15,290 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
Spin Hamiltonian Parameters for Cu(II)−Prion Peptide Complexes from L-Band Electron Paramagnetic Resonance Spectroscopy
Cu(II) is an essential element for life but is also associated with numerous and serious medical conditions, particularly neurodegeneration. Structural modeling of crystallization-resistant biological Cu(II) species relies on detailed spectroscopic analysis. Electron paramagnetic resonance (EPR) can, in principle, provide spin Hamiltonian parameters that contain information on the geometry and ligand atom complement of Cu(II). Unfortunately, EPR spectra of Cu(II) recorded at the traditional X-band frequency are complicated by (i) strains in the region of the spectrum corresponding to the g∥ orientation and (ii) potentially very many overlapping transitions in the g⊥ region. The rapid progress of density functional theory computation as a means to correlate EPR and structure, and the increasing need to study Cu(II) associated with biomolecules in more biologically and biomedically relevant environments such as cells and tissue, have spurred the development of a technique for the extraction of a more complete set of spin Hamiltonian parameters that is relatively straightforward and widely applicable. EPR at L-band (1−2 GHz) provides much enhanced spectral resolution and straightforward analysis via computer simulation methods. Herein, the anisotropic spin Hamiltonian parameters and the nitrogen coordination numbers for two hitherto incompletely characterized Cu(II)-bound species of a prion peptide complex are determined by analysis of their L-band EPR spectra
EPR Methods for Biological Cu(II): L-Band CW and NARS
Abstract: Copper has many roles in biology that involve the change of coordination sphere and/or oxidation state of the copper ion. Consequently, the study of copper in heterogeneous environments is an important area in biophysics. EPR is a primary technique for the investigation of paramagnetic copper, which is usually the isolated Cu(II) ion, but sometimes as Cu(II) in different oxidation states of multitransition ion clusters. The gross geometry of the coordination environment of Cu(II) can often be determined from a simple inspection of the EPR spectrum, recorded in the traditional X-band frequency range (9–10 GHz). Identification and quantitation of the coordinating ligand atoms, however, is not so straightforward. In particular, analysis of the superhyperfine structure on the EPR spectrum, to determine the number of coordinated nitrogen atoms, is fraught with difficulty at X-band, despite the observation that the overwhelming number of EPR studies of Cu(II) in the literature have been carried out at X-band. Greater reliability has been demonstrated at S-band (3–4 GHz), using the low-field parallel (gz) features. However, analysis relies on clear identification of the outermost superhyperfine line, which has the lowest intensity of all the spectral features. Computer simulations have subsequently indicated that the much more intense perpendicular region of the spectrum can be reliably interpreted at L-band (2 GHz). The present work describes the development of L-band EPR of Cu(II) into a routine method that is applicable to biological samples
Homogeneous geodesics of non-unimodular Lorentzian Lie groups and naturally reductive Lorentzian spaces in dimension three
We determine, for all three-dimensional non-unimodular Lie groups equipped
with a Lorentzian metric, the set of homogeneous geodesics through a point.
Together with the results of [C] and [CM2], this leads to the full
classification of three-dimensional Lorentzian g.o. spaces and naturally
reductive spaces
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
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