80 research outputs found
Opacity Effects on Pulsations of Main-Sequence A-Type Stars
Opacity enhancements for stellar interior conditions have been explored to
explain observed pulsation frequencies and to extend the pulsation instability
region for B-type main-sequence variable stars. For these stars, the pulsations
are driven in the region of the opacity bump of Fe-group elements at
200,000 K in the stellar envelope. Here we explore effects of opacity
enhancements for the somewhat cooler main-sequence A-type stars, in which
-mode pulsations are driven instead in the second helium ionization region
at 50,000 K. We compare models using the new LANL OPLIB vs. LLNL OPAL
opacities for the AGSS09 solar mixture. For models of 2 solar masses and
effective temperature 7600 K, opacity enhancements have only a mild effect on
pulsations, shifting mode frequencies and/or slightly changing kinetic-energy
growth rates. Increased opacity near the bump at 200,000 K can induce
convection that may alter composition gradients created by diffusive settling
and radiative levitation. Opacity increases around the hydrogen and 1st He
ionization region (13,000 K) can cause additional higher-frequency modes to
be excited, raising the possibility that improved treatment of these layers may
result in prediction of new modes that could be tested by observations. New or
wider convective zones and higher convective velocities produced by opacity
increases could also affect angular momentum transport during evolution. More
work needs to be done to quantify the effects of opacity on the boundaries of
the pulsation instability regions for A-type stars.Comment: 14 pages, 12 figures. Accepted version for MDPI Atoms Special Issue
"Atomic and Molecular Opacity Data for Astrophysics", Published 4 June 2018,
Atoms 2018, 6(2), 31, https://doi.org/10.3390/atoms602003
Model Atmospheres for X-ray Bursting Neutron Stars
The hydrogen and helium accreted by X-ray bursting neutron stars is
periodically consumed in runaway thermonuclear reactions that cause the entire
surface to glow brightly in X-rays for a few seconds. With models of the
emission, the mass and radius of the neutron star can be inferred from the
observations. By simultaneously probing neutron star masses and radii, X-ray
bursts are one of the strongest diagnostics of the nature of matter at
extremely high densities. Accurate determinations of these parameters are
difficult, however, due to the highly non-ideal nature of the atmospheres where
X-ray bursts occur. Observations from X-ray telescopes such as RXTE and NuStar
can potentially place strong constraints on nuclear matter once uncertainties
in atmosphere models have been reduced. Here we discuss current progress on
modeling atmospheres of X-ray bursting neutron stars and some of the challenges
still to be overcome.Comment: 25 pages, 14 figure
Monte Carlo Radiation Transport for Astrophysical Transients Powered by Circumstellar Interaction
In this paper, we introduce \texttt{SuperLite}, an open-source Monte Carlo
radiation transport code designed to produce synthetic spectra for
astrophysical transient phenomena affected by circumstellar interaction.
\texttt{SuperLite} utilizes Monte Carlo methods for semi-implicit,
semi-relativistic radiation transport in high-velocity shocked outflows,
employing multi-group structured opacity calculations. The code enables rapid
post-processing of hydrodynamic profiles to generate high-quality spectra that
can be compared with observations of transient events, including superluminous
supernovae, pulsational pair-instability supernovae, and other peculiar
transients. We present the methods employed in \texttt{SuperLite} and compare
the code's performance to that of other radiative transport codes, such as
\texttt{SuperNu} and CMFGEN. We show that \texttt{SuperLite} has successfully
passed standard Monte Carlo radiation transport tests and can reproduce spectra
of typical supernovae of Type Ia, Type IIP and Type IIn.Comment: Accepted for publication at the Astrophysics Journa
Composition Effects on Kilonova Spectra and Light Curves: I
The merger of neutron star binaries is believed to eject a wide range of
heavy elements into the universe. By observing the emission from this ejecta,
scientists can probe the ejecta properties (mass, velocity and composition
distributions). The emission (a.k.a. kilonova) is powered by the radioactive
decay of the heavy isotopes produced in the merger and this emission is
reprocessed by atomic opacities to optical and infra-red wavelengths.
Understanding the ejecta properties requires calculating the dependence of this
emission on these opacities. The strong lines in the optical and infra-red in
lanthanide opacities have been shown to significantly alter the light-curves
and spectra in these wavelength bands, arguing that the emission in these
wavelengths can probe the composition of this ejecta. Here we study variations
in the kilonova emission by varying individual lanthanide (and the actinide
uranium) concentrations in the ejecta. The broad forest of lanthanide lines
makes it difficult to determine the exact fraction of individual lanthanides.
Nd is an exception. Its opacities above 1 micron are higher than other
lanthanides and observations of kilonovae can potentially probe increased
abundances of Nd. Similarly, at early times when the ejecta is still hot (first
day), the U opacity is strong in the 0.2-1 micron wavelength range and kilonova
observations may also be able to constrain these abundances
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