37 research outputs found
HIRDES - The High-Resolution Double-Echelle Spectrograph for the World Space Observatory Ultraviolet (WSO/UV)
The World Space Observatory Ultraviolet (WSO/UV) is a multi-national project
grown out of the needs of the astronomical community to have future access to
the UV range. WSO/UV consists of a single UV telescope with a primary mirror of
1.7m diameter feeding the UV spectrometer and UV imagers. The spectrometer
comprises three different spectrographs, two high-resolution echelle
spectrographs (the High-Resolution Double-Echelle Spectrograph, HIRDES) and a
low-dispersion long-slit instrument. Within HIRDES the 102-310nm spectral band
is split to feed two echelle spectrographs covering the UV range 174-310nm and
the vacuum-UV range 102-176nm with high spectral resolution (R>50,000). The
technical concept is based on the heritage of two previous ORFEUS SPAS
missions. The phase-B1 development activities are described in this paper
considering performance aspects, design drivers, related trade-offs (mechanical
concepts, material selection etc.) and a critical functional and environmental
test verification approach. The current state of other WSO/UV scientific
instruments (imagers) is also described.Comment: Accepted for publication in Advances in Space Researc
A FUSE Survey of Molecular Hydrogen in Intermediate-Velocity Clouds in the Milky Way Halo
Far Ultraviolet Spectroscopic Explorer (FUSE) data is used to investigate the
molecular hydrogen (H_2) content of intermediate-velocity clouds (IVCs) in the
lower halo of the Milky Way. We analyze interstellar absorption towards 56
(mostly extragalactic) background sources to study H_2 absorption in the Lyman-
and Werner bands in 61 IVC components at H I column densities >10^19 cm^-2. For
data with good S/N (~9 per resolution element and higher), H_2 in IVC gas is
convincingly detected in 14 cases at column densities varying between ~10^14
and ~10^17 cm^-2. We find an additional 17 possible H_2 detections in IVCs in
FUSE spectra with lower S/N. The molecular hydrogen fractions, f, vary between
10^-6 and 10^-3, implying a dense, mostly neutral gas phase that is probably
related to the Cold Neutral Medium (CNM) in these clouds. If the H_2 stays in
formation-dissociation equlibrium, the CNM in these clouds can be characterized
by compact (D~0.1 pc) filaments with volume densities on the order of n_H~30
cm^-3. The relatively high detection rate of H_2 in IVC gas implies that the
CNM in these clouds is ubiquitous. More dense regions with much higher
molecular fractions may exist, but it would be difficult to detect them in
absorption because of their small size.Comment: 36 pages, 11 figures; accepted for publication in Ap
The FUSE Spectrum of PG0804+761: A Study of Atomic and MolecularGas in the Lower Galactic Halo and Beyond
We present an analysis of interstellar and intergalactic absorption lines in
the FUSE spectrum of the low-redshift quasar PG0804+761 (z=0.100) at
intermediate resolution (FWHM=25 km/s) in the direction l=138.3, b=31.0. With a
good signal-to-noise ratio and the presence of several interesting Galactic and
extragalactic absorption components along the sight line, this spectrum
provides a good opportunity to demonstrate the ability of FUSE to do both
interstellar and extragalactic science. Although the spectrum of PG0804+761 is
dominated by strong absorption from local Galactic gas, we concentrate our
study on absorption by molecular hydrogen and neutral neutral and ionized
metals related to an intermediate-velocity cloud in the lower Galactic halo at
-55 km/s, and on absorption from OVI extended to negative velocities. In the
IVC, weak molecular hydrogen absorption is found in 5 lines for rotational
levels 0 and 1, leading to a total H_2 column density of log N = 14.71(+-0.30).
We derive an OI gas-phase abundance for the IVC of 1.03(+0.71-0.42) solar.
Absorption by OVI is found at velocities as negative as -110 km/s, but no
absorption from any species is found at velocities of -180 km/ where absorption
from the nearby high-velocity Complex A would be expected. We suggest that the
extended OVI absorption traces hot gas above the Perseus spiral arm. Finally,
we find intergalactic absorption by an intervening HI Ly betax absorber at
z=0.019 and absorption by HI, CIII and OVI in an associated system at z=0.102.
