84 research outputs found
FUSE Observations of the Cygnus Loop: OVI Emission from a Nonradiative Shock
We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations of a
Balmer filament in the northeast region of the Cygnus Loop supernova remnant.
The data consist of one spectrum obtained through the 30"x30" (LWRS) aperture
and three spectra at adjacent positions obtained through the 4"x20" (MDRS)
aperture. The nonradiative shocks in the region giving rise to these faint
optical filaments produce strong OVI 1032,1038 emission, which is detected in
all the spectra. The OVI emission is resolved by FUSE into a strong component
centered at 0 km/s, and weaker components centered at +/- 140 km/s. The MDRS
spectra allow us to study the variation of OVI emission in the post-shock
structure. We find that the zero velocity emission is associated directly with
the Balmer filament shock, while the high velocity emission comes from a more
uniformly distributed component elsewhere along the line of sight. We also find
that the shocks producing the emission at +/- 140 km/s have velocities between
180 km/s and 220 km/s, if we assume that the ram pressure driving them is the
same as for the zero velocity component shock. In the context of the cavity
model for the Cygnus Loop, the interaction of the blast wave with the spherical
shell that forms most of the cavity wall can naturally give rise to the similar
red and blue-shifted components that are observed.Comment: LaTeX uses aaspp4.sty, 18 pages + 5 PostScript figures. Accepted for
publication in the Astrophysical Journa
Resonance Line Scattering in Supernova Remnant Shocks
We present a three dimensional radiative transfer model to examine the
effects of resonance line scattering in the post-shock flow behind a
non-radiative supernova remnant shock. For a rippled shock front viewed
edge-on, line scattering significantly reduces the observed flux of CIV 1549
and NV 1240, two important diagnostic lines in the ultraviolet spectra of
supernova remnants. The correction factor (defined to be the ratio of the line
flux that would be observed neglecting scattering, to the actual observed line
flux) is a function of position within the filament. For sufficiently large
regions that include crisp edges as well as more diffuse regions of the
filament structure, the CIV and NV correction factors are between about 1.5 and
3.5 (and the CIV correction factor is invariably larger than the NV correction
factor). The correction factors have a larger range when smaller regions are
considered. The CIV correction factor is about 6 at the filament edges, while
the NV correction factor is about 4. These simulations of resonance line
scattering will be useful for the analysis of supernova remnant shock spectra.Comment: LaTeX uses aaspp4.sty, 12 pages + 5 PostScript figures. Accepted for
publication in the Astrophysical Journa
An Extended FUSE Survey of Diffuse O VI Emission in the Interstellar Medium
We present a survey of diffuse O VI emission in the interstellar medium
obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE). Spanning 5.5
years of FUSE observations, from launch through 2004 December, our data set
consists of 2925 exposures along 183 sight lines, including all of those with
previously-published O VI detections. The data were processed using an
implementation of CalFUSE v3.1 modified to optimize the signal-to-noise ratio
and velocity scale of spectra from an aperture-filling source. Of our 183 sight
lines, 73 show O VI 1032 emission, 29 at > 3-sigma significance. Six of the
3-sigma features have velocities |v_LSR| > 120 km/s, while the others have
|v_LSR| < 50 km/s. Measured intensities range from 1800 to 9100 LU, with a
median of 3300 LU. Combining our results with published O VI absorption data,
we find that an O VI-bearing interface in the local ISM yields an electron
density n_e = 0.2--0.3 cm^-3^ and a path length of 0.1 pc, while O VI-emitting
regions associated with high-velocity clouds in the Galactic halo have
densities an order of magnitude lower and path lengths two orders of magnitude
longer. Though the O VI intensities along these sight lines are similar, the
emission is produced by gas with very different properties.Comment: 27 pages, 16 figures, accepted by Ap
Discovery of an OVI Emitting Nebula around the Hot White Dwarf KPD 0005+5106
A survey of diffuse interstellar sight lines observed with the Far
Ultraviolet Spectroscopic Explorer has led to the serendipitous discovery of a
high-ionization nebula around the hot white dwarf KPD 0005+5106. The nebula has
an OVI 1032A surface brightness of up to 25,000 photons/s/cm^2/sr, making it
the brightest region of extended OVI emission in our survey. Photoionization
models using the incident white dwarf continuum successfully reproduce the
observed OVI intensity. The OVI emission arises in the highly ionized inner
region of a planetary nebula around KPD 0005+5106. This newly discovered nebula
may be one member of a class of high-ionization planetary nebulae that are
difficult to detect in the optical, but which can be easily identified in the
ultraviolet.Comment: accepted for publication in ApJL, 11 pages including 2 figure
FUSE Detection of Galactic OVI Emission in the Halo above the Perseus Arm
Background observations obtained with the Far Ultraviolet Spectroscopic
Explorer (FUSE) toward l=95.4, b=36.1 show OVI 1032,1038 in emission. This
sight line probes a region of stronger-than-average soft X-ray emission in the
direction of high-velocity cloud Complex C above a part of the disk where
Halpha filaments rise into the halo. The OVI intensities, 1600+/-300
ph/s/cm^2/sr (1032A) and 800+/-300 ph/s/cm^2/sr (1038A), are the lowest
detected in emission in the Milky Way to date. A second sight line nearby
(l=99.3, b=43.3) also shows OVI 1032 emission, but with too low a
signal-to-noise ratio to obtain reliable measurements. The measured
intensities, velocities, and FWHMs of the OVI doublet and the CII* line at
1037A are consistent with a model in which the observed emission is produced in
the Galactic halo by hot gas ejected by supernovae in the Perseus arm. An
association of the observed gas with Complex C appears unlikely.Comment: accepted for publication in ApJL, 11 pages including 3 figure
Dust in a Type Ia Supernova Progenitor: Spitzer Spectroscopy of Kepler's Supernova Remnant
Characterization of the relatively poorly-understood progenitor systems of
Type Ia supernovae is of great importance in astrophysics, particularly given
the important cosmological role that these supernovae play. Kepler's Supernova
Remnant, the result of a Type Ia supernova, shows evidence for an interaction
with a dense circumstellar medium (CSM), suggesting a single-degenerate
progenitor system. We present 7.5-38 m infrared (IR) spectra of the
remnant, obtained with the {\it Spitzer Space Telescope}, dominated by emission
from warm dust. Broad spectral features at 10 and 18 m, consistent with
various silicate particles, are seen throughout. These silicates were likely
formed in the stellar outflow from the progenitor system during the AGB stage
of evolution, and imply an oxygen-rich chemistry. In addition to silicate dust,
a second component, possibly carbonaceous dust, is necessary to account for the
short-wavelength IRS and IRAC data. This could imply a mixed chemistry in the
atmosphere of the progenitor system. However, non-spherical metallic iron
inclusions within silicate grains provide an alternative solution. Models of
collisionally-heated dust emission from fast shocks ( 1000 km s)
propagating into the CSM can reproduce the majority of the emission associated
with non-radiative filaments, where dust temperatures are K, but
fail to account for the highest temperatures detected, in excess of 150 K. We
find that slower shocks (a few hundred km s) into moderate density
material ( cm) are the only viable source of heating
for this hottest dust. We confirm the finding of an overall density gradient,
with densities in the north being an order of magnitude greater than those in
the south.Comment: Accepted by ApJ. 11 pages, 5 figures, 1 table. Produced using
emulateapj forma
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