143 research outputs found
Far Ultraviolet Diffuse Emission from the Large Magellanic Cloud
We present the first observations of diffuse radiation in the far ultraviolet
(1000 -- 1150 \AA) from the Large Magellanic Cloud based on observations made
with the {\it Far Ultraviolet Spectroscopic Explorer}. The fraction of the
total radiation in the field emitted as diffuse radiation is typically 5 --
20\% with a high of 45\% near N70 where there are few exciting stars,
indicating that much of the emission is not due to nearby stars. Much less
light is scattered in the far ultraviolet than at longer wavelengths with the
stellar radiation going into heating the interstellar dust.Comment: Accepted in ApJ Letter
Spatially Resolved Far-Ultraviolet Spectroscopy of the Nuclear Region of NGC 1068
We carry out high-resolution FUSE spectroscopy of the nuclear region of NGC
1068. The first set of spectra was obtained with a 30" square aperture that
collects all emission from the narrow-line region. The data reveal a strong
broad OVI component of FWHM ~3500 kms-1 and two narrow OVI 1031/1037 components
of ~350 kms-1. The CIII 977 and NIII 991 emission lines in this spectrum can be
fitted with a narrow component of FWHM ~1000 kms-1 and a broad one of ~2500
kms-1. Another set of seven spatially resolved spectra were made using a long
slit of 1.25" X 20", at steps of ~1" along the axis of the emission-line cone.
We find that (1) Major emission lines in the FUSE wavelength range consist of a
broad and a narrow component; (2) There is a gradient in the velocity field for
the narrow OVI component of ~200 kms-1 from ~2" southwest of the nucleus to ~4"
northeast. A similar pattern is also observed with the broad OVI component,
with a gradient of ~3000 kms-1. These are consistent with the HST/STIS findings
and suggest a biconical structure in which the velocity field is mainly radial
outflow; (3) A major portion of the CIII and NIII line flux is produced in the
compact core. They are therefore not effective temperature diagnostics for the
conical region; and (4) The best-fitted UV continuum suggests virtually no
reddening, and the HeII 1085/1640 ratio suggests a consistently low extinction
factor across the cone.Comment: To appear in the Astrophysical Journal. 37 pages with 12 figure
A New Analysis of the O VI Emitting Nebula around KPD 0005+5106
We present observations of O VI 1032 emission around the helium white dwarf
KPD 0005+5106 obtained with the Far Ultraviolet Spectroscopic Explorer.
Previously published data, reprocessed with an updated version of the
calibration pipeline, are included along with new observations. The recent
upward revision of the white dwarf's effective temperature to 200,000 K has
motivated us to re-analyze all the data. We compare observations with
photoionization models and find that the density of the O VI nebula is about 10
cm^-3, and that the stellar flux must be attenuated by about 90% by the time it
impinges on the inner face of the nebula. We infer that this attenuation is due
to circumstellar material ejected by KPD 0005+5106 earlier in its evolution.Comment: 21 pages, including 3 figures and 2 tables. Accepted for publication
in the Astrophysical Journal. Minor change: a few uncited references remove
Observations and modeling of H_2 fluorescence with partial frequency redistribution in giant planet atmospheres
Partial frequency redistribution (PRD), describing the formation of the line
profile, has negligible observational effects for optical depths smaller than
~10^3, at the resolving power of most current instruments. However, when the
spectral resolution is sufficiently high, PRD modeling becomes essential in
interpreting the line shapes and determining the total line fluxes. We
demonstrate the effects of PRD on the H_2 line profiles observed at high
spectral resolution by the Far-Ultraviolet Spectroscopic Explorer (FUSE) in the
atmospheres of Jupiter and Saturn. In these spectra, the asymmetric shapes of
the lines in the Lyman (v"- 6) progression pumped by the solar Ly-beta are
explained by coherent scattering of the photons in the line wings. We introduce
a simple computational approximation to mitigate the numerical difficulties of
radiative transfer with PRD, and show that it reproduces the exact radiative
transfer solution to better than 10%. The lines predicted by our radiative
transfer model with PRD, including the H_2 density and temperature distribution
as a function of height in the atmosphere, are in agreement with the line
profiles observed by FUSE. We discuss the observational consequences of PRD,
and show that this computational method also allows us to include PRD in
modeling the continuum pumped H_2 fluorescence, treating about 4000 lines
simultaneously.Comment: 17 pages, accepted for publication in Ap
FUSE spectroscopy of sdOB primary of the post common-envelope binary LB 3459 (AA Dor)
LB 3459 (AA Dor) is an eclipsing, close, post common-envelope binary
consisting of an sdOB primary star and an unseen secondary with an
extraordinarly low mass - formally a brown dwarf. A recent NLTE spectral
analysis shows a discrepancy with the surface gravity, which is derived from
analyses of radial-velocity and lightcurves. We aim at precisely determing of
the photospheric parameters of the primary, especially of the surface gravity,
and searching for weak metal lines in the far UV. We performed a detailed
spectral analysis of the far-UV spectrum of LB 3459 obtained with FUSE by means
of state-of-the-art NLTE model-atmosphere techniques.
