607 research outputs found
The origin of peculiar molecular bands in cool DQ white dwarfs
The DQ white dwarfs are stars whose atmosphere is enriched with carbon, which
for cool stars () is indicated by the Swan bands of
in the optical part of their spectra. With decreasing effective
temperature these molecular bands undergo a significant blueshift (). The origin of this phenomenon has been disputed over the last
two decades and has remained unknown. We attempt to address this problem by
investigating the impact of dense helium on the spectroscopic properties of
molecular carbon under the physical conditions encountered inside helium-rich,
fluid-like atmospheres of cool DQ white dwarfs. We found that the electronic
transition energy increases monotonically with the helium density
(). This causes
the Swan absorption to occur at shorter wavelengths compared with unperturbed
. On the other hand the pressure-induced increase in the vibrational
frequency is insufficient to account for the observed Swan bands shifts. This
is consistent with the observations and indicates that the observed Swan-like
molecular bands are most likely the pressure-shifted bands of .Comment: 4 pages, 5 figures, accepted for publication in A&A letter
On the Dissociation Equilibrium of H2 in Very Cool, Helium-Rich White Dwarf Atmospheres
We investigate the dissociation equilibrium of in very cool,
helium-rich white dwarf atmospheres. We present the solution of the non-ideal
chemical equilibrium for the dissociation of molecular hydrogen in a medium of
dense helium. We find that at the photosphere of cool white dwarfs of , the non-ideality results in an increase of the mole fraction
of molecular hydrogen by up to a factor of , compared to the
equilibrium value for the ideal gas. This increases the CIA
opacity by an order of magnitude and will affect the determination of the
abundance of hydrogen in very cool, helium-rich white dwarfs.Comment: 9 pages, 5 figures, 1 table; Accepted for publication in The
Astrophysical Journa
Ab initio Stellar Astrophysics: Reliable Modeling of Cool White Dwarf Atmospheres
Over the last decade {\it ab initio} modeling of material properties has
become widespread in diverse fields of research. It has proved to be a powerful
tool for predicting various properties of matter under extreme conditions. We
apply modern computational chemistry and materials science methods, including
density functional theory (DFT), to solve lingering problems in the modeling of
the dense atmospheres of cool white dwarfs (). Our
work on the revision and improvements of the absorption mechanisms in the
hydrogen and helium dominated atmospheres resulted in a new set of atmosphere
models. By inclusion of the Ly- red wing opacity we successfully
fitted the entire spectral energy distributions of known cool DA stars. In the
subsequent work we fitted the majority of the coolest stars with hydrogen-rich
models. This finding challenges our understanding of the spectral evolution of
cool white dwarfs. We discuss a few examples, including the cool companion to
the pulsar PSR J0437-4715. The two problems important for the understanding of
cool white dwarfs are the behavior of negative hydrogen ion and molecular
carbon in a fluid-like, helium dominated medium. Using {\it ab initio} methods
we investigate the stability and opacity of these two species in dense helium.
Our investigation of indicates that the absorption features observed
in the ``peculiar'' DQp white dwarfs resemble the absorption of perturbed in dense helium.Comment: 6 pages, 4 figures, submitted to proceedings of 17th European White
Dwarf Workshop, Tuebingen, Germany 201
The Pseudo-continuum Bound-free Opacity of Hydrogen and its Importance in Cool White Dwarf Atmospheres
We investigate the importance of the pseudo-continuum bound-free opacity from
hydrogen atoms in the atmospheres of cool white dwarfs. This source of
absorption, when calculated by the occupation probability formalism applied in
the modeling of white dwarf atmospheres with ,
dominates all other sources of opacity at optical wavelengths. This is
unrealistic and not observed. On the other hand, a significant flux suppression
in the blue part of the spectra of cool white dwarfs has been reported, and
mainly interpreted as a result of the pseudo-continuum absorption from atomic
hydrogen. We investigate this problem by proposing a new, more realistic
approach to calculating this source of opacity. We show that this absorption is
orders of magnitude smaller than that predicted by current methods. Therefore,
we rule out the pseudo-continuum opacity as a source of the flux deficiency
observed in the spectra of cool white dwarfs.Comment: 11 pages, 5 gigures, accepted for publication in the Astrophysical
Journa
A New Generation of Cool White Dwarf Atmosphere Models Using Ab Initio Calculations
Due to their high photospheric density, cool helium-rich white dwarfs
(particularly DZ, DQpec and ultracool) are often poorly described by current
atmosphere models. As part of our ongoing efforts to design atmosphere models
suitable for all cool white dwarfs, we investigate how the ionization ratio of
heavy elements and the H-He collision-induced absorption (CIA) spectrum are
altered under fluid-like densities. For the conditions encountered at the
photosphere of cool helium-rich white dwarfs, our ab initio calculations show
that the ionization of most metals is inhibited and that the H-He CIA
spectrum is significantly distorted for densities higher than 0.1 g/cm.Comment: 4 pages, 2 figures, submitted for publication in the proceedings of
the 20th European Workshop on White Dwarf
Ab initio prediction of equilibrium boron isotope fractionation between minerals and aqueous fluids at high P and T
Over the last decade experimental studies have shown a large B isotope
fractionation between materials carrying boron incorporated in trigonally and
tetrahedrally coordinated sites, but the mechanisms responsible for producing
the observed isotopic signatures are poorly known. In order to understand the
boron isotope fractionation processes and to obtain a better interpretation of
the experimental data and isotopic signatures observed in natural samples, we
use first principles calculations based on density functional theory in
conjunction with ab initio molecular dynamics and a new pseudofrequency
analysis method to investigate the B isotope fractionation between B-bearing
minerals (such as tourmaline and micas) and aqueous fluids containing H_3BO_3
and H_4BO_4- species. We confirm the experimental finding that the isotope
fractionation is mainly driven by the coordination of the fractionating boron
atoms and have found in addition that the strength of the produced isotopic
signature is strongly correlated with the B-O bond length. We also demonstrate
the ability of our computational scheme to predict the isotopic signatures of
fluids at extreme pressures by showing the consistency of computed
pressure-dependent beta factors with the measured pressure shifts of the B-O
vibrational frequencies of H_3BO_3 and H_4BO_4- in aqueous fluid. The
comparison of the predicted with measured fractionation factors between
boromuscovite and neutral fluid confirms the existence of the admixture of
tetrahedral boron species in neutral fluid at high P and T found
experimentally, which also explains the inconsistency between the various
measurements on the tourmaline-mica system reported in the literature. Our
investigation shows that the calculated equilibrium isotope fractionation
factors have an accuracy comparable to the experiments.Comment: 19 pages, 11 figures, Accepted for publication in Geochimica et
Cosmochimica Act
Pressure Distortion of the H-He Collision-Induced Absorption at the Photosphere of Cool White Dwarf Stars
Collision-induced absorption (CIA) from molecular hydrogen is a dominant
opacity source in the atmosphere of cool white dwarfs. It results in a
significant flux depletion in the near-IR and IR parts of their spectra.