No intervening OVI absorbers are seen in the spectrum of PG0804+761.Comment: 27 pages, 6 figures; accepted for publication in Ap
FUSE Observations of Atomic Abundances and Molecular Hydrogen in the Leading Arm of the Magellanic Stream
We present Far Ultraviolet Spectroscopic Explorer observations of the atomic
and molecular absorption in high velocity cloud HVC 287.5+22.5+240, which lies
in front of the ultraviolet-bright nucleus of the Seyfert 1 galaxy NGC 3783. We
detect H2, N I, N II, Si II, and Fe II absorption and set limits on the amount
of absorption due to P III, Ar I, and Fe III. We extend the earlier metallicity
and dust-depletion measurements made by Lu and collaborators by examining the
relative gas-phase abundances of Si, P, S, and Fe. Corrections to the derived
gas-phase abundances due to ionized gas in the HVC are less than 15%. The HVC
has a metallicity of 0.2-0.4 solar, similar to that of the Small Magellanic
Cloud. The relative abundance pattern for the elements studied resembles that
of warm gas in the SMC, which supports the idea that this HVC is part of the
tidally stripped Leading Arm of the Magellanic Stream. The abundance pattern
implies that the HVC contains dust grains that have been processed
significantly. It is likely that the grain mantles have been modified or
stripped back to expose the grain cores. We have identified more than 30 lines
of H2 arising in the HVC from rotational levels J = 0 to J = 3. Synthetic
spectra and a curve-of-growth fit to these lines with b = 12 km/sec indicate
that log N(H2) = 16.80+/-0.10 and f(H2) = 2N(H2)/[N(H I)+2N(H2)] = 0.0016. From
an analysis of the H2 rotational populations, we find an absorption rate (at
1000 A) that is less than one-tenth the average value in the solar
neighborhood. The presence of molecular gas in the HVC requires that either the
H2 formed in situ or that molecules formed within the SMC survived tidal
stripping. We favor the latter possibility because of the long formation time
derived for molecules in this HVC.Comment: 28 pages (includes 6 figures). AASTeX preprint format. Accepted for
publication in the February 2001 issue of the Astronomical Journa
Discovery of molecular hydrogen in a high-velocity cloud of the Galactic halo
We report the discovery of molecular hydrogen absorption in a Galactic
high-velocity cloud (HVC) in the direction of the Large Magellanic Cloud. For
the same HVC we derive an iron abundance which is half of the solar value.
Thus, all evidence points to a Galactic origin for high-velocity cloud complex
in front of the LMC.Comment: Published in Nature, this week; 14 pages, 3 figure
ORFEUS echelle spectra: Molecular hydrogen in disk, IVC, and HVC gas in front of the LMC
In front of the LMC molecular hydrogen is found in absorption near 0 km/s,
being local disk gas, near +60 km/s in an intermediate velocity cloud, and near
+120 km/s, in a high velocity halo cloud. The nature of the gas is discussed
based on four ORFEUS far UV spectra of LMC stars and including data from the
ground and from the IUE satellite. The local gas is cool and, given a span of
sight lines of only 2.5 deg, rather fluffy. The fractional abundance of H_2
varies from log(f)=-5.4 to -3.3. Metal depletions (up to -1.7 dex for Fe) are
typical for galactic disk gas. In the IV and HV gas an apparent underabundance
of neutral oxygen points to an ionization level of the gas of about 90%. H_2 is
detected in IV and HV gas toward HD 269546. In the IV gas we find an H_2 column
density of log(N)\simeq15.6. The H_2 excitation indicates that the line of
sight samples a cloud at a temperature below 150 K. Column densities are too
small to detect the higher UV pumped excitation levels. The high velocity H_2
(log(N)\simeq15.6) is highly excited and probably exposed to a strong radiation
field. Its excitation temperature exceeds 1000 K. Due to the radial velocity
difference between the halo gas and the Milky Way disk, the unattenuated disk
radiation is available for H_2 excitation in the halo. We do not find evidence
for an intergalactic origin of this gas; a galactic as well as a Magellanic
Cloud origin is possible.Comment: 12 pages, 5 figures, accepted for publication in A&