523 research outputs found
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
H_2 Absorption and Fluorescence for Gamma Ray Bursts in Molecular Clouds
If a gamma ray burst with strong UV emission occurs in a molecular cloud,
there will be observable consequences resulting from excitation of the
surrounding H2. The UV pulse from the GRB will pump H2 into
vibrationally-excited levels which produce strong absorption at wavelengths <
1650 A. As a result, both the prompt flash and later afterglow will exhibit
strong absorption shortward of 1650 A, with specific spectroscopic features.
Such a cutoff in the emission from GRB 980329 may already have been observed by
Fruchter et al.; if so, GRB 980329 was at redshift 3.0 < z < 4.4 . BVRI
photometry of GRB 990510 could also be explained by H2 absorption if GRB 990510
is at redshift 1.6 < z < 2.3. The fluorescence accompanying the UV pumping of
the H2 will result in UV emission from the GRB which can extend over days or
months, depending on parameters of the ambient medium and beaming of the GRB
flash. The 7.5-13.6 eV fluorescent luminosity is \sim 10^{41.7} erg/s for
standard estimates of the parameters of the GRB and the ambient medium.
Spectroscopy can distinguish this fluorescent emission from other possible
sources of transient optical emission, such as a supernova.Comment: 13 pages, including 4 figures. submitted to Ap.J.(Letters
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
Molecular Hydrogen Emission Lines in Far Ultraviolet Spectroscopic Explorer Observations of Mira B
We present new Far Ultraviolet Spectroscopic Explorer (FUSE) observations of
Mira A's wind-accreting companion star, Mira B. We find that the strongest
lines in the FUSE spectrum are H2 lines fluoresced by H I Lyman-alpha. A
previously analyzed Hubble Space Telescope (HST) spectrum also shows numerous
Lyman-alpha fluoresced H2 lines. The HST lines are all Lyman band lines, while
the FUSE H2 lines are mostly Werner band lines, many of them never before
identified in an astrophysical spectrum. We combine the FUSE and HST data to
refine estimates of the physical properties of the emitting H2 gas. We find
that the emission can be reproduced by an H2 layer with a temperature and
column density of T=3900 K and log N(H2)=17.1, respectively. Another similarity
between the HST and FUSE data, besides the prevalence of H2 emission, is the
surprising weakness of the continuum and high temperature emission lines,
suggesting that accretion onto Mira B has weakened dramatically. The UV fluxes
observed by HST on 1999 August 2 were previously reported to be over an order
of magnitude lower than those observed by HST and the International Ultraviolet
Explorer (IUE) from 1979--1995. Analysis of the FUSE data reveals that Mira B
was still in a similarly low state on 2001 November 22.Comment: 23 pages, 6 figures; AASTEX v5.0 plus EPSF extensions in mkfig.sty;
accepted by Ap
The Deuterium, Oxygen, and Nitrogen Abundance Toward LSE 44
We present measurements of the column densities of interstellar DI, OI, NI,
and H2 made with FUSE, and of HI made with IUE toward the sdO star LSE 44, at a
distance of 554+/-66 pc. This target is among the seven most distant Galactic
sight lines for which these abundance ratios have been measured. The column
densities were estimated by profile fitting and curve of growth analyses. We
find D/H = (2.24 +1.39 -1.32)E-5, D/O = (1.99 +1.30 -0.67)E-2, D/N = (2.75
+1.19 -0.89)E-1, and O/H = (1.13 +0.96 -0.71)E-3 (2 sigma). Of the most distant
Galactic sight lines for which the deuterium abundance has been measured LSE 44
is one of the few with D/H higher than the Local Bubble value, but D/O toward
all these targets is below the Local Bubble value and more uniform than the D/H
distribution. (Abstract abridged.)Comment: 20 pages, including 9 figures. Accepted for publication in Ap
Progress in Atomic Fountains at LNE-SYRTE
We give an overview of the work done with the Laboratoire National de
M\'etrologie et d'Essais-Syst\`emes de R\'ef\'erence Temps-Espace (LNE-SYRTE)
fountain ensemble during the last five years. After a description of the clock
ensemble, comprising three fountains, FO1, FO2, and FOM, and the newest
developments, we review recent studies of several systematic frequency shifts.
This includes the distributed cavity phase shift, which we evaluate for the FO1
and FOM fountains, applying the techniques of our recent work on FO2. We also
report calculations of the microwave lensing frequency shift for the three
fountains, review the status of the blackbody radiation shift, and summarize
recent experimental work to control microwave leakage and spurious phase
perturbations. We give current accuracy budgets. We also describe several
applications in time and frequency metrology: fountain comparisons,
calibrations of the international atomic time, secondary representation of the
SI second based on the 87Rb hyperfine frequency, absolute measurements of
optical frequencies, tests of the T2L2 satellite laser link, and review
fundamental physics applications of the LNE-SYRTE fountain ensemble. Finally,
we give a summary of the tests of the PHARAO cold atom space clock performed
using the FOM transportable fountain.Comment: 19 pages, 12 figures, 5 tables, 126 reference
Variations in the D/H ratio of extended sightlines from FUSE observations
We use new FUSE data to determine the column densities of interstellar DI,
NI, OI, FeII, and H2 along the HD41161 and HD53975 sightlines. Together with
N(HI) from the literature (derived from Copernicus and IUE data) we derive D/H,
N/H, and O/H ratios. These high column density sightlines have both log
H(HI)>21.00 and allow us to probe gas up to 1300 pc. In particular these
sightlines allow us to determine the gas phase D/H ratio in a hydrogen column
density range, log N(H)>20.70, where the only five measurements available in
the literature yield a weighted average of D/H = (0.86 +/- 0.08)E-5. We find
D/H=(2.14+ 0.51 - 0.43)E-5 along the HD41161 sightline. This ratio is 3sigma
higher than the weighted mean D/H ratio quoted above, for sightlines with log
N(H)>20.70, while the D/H ratio for the HD53975 line of sight, D/H = (1.02
+0.23 -0.20)E-5, agrees within the 1sigma uncertainties. Our D/H measurement
along the HD 41161 sightline presents the first evidence of variations of D/H
at high N(H). Our result seems to indicate that either the long sightlines that
according to the deuterium depletion model are dominated by cold undisturbed
gas where deuterium would be depleted onto carbonaceous grains occur at higher
N(H) than previously thought or that the clumping of low D/H values in the
literature for the long sightlines has another explanation. In addition, the
relatively high signal-to-noise ratio of the HD41161 data allows us to place
constraints on the f-values of some neutral chlorine transitions, present in
the FUSE bandpass, for which only theoretical values are available.Comment: Accepted for publication on the Dec 10 2006 issue of the Ap
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