688 research outputs found
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
Variations in D/H and D/O from New FUSE Observations
We use data obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE)
to determine the interstellar abundances of DI, NI, OI, FeII, and H2 along the
sigh tlines to WD1034+001, BD+393226, and TD132709. Our main focus is on
determining the D/H, N/H, O/H, and D/O ratios along these sightlines, with log
N(H) > 20.0, that probe gas well outside of the Local Bubble. Hubble Space
Telescope (HST) and International Ultraviolet Explorer (IUE) archival data are
used to determine the HI column densities along the WD1034+001 and TD132709
sightlines, respectively. For BD+393226, a previously published N(HI) is used.
We find (D/H)x10^5 = 2.14 + 0.53 - 0.45, 1.17 + 0.31 - 0.25, and 1.86 + 0.53 -
0.43, and (D/O)x10^2 = 6.31 + 1.79 - 1.38, 5.62 + 1.61 - 1.31, and 7.59 + 2.17
- 1.76, for the WD1034+001, BD+393226, and TD132709 sightlines, respectively
(all 1 si gma). The scatter in these three D/H ratios exemplifies the scatter
that has been found by other authors for sightlines with column densities in
the range 19.2 < log N(H) < 20.7. The D/H ratio toward WD1034+001 and all the
D/O ratios derived here are inconsistent with the Local Bubble value and are
some of the highest in the literature. We discuss the implications of our
measurements for the determination of the present-epoch abundance of deuterium,
and for the different scenarios that try to explain the D/H variations. We
present a study of D/H as a function of the average sightline gas density,
using the ratios derived in this work as well as ratios from the literature,
which suggests that D/H decreases with increasing gas volume density. Similar
behaviors by other elements such Fe and Si have been interpreted as the result
of depletion into dust grains.Comment: Accepted for publication in the Ap
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
Spatial Variability in the Ratio of Interstellar Atomic Deuterium to Hydrogen. I. Observations toward delta Orionis by the Interstellar Medium Absorption Profile Spectrograph
Studies of the abundances of deuterium in different astrophysical sites are
of fundamental importance to answering the question about how much deuterium
was produced during big bang nucleosynthesis and what fraction of it was
destroyed later. With this in mind, we used the Interstellar Medium Absorption
Profile Spectrograph (IMAPS) on the ORFEUS-SPAS II mission to observe at a
wavelength resolution of 4 km/s (FWHM) the L-delta and L-epsilon absorption
features produced by interstellar atomic deuterium in the spectrum of delta Ori
A. A chi-square analysis indicated that 0.96 < N(D I)< 1.45e15 cm^{-2} at a 90%
level of confidence, and the gas is at a temperature of about 6000K. To obtain
an accurate value of N(H I) needed for a determination of the atomic ratio of D
to H, we measured the L-alpha absorption features in 57 spectra of delta Ori in
the IUE archive. From our measurement of N(H I)= 1.56e20 cm^{-2}, we found that
N(D I)/N(H I)= 7.4(+1.9,-1.3)e-6 (90% confidence). Our result for D/H contrasts
with the more general finding along other lines of sight that D/H is
approximately 1.5e-5. The underabundance of D toward delta Ori A is not
accompanied by an overabundance of N or O relative to H, as one might expect if
the gas were subjected to more stellar processing than usual.Comment: 37 pages, 6 figures. Submitted to the Astrophysical Journa
The D/H Ratio in the Interstellar Medium toward the White Dwarf PG0038+199
We determine the D/H ratio in the interstellar medium toward the DO white
dwarf PG0038+199 using spectra from the Far Ultraviolet Spectroscopic Explorer
(FUSE), with ground-based support from Keck HIRES. We employ curve of growth,
apparent optical depth and profile fitting techniques to measure column
densities and limits of many other species (H2, NaI, CI, CII, CIII, NI, NII,
OI, SiII, PII, SIII, ArI and FeII) which allow us to determine related ratios
such as D/O, D/N and the H2 fraction. Our efforts are concentrated on measuring
