953 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
Non-WKB Models of the FIP Effect: The Role of Slow Mode Waves
A model for element abundance fractionation between the solar chromosphere
and corona is further developed. The ponderomotive force due to Alfven waves
propagating through, or reflecting from the chromosphere in solar conditions
generally accelerates chromospheric ions, but not neutrals, into the corona.
This gives rise to what has become known as the First Ionization Potential
(FIP) Effect. We incorporate new physical processes into the model. The
chromospheric ionization balance is improved, and the effect of different
approximations is discussed. We also treat the parametric generation of slow
mode waves by the parallel propagating Alfven waves. This is also an effect of
the ponderomotive force, arising from the periodic variation of the magnetic
pressure driving an acoustic mode, which adds to the background longitudinal
pressure. This can have subtle effects on the fractionation, rendering it
quasi-mass independent in the lower regions of the chromosphere. We also
briefly discuss the change in the fractionation with Alfven wave frequency,
relative to the frequency of the overlying coronal loop resonance.Comment: 32 pages, 8 figures, accepted by Ap
On the Origin of the Slow Speed Solar Wind: Helium Abundance Variations
The First Ionization Potential (FIP) effect is the by now well known
enhancement in abundance over photospheric values of Fe and other elements with
first ionization potential below about 10 eV observed in the solar corona and
slow speed solar wind. In our model, this fractionation is achieved by means of
the ponderomotive force, arising as Alfv\'en waves propagate through or reflect
from steep density gradients in the solar chromosphere. This is also the region
where low FIP elements are ionized, and high FIP elements are largely neutral
leading to the fractionation as ions interact with the waves but neutrals do
not. Helium, the element with the highest FIP and consequently the last to
remain neutral as one moves upwards can be depleted in such models. Here, we
investigate this depletion for varying loop lengths and magnetic field
strengths.
Variations in this depletion arise as the concentration of the ponderomotive
force at the top of the chromosphere varies in response to Alfv\'en wave
frequency with respect to the resonant frequency of the overlying coronal loop,
the magnetic field, and possibly also the loop length. We find that stronger
depletions of He are obtained for weaker magnetic field, at frequencies close
to or just above the loop resonance. These results may have relevance to
observed variations of the slow wind solar He abundance with wind speed, with
slower slow speed solar wind having a stronger depletion of He.Comment: 28 pages, 12 figures, accepted to Ap
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