32 research outputs found
On density and temperature-dependent ground-state and continuum effects in the equation of state for stellar interiors -- A Comment on the paper by S. Arndt, W. Dappen and A. Nayfonov 1998, ApJ 498, 349
Misunderstandings have occurred regarding the conclusions of the paper by S.
Arndt, W. Dappen and A. Nayfonov 1998, ApJ 498, 349. At occasions, its results
were interpreted as if it had shown basic flaws in the general theory of
dynamical screening. The aim of this comment is to emphasize in which
connection the conclusions of the paper have to be understood in order to avoid
misinterpretations.Comment: Astrophysical Journal (to appear 1 May 1999), 5 page
Inconsistency in Fermi's probability of the quantum states
We point out an important hidden inconsistency in Fermi's probability of the
quantum states that engendered inconsistent/inaccurate equations-of-state
extensively used in the literature to model nonideal plasma systems. The
importance of this amendment goes beyond rectifying our comprehension and
foundation of an important physical problem to influencing contemporary
research results.Comment: Accepted for Publicatio
Quantum Langevin molecular dynamics determination of the solar-interior equation of state
The equation of state (EOS) of the solar interior is accurately and smoothly
determined from \textit{ab initio} simulations named quantum Langevin molecular
dynamics (QLMD) in the pressure range of Mbar at
the temperature range of eV. The central pressure is
calculated, and compared with other models. The effect of heavy elements such
as carbon and oxygen on the EOS is also discussed.Comment: to publish in AP
New Insights into the Problem of the Surface Gravity Distribution of Cool DA White Dwarfs
We review at length the longstanding problem in the spectroscopic analysis of
cool hydrogen-line (DA) white dwarfs (Teff < 13,000 K) where gravities are
significantly higher than those found in hotter DA stars. The first solution
that has been proposed for this problem is a mild and systematic helium
contamination from convective mixing that would mimic the high gravities. We
constrain this scenario by determining the helium abundances in six cool DA
white dwarfs using high-resolution spectra from the Keck I 10-m telescope. We
obtain no detections, with upper limits as low as He/H = 0.04 in some cases.
This allows us to put this scenario to rest for good. We also extend our model
grid to lower temperatures using improved Stark profiles with non-ideal gas
effects from Tremblay & Bergeron and find that the gravity distribution of cool
objects remains suspiciously high. Finally, we find that photometric masses
are, on average, in agreement with expected values, and that the high-log g
problem is so far unique to the spectroscopic approach.Comment: 44 pages, 14 figures, accepted for publication in the Astrophysical
Journa
Spectroscopic Analysis of DA White Dwarfs: Stark Broadening of Hydrogen Lines Including Non-Ideal Effects
We present improved calculations for the Stark broadening of hydrogen lines
in dense plasmas typical of white dwarf atmospheres. Our new model is based on
the unified theory of Stark broadening from Vidal, Cooper, & Smith. For the
first time, we account for the non-ideal effects in a consistent way directly
inside the line profile calculations. The Hummer & Mihalas theory is used to
describe the non-ideal effects due to perturbations on the absorber from
protons and electrons. We use a truncation of the electric microfield
distribution in the quasi-static proton broadening to take into account the
fact that high electric microfields dissociate the upper state of a transition.
This approach represents a significant improvement over previous calculations
that relied on the use of an ad hoc parameter to mimic these non-ideal effects.
