAS/480/154
Theory of Structural Glasses and Supercooled Liquids
We review the Random First Order Transition Theory of the glass transition,
emphasizing the experimental tests of the theory. Many distinct phenomena are
quantitatively predicted or explained by the theory, both above and below the
glass transition temperature . These include: the viscosity catastrophe
and heat capacity jump at , and their connection; the non-exponentiality
of relaxations and their correlation with the fragility; dynamic heterogeneity
in supercooled liquids owing to the mosaic structure; deviations from the
Vogel-Fulcher law, connected with strings or fractral cooperative
rearrangements; deviations from the Stokes-Einstein relation close to ;
aging, and its correlation with fragility; the excess density of states at
cryogenic temperatures due to two level tunneling systems and the Boson Peak.Comment: submitted to Ann. Rev. Phys. Che
Discovery of an ultramassive pulsating white dwarf
We announce the discovery of the most massive pulsating hydrogen-atmosphere
(DA) white dwarf (WD) ever discovered, GD 518. Model atmosphere fits to the
optical spectrum of this star show it is a 12,030 +/- 210 K WD with a log(g) =
9.08 +/- 0.06, which corresponds to a mass of 1.20 +/- 0.03 Msun. Stellar
evolution models indicate that the progenitor of such a high-mass WD endured a
stable carbon-burning phase, producing an oxygen-neon-core WD. The discovery of
pulsations in GD 518 thus offers the first opportunity to probe the interior of
a WD with a possible oxygen-neon core. Such a massive WD should also be
significantly crystallized at this temperature. The star exhibits
multi-periodic luminosity variations at timescales ranging from roughly 425-595
s and amplitudes up to 0.7%, consistent in period and amplitude with the
observed variability of typical ZZ Ceti stars, which exhibit non-radial g-mode
pulsations driven by a hydrogen partial ionization zone. Successfully
unraveling both the total mass and core composition of GD 518 provides a unique
opportunity to investigate intermediate-mass stellar evolution, and can
possibly place an upper limit to the mass of a carbon-oxygen-core WD, which in
turn constrains SNe Ia progenitor systems.Comment: 5 pages, 3 figures, Astrophysical Journal Letters, 771, L2 (2013
Prehistory of Transit Searches
Nowadays the more powerful method to detect extrasolar planets is the transit
method. We review the planet transits which were anticipated, searched, and the
first ones which were observed all through history. Indeed transits of planets
in front of their star were first investigated and studied in the solar system.
The first observations of sunspots were sometimes mistaken for transits of
unknown planets. The first scientific observation and study of a transit in the
solar system was the observation of Mercury transit by Pierre Gassendi in 1631.
Because observations of Venus transits could give a way to determine the
distance Sun-Earth, transits of Venus were overwhelmingly observed. Some
objects which actually do not exist were searched by their hypothetical
transits on the Sun, as some examples a Venus satellite and an infra-mercurial
planet. We evoke the possibly first use of the hypothesis of an exoplanet
transit to explain some periodic variations of the luminosity of a star, namely
the star Algol, during the eighteen century. Then we review the predictions of
detection of exoplanets by their transits, those predictions being sometimes
ancient, and made by astronomers as well as popular science writers. However,
these very interesting predictions were never published in peer-reviewed
journals specialized in astronomical discoveries and results. A possible
transit of the planet beta Pic b was observed in 1981. Shall we see another
transit expected for the same planet during 2018? Today, some studies of
transits which are connected to hypothetical extraterrestrial civilisations are
published in astronomical refereed journals. Some studies which would be
classified not long ago as science fiction are now considered as scientific
ones.Comment: Submiited to Handbook of Exoplanets (Springer
An asteroseismic test of diffusion theory in white dwarfs
The helium-atmosphere (DB) white dwarfs are commonly thought to be the
descendants of the hotter PG1159 stars, which initially have uniform He/C/O
atmospheres. In this evolutionary scenario, diffusion builds a pure He surface
layer which gradually thickens as the star cools. In the temperature range of
the pulsating DB white dwarfs (T_eff ~ 25,000 K) this transformation is still
taking place, allowing asteroseismic tests of the theory. We have obtained
dual-site observations of the pulsating DB star CBS114, to complement existing
observations of the slightly cooler star GD358. We recover the 7 independent
pulsation modes that were previously known, and we discover 4 new ones to
provide additional constraints on the models. We perform objective global
fitting of our updated double-layered envelope models to both sets of
observations, leading to determinations of the envelope masses and pure He
surface layers that qualitatively agree with the expectations of diffusion
theory. These results provide new asteroseismic evidence supporting one of the
central assumptions of spectral evolution theory, linking the DB white dwarfs
to PG1159 stars.Comment: 7 pages, 3 figures, 3 tables, accepted for publication in A&
A comparative analysis of the observed white dwarf cooling sequence from globular clusters
We report our study of features at the observed red end of the white dwarf
cooling sequences for three Galactic globular clusters: NGC\,6397, 47\,Tucanae
and M\,4. We use deep colour-magnitude diagrams constructed from archival
Hubble Space Telescope (ACS) to systematically investigate the blue turn at
faint magnitudes and the age determinations for each cluster. We find that the
age difference between NGC\,6397 and 47\,Tuc is 1.98\,Gyr,
consistent with the picture that metal-rich halo clusters were formed later
than metal-poor halo clusters. We self-consistently include the effect of
metallicity on the progenitor age and the initial-to-final mass relation. In
contrast with previous investigations that invoked a single white dwarf mass
for each cluster, the data shows a spread of white dwarf masses that better
reproduce the shape and location of the blue turn. This effect alone, however,
does not completely reproduce the observational data - the blue turn retains
some mystery. In this context, we discuss several other potential problems in
the models. These include possible partial mixing of H and He in the atmosphere
of white dwarf stars, the lack of a good physical description of the
collision-induced absorption process and uncertainties in the opacities at low
temperatures. The latter are already known to be significant in the description
of the cool main sequence. Additionally, we find that the present day local
mass function of NGC\,6397 is consistent with a top-heavy type, while 47\,Tuc
presents a bottom-heavy profile.Comment: Accepted for publication in MNRAS (16 pages, 19 figures
On the nature of long-range contributions to pair interactions between charged colloids in two dimensions
We perform a detailed analysis of solutions of the inverse problem applied to
experimentally measured two-dimensional radial distribution functions for
highly charged latex dispersions. The experiments are carried out at high
colloidal densities and under low-salt conditions. At the highest studied
densities, the extracted effective pair potentials contain long-range
attractive part. At the same time, we find that for the best distribution
functions available the range of stability of the solutions is limited by the
nearest neighbour distance between the colloidal particles. Moreover, the
measured pair distribution functions can be explained by purely repulsive pair
potentials contained in the stable part of the solution.Comment: 6 pages, 5 figure
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