78 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
X-ray Observations of Young Supernova Remnants
This brief review of recent X-ray observations of young supernova remnants
highlights results obtained by the Chandra and XMM Newton Observatories since
their launch last year. Their impressive capabilities are illustrated by
results for spectral imaging, and for spatially resolved spectroscopy that
isolates emission from individual ejecta knots and from the forward shock. I
also review X-ray dynamical studies of supernova remnants, which should undergo
significant advances during this new era.Comment: 10 pages LaTeX, with postscript figures; to appear in Proceedings of
the 11th Astrophysics Conference in Maryland: Young Supernova Remnant
Interstellar and Ejecta Dust in the Cas A Supernova Remnant
Infrared continuum observations provide a means of investigating the physical
composition of the dust in the ejecta and swept up medium of the Cas A
supernova remnant. Using low resolution Spitzer IRS spectra (5-35 m), and
broad-band Herschel PACS imaging (70, 100, and 160 m), we identify
characteristic dust spectra, associated with ejecta layers that underwent
distinct nuclear burning histories. The most luminous spectrum exhibits strong
emission features at and 21 m and is closely associated with
ejecta knots with strong Ar emission lines. The dust features can be reproduced
by magnesium silicate grains with relatively low Mg to Si ratios. Another dust
spectrum is associated with ejecta having strong Ne emission lines. It has no
indication of any silicate features, and is best fit by AlO dust. A
third characteristic dust spectrum shows features that are best matched by
magnesium silicates with a relatively high Mg to Si ratio. This dust is
primarily associated with the X-ray emitting shocked ejecta, but it is also
evident in regions where shocked interstellar or circumstellar material is
expected. However, the identification of dust composition is not unique, and
each spectrum includes an additional featureless dust component of unknown
composition. Colder dust of indeterminate composition is associated with
emission from the interior of the SNR, where the reverse shock has not yet
swept up and heated the ejecta. Most of the dust mass in Cas A is associated
with this unidentified cold component, which is . The
mass of warmer dust is only .Comment: 45 pages. 21 Figures. Accepted for publication in Ap
A Chandra X-ray Survey of Ejecta in the Cassiopeia A Supernova Remnant
We present a survey of the X-ray emitting ejecta in the Cassiopeia A
supernova remnant based on an extensive analysis of over 6000 spectral regions
extracted on 2.5-10" angular scales using the Chandra 1 Ms observation. We
interpret these results in the context of hydrodynamical models for the
evolution of the remnant. The distributions of fitted temperature and
ionization age, and the implied mass coordinates, are highly peaked and suggest
that the ejecta were subjected to multiple secondary shocks following reverse
shock interaction with ejecta inhomogeneities. Based on the fitted emission
measure and element abundances, and an estimate of the emitting volume, we
derive masses for the X-ray emitting ejecta and also show the distribution of
the mass of various elements over the remnant. An upper limit to the total
shocked Fe mass visible in X-rays appears to be roughly 0.13 M_sun, which
accounts for nearly all of the mass expected in Fe ejecta. We find two
populations of Fe ejecta, that associated with normal Si-burning and that
possibly associated with alpha-rich freeze-out, with a mass ratio of
approximately 2:1. Essentially all of the observed Fe (both components) lies
well outside the central regions of the SNR, possibly having been ejected by
hydrodynamic instabilities during the explosion. We discuss this, and its
implications for the neutron star kick.Comment: ApJ, in press; 22 pages, 6 figures with multiple part
Discovery of X-Ray-Emitting O-Ne-Mg-Rich Ejecta in the Galactic Supernova Remnant Puppis A
We report on the discovery of X-ray--emitting O-Ne-Mg-rich ejecta in the
middle-aged Galactic O-rich supernova remnant Puppis A with Chandra and
XMM-Newton. We use line ratios to identify a low-ionization filament running
parallel to the northeastern edge of the remnant that requires supersolar
abundances, particularly for O, Ne, and Mg, which we interpret to be from
O-Ne-Mg-rich ejecta. Abundance ratios of Ne/O, Mg/O, and Fe/O are measured to
be ~2, ~2, and <0.3 times the solar values. Our spatially-resolved spectral
analysis from the northeastern rim to the western rim otherwise reveals
sub-solar abundances consistent with those in the interstellar medium. The
filament is coincident with several optically emitting O-rich knots with high
velocities. If these are physically related, the filament would be a peculiar
fragment of ejecta. On the other hand, the morphology of the filament suggests
that it may trace ejecta heated by a shock reflected strongly off the dense
ambient clouds near the northeastern rim.Comment: Published onlin
The Polar Regions of Cassiopeia A: The Aftermath of a Gamma Ray Burst?
Probably not, but it is interesting nevertheless to investigate just how
close Cas A might have come to generating such an event. Focusing on the
northeast jet filaments, we analyze the polar regions of the recently acquired
very deep 1 Ms Chandra X-ray observation. We infer that the so-called "jet"
regions are indeed due to jets emanating from the explosion center, and not due
to polar cavities in the circumstellar medium at the time of explosion. We
place limits on the equivalent isotropic explosion energy in the polar regions
(around 2.3 x 10^52 ergs), and the opening angle of the x-ray emitting ejecta
(around 7 degrees), which give a total energy in the NE jet of order 10^50
ergs; an order of magnitude or more lower than inferred for "typical" GRBs.
While the Cas A progenitor and explosion exhibit many of the features
associated with GRB hosts, e.g. extensive presupernova mass loss and rotation,
and jets associated with the explosion, we speculate that the recoil of the
compact central object, with velocity 330 km/s, may have rendered the jet
unstable. In such cases the jet rapidly becomes baryon loaded, if not truncated
altogether. Although unlikely to have produced a gamma ray burst, the jets in
Cas A suggest that such outflows may be common features of core-collapse SNe.Comment: 35 pages, 7 figures, accepted by Ap
Chandra X-ray Observation of a Mature Cloud-Shock Interaction in the Bright Eastern Knot Region of Puppis A
We present Chandra X-ray images and spectra of the most prominent cloud-shock
interaction region in the Puppis A supernova remnant. The Bright Eastern Knot
(BEK) has two main morphological components: (1) a bright compact knot that
lies directly behind the apex of an indentation in the eastern X-ray boundary
and (2) lying 1' westward behind the shock, a curved vertical structure (bar)
that is separated from a smaller bright cloud (cap) by faint diffuse emission.
Based on hardness images and spectra, we identify the bar and cap as a single
shocked interstellar cloud. Its morphology strongly resembles the ``voided
sphere'' structures seen at late times in Klein et al.'s experimental
simulations of cloud-shock interactions, when the crushing of the cloud by
shear instabilities is well underway. We infer an interaction time of roughly 3
cloud-crushing timescales, which translates to 2000-4000 years, based on the
X-ray temperature, physical size, and estimated expansion of the shocked cloud.
This is the first X-ray identified example of a cloud-shock interaction in this
advanced phase. Closer to the shock front, the X-ray emission of the compact
knot in the eastern part of the BEK region implies a recent interaction with
relatively denser gas, some of which lies in front of the remnant. The complex
spatial relationship of the X-ray emission of the compact knot to optical [O
III] emission suggests that there are multiple cloud interactions occurring
along the line of sight.Comment: 22 pages LaTeX with multiple figures, to appear in Ap
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