We obtain the first model spectra with line profiles that are consistent with
the equation of state. We revisit the properties of DA stars in the range
40,000 K > Teff > 13,000 K by analyzing the optical spectra with our improved
models. The updated atmospheric parameters are shown to differ substantially
from those published in previous studies, with a mean mass shifted by +0.034
Msun. We also show that these revised atmospheric parameters yield absolute
visual magnitudes that remain in excellent agreement with trigonometric
parallax measurements.Comment: 44 pages, 14 figures, accepted for publication in the Astrophysical
Journa
A comparison of Rosseland-mean opacities from OP and OPAL
Monochromatic opacities from the Opacity Project (OP) (Seaton et al.) have
been augmented by hitherto missing inner-shell contributions (Badnell &
Seaton). OP Rosseland-mean opacities are compared with results from OPAL
(Iglesias & Rogers) for the elements H, He, C, O, S and Fe. Overall, there is
good agreement between OP and OPAL Rosseland-mean opacities for the 6-elements,
but there are some differences. In particular, recent work (Bahcall et al.) has
shown that helioseismology measurements give a very accurate value for the
depth of the solar convection zone and that solar models give agreement with
that value only if opacities there are about 7% larger than OPAL values. For
the 6-element mix, here, we obtain Rosseland-mean opacities to be larger than
OPAL by 5%.Comment: 14 pages, 11 figures, submitted to MNRAS, uses personal Latex style
fil
New Insights into White-Light Flare Emission from Radiative-Hydrodynamic Modeling of a Chromospheric Condensation
(abridged) The heating mechanism at high densities during M dwarf flares is
poorly understood. Spectra of M dwarf flares in the optical and
near-ultraviolet wavelength regimes have revealed three continuum components
during the impulsive phase: 1) an energetically dominant blackbody component
with a color temperature of T 10,000 K in the blue-optical, 2) a smaller
amount of Balmer continuum emission in the near-ultraviolet at lambda 3646
Angstroms and 3) an apparent pseudo-continuum of blended high-order Balmer
lines. These properties are not reproduced by models that employ a typical
"solar-type" flare heating level in nonthermal electrons, and therefore our
understanding of these spectra is limited to a phenomenological interpretation.
We present a new 1D radiative-hydrodynamic model of an M dwarf flare from
precipitating nonthermal electrons with a large energy flux of erg
cm s. The simulation produces bright continuum emission from a
dense, hot chromospheric condensation. For the first time, the observed color
temperature and Balmer jump ratio are produced self-consistently in a
radiative-hydrodynamic flare model. We find that a T 10,000 K
blackbody-like continuum component and a small Balmer jump ratio result from
optically thick Balmer and Paschen recombination radiation, and thus the
properties of the flux spectrum are caused by blue light escaping over a larger
physical depth range compared to red and near-ultraviolet light. To model the
near-ultraviolet pseudo-continuum previously attributed to overlapping Balmer
lines, we include the extra Balmer continuum opacity from Landau-Zener
transitions that result from merged, high order energy levels of hydrogen in a
dense, partially ionized atmosphere. This reveals a new diagnostic of ambient
charge density in the densest regions of the atmosphere that are heated during
dMe and solar flares.Comment: 50 pages, 2 tables, 13 figures. Accepted for publication in the Solar
Physics Topical Issue, "Solar and Stellar Flares". Version 2 (June 22, 2015):
updated to include comments by Guest Editor. The final publication is
available at Springer via http://dx.doi.org/10.1007/s11207-015-0708-
Improved synthetic spectra of helium-core white dwarf stars
We examine the emergent fluxes from helium-core white dwarfs following their
evolution from the end of pre-white dwarf stages down to advanced cooling
stages. For this purpose, we include a detailed treatment of the physics of the
atmosphere, particularly an improved representation of the state of the gas by
taking into account non-ideal effects according to the so-called occupation
probability formalism. The present calculations also incorporate hydrogen line
opacity from Lyman, Balmer and Paschen series, pseudo-continuum absorptions and
new updated induced-dipole absorption from H-H, H-He and H-He
pairs. We find that the non-ideal effects and line absorption alter the
appearance of the stellar spectrum and have a significant influence upon the
photometric colours in the UBVRI-JHKL system. This occurs specially for hot
models T_{\rm eff}\ga 8000 due to line and pseudo-continuum opacities, and
for cool models T_{\rm eff}\la 4000 where the perturbation of atoms and
molecules by neighbour particles affects the chemical equilibrium of the gas.
In the present study, we also include new cooling sequences for helium-core
white dwarfs of very low mass (0.160 and 0.148 M) with metallicity
. These computations provide theoretical support to search for and
identify white dwarfs of very low mass, specially useful for recent and future
observational studies of globular cluster, where these objects have began to be
detected.Comment: 15 pages. Accepted for publication in MNRA