146 research outputs found
The Circumstellar Medium of Cassiopeia A Inferred from the Outer Ejecta Knot Properties
We investigate the effect of the circumstellar medium density profile on the
X-ray emission from outer ejecta knots in the Cassiopeia A supernova remnant
using the 1 Ms Chandra observation. The spectra of a number of radial series of
ejecta knots at various positions around the remnant are analyzed using
techniques similar to those devised in previous papers. We can obtain a
reasonable match to our data for a circumstellar density profile proportional
to r^-2 as would arise from the steady dense wind of a red supergiant, but the
agreement is improved if we introduce a small (0.2-0.3 pc) central cavity
around the progenitor into our models. Such a profile might arise if the
progenitor emitted a fast tenuous stellar wind for a short period immediately
prior to explosion. We review other lines of evidence supporting this
conclusion. The spectra also indicate the widespread presence of Fe-enriched
plasma that was presumably formed by complete Si burning during the explosion,
possibly via alpha-rich freezeout. This component is typically associated with
hotter and more highly ionized gas than the bulk of the O- and Si-rich ejecta.Comment: 12 pages, 3 figures; ApJ in pres
Where was the Iron Synthesized in Cassiopeia A?
We investigate the properties of Fe-rich knots on the east limb of the
Cassiopeia A supernova remnant using observations with Chandra/ACIS and
analysis methods developed in a companion paper. We use the fitted ionization
age and electron temperature of the knots to constrain the ejecta density
profile and the Lagrangian mass coordinates of the knots. Fe-rich knots which
also have strong emission from Si, S, Ar, and Ca are clustered around mass
coordinates q~0.35-0.4 in the shocked ejecta; for ejecta mass 2M_sun, this
places the knots 0.7-0.8 M_sun out from the center (or 2-2.1 M_sun, allowing
for a 1.3 M_sun compact object). We also find an Fe clump that is evidently
devoid of line emission from lower mass elements, as would be expected if it
were the product of alpha-rich freeze out; the mass coordinate of this clump is
similar to those of the other Fe knots.Comment: submitted to ApJ, companion to Laming & Hwang; 25 pages, 6 figure
Uncertainties in Dielectronic Recombination Rate Coefficients: Effects on Solar and Stellar Upper Atmosphere Abundance Determinations
We have investigated how the relative elemental abundances inferred from the
solar upper atmosphere are affected by uncertainties in the dielectronic
recombination (DR) rate coefficients used to analyze the spectra. We find that
the inferred relative abundances can be up to a factor of ~5 smaller or ~1.6
times larger than those inferred using the currently recommended DR rate
coefficients. We have also found a plausible set of variations to the DR rate
coefficients which improve the inferred (and expected) isothermal nature of
solar coronal observations at heights of >~ 50 arcsec off the solar limb. Our
results can be used to help prioritize the enormous amount of DR data needed
for modeling solar and stellar upper atmospheres. Based on the work here, our
list of needed rate coefficients for DR onto specific isoelectronic sequences
reads, in decreasing order of importance, as follows: O-like, C-like, Be-like,
N-like, B-like, F-like, Li-like, He-like, and Ne-like. It is our hope that this
work will help to motivate and prioritize future experimental and theoretical
studies of DR.Comment: 33 pages, including 3 figures and 4 tables. To be published in Ap
On the Detectability of 57Fe Axion-Photon Mode Conversion in the Sun
The purpose of this paper is to assess the feasibility of axion detection by
X-ray spectroscopy of the sun. We review the theory of axion-photon mode
conversion with special attention to axions emitted in the 14.4 keV M1 decay of
57Fe at the solar center. These then mode convert to photons in the outer
layers of the solar envelope, and may in principle be detected subsequently as
X-rays. For axion masses above about 10^-4 eV, resonant mode conversion at a
layer where the axion mass matches the local electron plasma frequency is
necessary. For axion masses above about 10^-2 eV, this mode conversion occurs
too deep in the solar atmosphere for the resulting photon to escape the solar
surface and be detected before Compton scattering obscures the line. At the
(detectable) axion masses below this, the flux of mode converted photons
predicted by axion models appears to be too low for detection to be feasible
with current instrumentation. Nonresonant mode conversion for axion masses
below 10^-4 eV is also plausible, but with still lower predicted fluxes, since
the axion coupling constant is related to it mass. Prospects for meaningful
constraints on massive axion parameters from X-ray observations of this
transition from the Sun do not appear to be promising. However parameters for
massless counterparts (e.g. the "arion") may still result from such
observations. It may mode convert in the outer layers of the solar atmosphere,
but is not restricted by this to have a small coupling constant.Comment: 5 pages, 4 figures, accepted by Astronomy and Astrophysic
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