Because of the extreme conditions of helium-rich atmospheres (where the density
can be as high as a few g/cm), this opacity source is expected to undergo
strong pressure distortion and the currently used opacities have not been
validated at such extreme conditions. To check the distortion of the CIA
opacity we applied state-of-the-art ab initio methods of computational quantum
chemistry to simulate the CIA opacity at high densities. The results show that
the CIA profiles are significantly distorted above densities of in a way that is not captured by the existing models. The
roto-translational band is enhanced and shifted to higher frequencies as an
effect of the decrease of the interatomic separation of the H molecule. The
vibrational band is blueward shifted and split into and branches,
separated by a pronounced interference dip. Its intensity is also substantially
reduced. The distortions result in a shift of the maximum of the absorption
from to , which could potentially explain the
spectra of some very cool, helium-rich white dwarfs.Comment: 12 pages, 13 figures. Accepted for publication in The Astrophysical
Journa
Towards an Understanding of the Atmospheres of Cool White Dwarfs
Cool white dwarfs with Teff < 6000 K are the remnants of the oldest stars
that existed in our Galaxy. Their atmospheres, when properly characterized, can
provide valuable information on white dwarf evolution and ultimately star
formation through the history of the Milky Way. Understanding the atmospheres
of these stars requires joined observational effort and reliable atmosphere
modeling. We discuss and analyze recent observations of the near-ultraviolet
(UV) and near-infrared (IR) spectrum of several cool white dwarfs including
DQ/DQp stars showing carbon in their spectra. We present fits to the entire
spectral energy distribution (SED) of selected cool stars, showing that the
current pure-hydrogen atmosphere models are quite reliable, especially in the
near-UV spectral region. Recently, we also performed an analysis of the coolest
known DQ/DQp stars investigating further the origin of the C2 Swan bands-like
spectral features that characterize the DQp stars. We show that the carbon
abundances derived for DQp stars fit the trend of carbon abundance with Teff
seen in normal cool DQ stars. This further supports the recent conclusion of
Kowalski A&A (2010) that DQp stars are DQ stars with pressure distorted Swan
bands. However, we encounter some difficulties in reproducing the IR part of
the SED of stars having a mixed He/H atmosphere. This indicates limitations in
current models of the opacity in dense He/H fluids.Comment: 6 pages, 4 figures, to appear in the proceedings of the "18th
European White Dwarf Workshop" in Krakow, Poland (2012
Ideal, Defective, and Gold--Promoted Rutile TiO2(110) Surfaces: Structures, Energies, Dynamics, and Thermodynamics from PBE+U
Extensive first principles calculations are carried out to investigate
gold-promoted TiO2(110) surfaces in terms of structure optimizations,
electronic structure analyses, ab initio thermodynamics calculations of surface
phase diagrams, and ab initio molecular dynamics simulations. All computations
rely on density functional theory in the generalized gradient approximation
(PBE) and account for on-site Coulomb interactions via inclusion of a Hubbard
correction, PBE+U, where U is computed from linear response theory. This
approach is validated by investigating the interaction between TiO2(110)
surfaces and typical probe species (H, H2O, CO). Relaxed structures and binding
energies are compared to both data from the literature and plain PBE results.
The main focus of the study is on the properties of gold-promoted titania
surfaces and their interactions with CO. Both PBE+U and PBE optimized
structures of Au adatoms adsorbed on stoichiometric and reduced TiO2 surfaces
are computed, along with their electronic structure. The charge rearrangement
induced by the adsorbates at the metal/oxide contact are also analyzed and
discussed. By performing PBE+U ab initio molecular dynamics simulations, it is
demonstrated that the diffusion of Au adatoms on the stoichiometric surface is
highly anisotropic. The metal atoms migrate either along the top of the
bridging oxygen rows, or around the area between these rows, from one bridging
position to the next along the [001] direction. Approximate ab initio
thermodynamics predicts that under O-rich conditions, structures obtained by
substituting a Ti5c atom with an Au atom are thermodynamically stable over a
wide range of temperatures and pressures.Comment: 20 pages, 12 figures, accepted for publication in Phys. Rev.
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