gas-phase D/H, which is key to understanding Galactic chemical evolution and
comparing it to predictions from Big Bang nucleosynthesis. We find column
densities log N(HI) = 20.41+-0.08, log N(DI)=15.75+-0.08 and log N(H2) =
19.33+-0.04, yielding a molecular hydrogen fraction of 0.14+-0.02 (2 sigma
errors), with an excitation temperature of 143+-5K. The high HI column density
implies that PG0038+199 lies outside of the Local Bubble; we estimate its
distance to be 297 (+164,-104)pc (1 sigma). D/[HI+2H2] toward PG0038+199 is
1.91(+0.52,-0.42) e-5 (2 sigma). There is no evidence of component structure on
the scale of Delta v > 8 km/s based on NaI, but there is marginal evidence for
structure on smaller scales. The D/H value is high compared to the majority of
recent D/H measurements, but consistent with the values for two other
measurements at similar distances. D/O is in agreement with other distant
measurements. The scatter in D/H values beyond ~100pc remains a challenge for
Galactic chemical evolution.Comment: 59 pages, 7 tables, 18 figures (1 standalone), accepted by ApJ v2
minor typos correcte
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
Oxygen Gas Phase Abundance Revisited
We present new measurements of the interstellar gas-phase oxygen abundance
along the sight lines towards 19 early-type galactic stars at an average
distance of 2.6 kpc. We derive O {\small I} column densities from {\it
HST}/STIS observations of the weak 1355 \AA intersystem transition. We derive
total hydrogen column densities [N(H {\small I})+2N(H)] using {\it
HST}/STIS observations of \lya and {\it FUSE} observations of molecular
hydrogen. The molecular hydrogen content of these sight lines ranges from
f(H) = 2N(H)/[N(H {\small I})+2N(H)] = 0.03 to 0.47. The average
of 6.3 cm mag with a standard
deviation of 15% is consistent with previous surveys. The mean oxygen abundance
along these sight lines, which probe a wide range of galactic environments in
the distant ISM, is 10 \oh = (1 in the mean). %(1 ). We see no evidence for decreasing
gas-phase oxygen abundance with increasing molecular hydrogen fraction and the
relative constancy of \oh suggests that the component of dust containing the
oxygen is not readily destroyed. We estimate that, if 60% of the dust grains
are resilient against destruction by shocks, the distant interstellar total
oxygen abundance can be reconciliated with the solar value derived from the
most recent measurements %by Holweger and by Allende Prieto, Lambert & Asplund:
of 10 \oh = 517 58 (1 ). We note that the smaller
oxygen abundances derived for the interstellar gas within 500 pc %by Meyer,
Cardelli & Jura or from nearby B star surveys are consistent with a local
elemental deficit.Comment: 9 figures, 37 page
Gamma-Ray Burst in a Molecular Cloud: Destruction of Dust and H2, and Emergent Spectrum
A gamma ray burst with strong optical-UV emission occuring in a molecular
cloud will photodissociate H2, photoionize H2, H, and He, and destroy dust
grains. We model these processes, including time-dependent radiative transfer
in both continuum radiation and the resonance lines of H2. The UV will pump H2
into vibrationally-excited levels. We calculate the absorption spectrum
imprinted on radiation from the GRB at various times. In addition to the strong
absorption lines of H2(v=0) at lambda < 1110 Angstrom, due to cold ambient gas,
we find that radiation reaching us from the GRB and its afterglow will show
strong absorption lines due to vibrationally-excited H2 at 1110 < lambda < 1705
Angstrom. These absorption lines, if observed, would provide unequivocal
evidence for association of the GRB with molecular gas. Low-resolution spectra
will exhibit conspicuous features due to clustering of individual lines; a list
of the strongest such absorption features is given for spectral resolution R
approx 350 characteristic of the grism on the Swift UV-Optical Telescope.Comment: submitted to ApJ, 27 pages, 11 figures, Late
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