3,906 research outputs found
Small-Scale Structure of O VI Interstellar Gas in the Direction of the Globular Cluster NGC 6752
In order to study the small-scale structure of the hot interstellar gas, we
obtained Far Ultraviolet Spectroscopic Explorer interstellar O VI interstellar
absorption spectra of 4 four post-extreme horizontal branch stars in the
globular cluster NGC 6752 [(l,b) = (336.50,-25.63), d = 3.9 kpc, z= -1.7 kpc].
The good quality spectra of these stars allow us to measure both lines of the O
VI doublet at 1031.926 \AA and 1037.617 \AA. The close proximity of these stars
permits us to probe the hot interstellar gas over angular scale of only
2\farcm2 - 8\farcm9, corresponding to spatial scales \la 2.5-10.1 pc. On
these scales we detect no variations in the O VI column density and velocity
distribution. The average column density is log = 14.34 \pm 0.02 (log
= 13.98). The measured velocity dispersions of the O VI
absorption are also indistinguishable. Including the earlier results of Howk et
al., this study suggests that interstellar O VI is smooth on scales \Delta
\theta \la 12\arcmin, corresponding to a spatial scale of less than 10 pc, and
quite patchy at larger scales. Although such small scales are only probed in a
few directions, this suggests a characteristic size scale for the regions
producing collisionally-ionized O VI in the Galaxy.Comment: Accepted for publication in the PASP (to appear in the October 2004
issue
A Reservoir of Ionized Gas in the Galactic Halo to Sustain Star Formation in the Milky Way
Without a source of new gas, our Galaxy would exhaust its supply of gas
through the formation of stars. Ionized gas clouds observed at high velocity
may be a reservoir of such gas, but their distances are key for placing them in
the Galactic halo and unraveling their role. We have used the Hubble Space
Telescope to blindly search for ionized high-velocity clouds (iHVCs) in the
foreground of Galactic stars. We show that iHVCs with 90 < |v_LSR| < 170 km/s
are within one Galactic radius of the sun and have enough mass to maintain star
formation, while iHVCs with |v_LSR|>170 km/s are at larger distances. These may
be the next wave of infalling material.Comment: This paper is part of a set of three papers on circumgalactic gas
observed with COS and STIS on HST, to be published in Science, together with
related papers by Tripp et al. and Tumlinson et al., in the November 18, 2011
edition. This version has not undergone final copyediting. Please see Science
online for the final printed versio
FUSE Observations of the Magellanic Bridge Gas toward Two Early-Type Stars: Molecules, Physical Conditions, and Relative Abundance
We discuss FUSE observations of two early-type stars, DI1388 and DGIK975, in
the low density and low metallicity gas of Magellanic Bridge (MB). Toward
DI1388, the FUSE observations show molecular hydrogen, O VI, and numerous other
atomic or ionic transitions in absorption, implying the presence of multiple
gas phases in a complex arrangement. The relative abundance pattern in the MB
is attributed to varying degrees of depletion onto dust similar to that of halo
clouds. The N/O ratio is near solar, much higher than N/O in damped Ly-alpha
systems, implying subsequent stellar processing to explain the origin of
nitrogen in the MB. The diffuse molecular cloud in this direction has a low
column density and low molecular fraction. H2 is observed in both the
Magellanic Stream and the MB, yet massive stars form only in the MB, implying
significantly different physical processes between them. In the MB some of the
H2 could have been pulled out from the SMC via tidal interaction, but some also
could have formed in situ in dense clouds where star formation might have taken
place. Toward DGIK975, the presence of neutral, weakly and highly ionized
species suggest that this sight line has also several complex gas phases. The
highly ionized species of O VI, C IV, and Si IV toward both stars have very
broad features, indicating that multiple components of hot gas at different
velocities are present. Several sources (a combination of turbulent mixing
layer, conductive heating, and cooling flows) may be contributing to the
production of the highly ionized gas in the MB. Finally, this study has
confirmed previous results that the high-velocity cloud HVC 291.5-41.2+80 is
mainly ionized composed of weakly and highly ions. The high ion ratios are
consistent with a radiatively cooling gas in a fountain flow model.Comment: Accepted for publication in the ApJ (October 10, 2002). Added
reference (Gibson et al. 2000
Deuterium toward the WD0621-376 sight line: Results from the Far Ultraviolet Spectroscopic Explorer (FUSE) Mission
Far Ultraviolet Spectroscopic Explorer observations are presented for
WD0621-376, a DA white dwarf star in the local interstellar medium (LISM) at a
distance of about 78 pc. The data have a signal-to-noise ratio of about 20-40
per 20 km/s resolution element and cover the wavelength range 905-1187 \AA.