A strong contamination of the far-UV spectrum of LB 3459 by interstellar line
absorption hampers a precise determination of the photospheric properties of
its primary star. Its effective temperature (42 kK) was confirmed by the
evaluation of new ionization equilibria. For the first time, phosphorus and
sulfur have been identified in the spectrum of LB 3459. Their photospheric
abundances are solar and 0.01 times solar, respectively. From the C III
1174-1177A multiplet, we can measure the rotational velocity of 35 +/- 5 km/sec
of the primary of LB 3459 and confirm that the rotation is bound. From a
re-analysis of optical and UV spectra, we determine a higher log g = 5.3 (cgs)
that reduces the discrepancy in mass determination in comparison to analyses of
radial-velocity and lightcurves. However, the problem is not completely solved.Comment: 10 pages, 15 figure
Noise and dark performance for FIREBall-2 EMCCD delta-doped CCD detector
The Faint Intergalactic-medium Redshifted Emission Balloon (FIREBall-2) is an experiment designed to observe low density emission from HI, CIV, and OVI in the circum-galactic medium around low-redshift galaxies. To detect this diffuse emission, we use a high-efficiency photon-counting EMCCD as part of FIREBall-2's detector. The flight camera system includes a custom printed circuit board, a mechanical cryo-cooler, zeolite and charcoal getters, and a NĂŒvĂŒ controller, for fast read-out speeds and waveform shaping. Here we report on overall detector system performance, including pressure and temperature stability. We describe dark current and CIC measurements at several temperatures and substrate voltages, with the flight set-up
Chandra and FUSE spectroscopy of the hot bare stellar core H1504+65
H1504+65 is an extremely hot hydrogen-deficient white dwarf with an effective
temperature close to 200,000 K. We present new FUV and soft X-ray spectra
obtained with FUSE and Chandra, which confirm that H1504+65 has an atmosphere
primarily composed of carbon and oxygen. The Chandra LETG spectrum (60-160
Angstroem) shows a wealth of photospheric absorption lines from highly ionized
oxygen, neon, and - for the first time identified in this star - magnesium and
suggests relatively high Ne and Mg abundances. This corroborates an earlier
suggestion that H1504+65 represents a naked C/O stellar core or even the C/O
envelope of an O-Ne-Mg white dwarf.Comment: 15 pages, 10 figures, accepted for publication in A&
Opacity in the upper atmospheres of active stars II. AD Leonis
We present FUV and UV spectroscopic observations of AD Leonis, with the aim
of investigating opacity effects in the transition regions of late-type stars.
The C III lines in FUSE spectra show significant opacity during both the
quiescent and flaring states of AD Leonis, with up to 30% of the expected flux
being lost during the latter. Other FUSE emission lines tested for opacity
include those of O VI, while C IV, Si IV and N V transitions observed with STIS
are also investigated. These lines only reveal modest amounts of opacity with
losses during flaring of up to 20%. Optical depths have been calculated for
homogeneous and inhomogeneous geometries, giving path lengths of ~20-60 km and
\~10-30 km, respectively, under quiescent conditions. However path lengths
derived during flaring are ~2-3 times larger. These values are in excellent
agreement with both estimates of the small-scale structure observed in the
solar transition region, and path lengths derived previously for several other
active late-type stars.Comment: 13 pages, 4 figures, 4 Tabels, accepted A&
A search for line intensity enhancements in the far-UV spectra of active late-type stars arising from opacity
Radiative transfer calculations have predicted intensity enhancements for
optically thick emission lines, as opposed to the normal intensity reductions,
for astrophysical plasmas under certain conditions. In particular, the results
are predicted to be dependent both on the geometry of the emitting plasma and
the orientation of the observer. Hence in principle the detection of intensity
enhancement may provide a way of determining the geometry of an unresolved
astronomical source. To investigate such enhancements we have analyzed a sample
of active late-type stars observed in the far ultraviolet spectral region.
Emission lines of O VI in the FUSE satellite spectra of epsilon Eri, II Peg and
Prox Cen were searched for intensity enhancements due to opacity. We have found
strong evidence for line intensity enhancements due to opacity during active or
flare-like activity for all three stars. The O VI 1032/1038 line intensity
ratios, predicted to have a value of 2.0 in the optically thin case, are found
to be up to ~30% larger during several orbital phases. Our measurements,
combined with radiative transfer models, allow us to constrain both the
geometry of the O VI emitting regions in our stellar sources and the
orientation of the observer. A spherical emitting plasma can be ruled out, as
this would lead to no intensity enhancement. In addition, the theory tells us
that the line-of-sight to the plasma must be close to perpendicular to its
surface, as observations at small angles to the surface lead to either no
intensity enhancement or the usual line intensity decrease over the optically
thin value. For the future, we outline a laboratory experiment, that could be
undertaken with current facilities, which would provide an unequivocal test of
predictions of line intensity enhancement due to opacity, in particular the
dependence on plasma geometry.Comment: 10 Pages, 8 Figures, and 2 Tables; Accepted in A&
- âŠ