13 research outputs found
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
The Advanced Spectral Library (ASTRAL): Reference Spectra for Evolved M Stars
The HST (Hubble Space Telescope) Treasury Program Advanced Spectral Library Project: Cool Stars was designed to collect representative, high-quality ultraviolet spectra of eight evolved F-M type cool stars. The Space Telescope Imaging Spectrograph (STIS) echelle spectra of these objects enable investigations of a broad range of topics, including stellar and interstellar astrophysics. This paper provides a guide to the spectra of the two evolved M stars, the M2 Iab super giant alpha Ori and the M3.4 giant gamma Cru, with comparisons to the prototypical K1.5 giant alpha Boo. It includes identifications of the significant atomic and molecular emission and absorption features and discusses the character of the photospheric and chromospheric continua and line spectra. The fluorescent processes responsible for a large portion of the emission-line spectrum, the characteristics of the stellar winds, and the available diagnostics for hot and cool plasmas are also summarized. This analysis will facilitate the future study of the spectra, outer atmospheres, and winds, not only of these objects but of numerous other cool, low-gravity stars, for years to come
The Mass of the Cepheid V350 Sgr
V350 Sgr is a classical Cepheid suitable for mass determination. It has a hot
companion which is prominent in the ultraviolet and which is not itself a
binary. We have obtained two high resolution echelle spectra of the companion
at orbital velocity maximum and minimum with the Hubble Space Telescope (HST)
Space Telescope Imaging Spectrograph (STIS) in the 1320 to 1510 \AA\/ region.
By cross-correlating these spectra we obtained the orbital velocity amplitude
of the companion with an uncertainty in the companion amplitude of 1.9 km
sec. This provides a mass ratio of the Cepheid to the companion of 2.1.
The ultraviolet energy distribution of the companion provides the mass of the
companion, yielding a Cepheid mass of 5.2 0.3 M. This mass
requires some combination of moderate main sequence core convective overshoot
and rotation to match evolutionary tracks.Comment: Accepted by Ap
X-ray Spectral Variation of Eta Carinae through the 2003 X-ray Minimum
We report the results of an X-ray observing campaign on the massive, evolved
star Eta Carinae, concentrating on the 2003 X-ray minimum as seen by the
XMM-Newton observatory. These are the first spatially-resolved X-ray monitoring
observations of the stellar X-ray spectrum during the minimum. The hard X-ray
emission, believed to be associated with the collision of Eta Carinae's wind
with the wind from a massive companion star, varied strongly in flux on
timescales of days, but not significantly on timescales of hours. The lowest
X-ray flux in the 2-10 keV band seen by XMM-Newton was only 0.7% of the maximum
seen by RXTE just before the X-ray minimum. The slope of the X-ray continuum
above 5 keV did not vary in any observation, which suggests that the electron
temperature of the hottest plasma associated with the stellar source did not
vary significantly at any phase. Through the minimum, the absorption to the
stellar source increased by a factor of 5-10 to NH ~3-4E23 cm-2. The thermal Fe
XXV emission line showed significant excesses on both the red and blue sides of
the line outside the minimum and exhibited an extreme red excess during the
minimum. The Fe fluorescence line at 6.4 keV increased in equivalent width from
100 eV outside the minimum to 200 eV during the minimum. From these observed
features, we discuss two possible causes of the X-ray minimum; the eclipse of
the X-ray plasma and an intrinsic fading of the X-ray emissivity. The drop in
the colliding wind X-ray emission also revealed the presence of an additional
X-ray component which exhibited no variation on timescales of weeks to years.
This component may be produced by the collision of high speed outflows at v
\~1000-2000 km s-1 from Eta Carinae with ambient gas within a few thousand AU
from the star.Comment: 35 pages, 14 figures, accepted for publication in Ap
The Advanced Spectral Library (ASTRAL): Reference Spectra for Evolved M Stars
The HST Treasury Program Advanced Spectral Library Project: Cool Stars was designed to collect representative, high-quality UV spectra of eight evolved FM type cool stars. The Space Telescope Imaging Spectrograph (STIS)echelle spectra of these objects enable investigations of a broad range of topics, including stellar and interstellar astrophysics. This paper provides a guide to the spectra of the two evolved M stars, the M2 Iab supergiant Oriand the M3.4 giant Cru, with comparisons to the prototypical K1.5 giant Boo. It includes identifications of the significant atomic and molecular emission and absorption features and discusses the character of the photospheric and chromospheric continua and line spectra. The fluorescent processes responsible for a large portion of the emission-line spectrum, the characteristics of the stellar winds, and the available diagnostics for hot and cool plasmas are also summarized. This analysis will facilitate the future study of the spectra, outer atmospheres, and winds, not only of these objects but of numerous other cool, low-gravity stars, for years to come