LISM absorption is detected in the lines of D I, C II, C II*, C III, N I, N II,
N III, O I, Ar I, and Fe II. This sight line is partially ionized, with an
ionized nitrogen fraction of > 0.23. We determine the ratio (2). Assuming a standard interstellar
oxygen abundance, we derive . Using the
value of N(H I) derived from EUVE data gives a similar D/H ratio. The D I/N I
ratio is (2).Comment: accepted for publication in the ApJ
The Magellanic System: What have we learnt from FUSE?
I review some of the findings on the Magellanic System produced by the Far
Ultraviolet Spectroscopic Explorer (FUSE) during and after its eight years of
service. The Magellanic System with its high-velocity complexes provides a
nearby laboratory that can be used to characterize phenomena that involve
interaction between galaxies, infall and outflow of gas and metals in galaxies.
These processes are crucial for understanding the evolution of galaxies and the
intergalactic medium. Among the FUSE successes I highlight are the coronal gas
about the LMC and SMC, and beyond in the Stream, the outflows from these
galaxies, the discovery of molecules in the diffuse gas of the Stream and the
Bridge, an extremely sub-solar and sub-SMC metallicity of the Bridge, and a
high-velocity complex between the Milky Way and the Clouds.Comment: A contributed paper to the FUSE Annapolis Conference "Future
Directions in Ultraviolet Spectroscopy.", 5 pages. To appear as an AIP
Conference Proceedin
Low Redshift Intergalactic Absorption Lines in the Spectrum of HE0226-4110
We present an analysis of the FUSE and STIS E140M spectra of HE0226-4110
(z=0.495). We detect 56 Lyman absorbers and 5 O VI absorbers. The number of
intervening O VI systems per unit redshift with W>50 m\AA is dN(O VI)/dz~ 11.
The O VI systems unambiguously trace hot gas only in one case. For the 4 other
O VI systems, photoionization and collisional ionization models are viable
options to explain the observed column densities of the O VI and the other
ions. If the O VI systems are mostly photoionized, only a fraction of the
observed O VI will contribute to the baryonic density of the warm-hot ionized
medium (WHIM) along this line of sight. Combining our results with previous
ones, we show that there is a general increase of N(O VI) with increasing b(O
VI). Cooling flow models can reproduce the N-b distribution but fail to
reproduce the observed ionic ratios. A comparison of the number of O I, O II, O
III, O IV, and O VI systems per unit redshift show that the low-z IGM is more
highly ionized than weakly ionized. We confirm that photoionized O VI systems
show a decreasing ionization parameter with increasing H I column density. O VI
absorbers with collisional ionization/photoionization degeneracy follow this
relation, possibly suggesting that they are principally photoionized. We find
that the photoionized O VI systems in the low redshift IGM have a median
abundance of 0.3 solar. We do not find additional Ne VIII systems other than
the one found by Savage et al., although our sensitivity should have allowed
the detection of Ne VIII in O VI systems at T~(0.6-1.3)x10^6 K (if CIE
applies). Since the bulk of the WHIM is believed to be at temperatures T>10^6
K, the hot part of the WHIM remains to be discovered with FUV--EUV metal-line
transitions.Comment: Accepted for publication in the ApJS. Full resolution figures
available at
http://www.journals.uchicago.edu/ApJ/journal/preprints/ApJS63975.preprint.pd
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