694 research outputs found
Molecular Hydrogen Optical Depth Templates for FUSE Data Analysis
The calculation and use of molecular hydrogen optical depth templates to
quickly identify and model molecular hydrogen absorption features longward of
the Lyman edge at 912 Angstroms are described. Such features are commonly
encountered in spectra obtained by the Far Ultraviolet Spectroscopic Explorer
and also in spectra obtained by the Space Telescope Imaging Spectrograph,
albeit less commonly. Individual templates are calculated containing all the
Lyman and Werner transitions originating from a single rotational state (J'')
of the 0th vibrational level (v'') of the ground electronic state. Templates
are provided with 0.01 Angstrom sampling for doppler parameters ranging from 2
<= b <= 20 km s^-1 and rotational states 0 <= J'' <= 15. Optical depth
templates for excited vibrational states are also available for select doppler
parameters. Each template is calculated for a fiducial column density of
log[N(cm^-2)] = 21 and may be scaled to any column less than this value without
loss of accuracy. These templates will facilitate the determination of the
distribution of molecular hydrogen column density as a function of rotational
level. The use of these templates will free the user from the computationally
intensive task of calculating profiles for a large number of lines and allow
concentration on line profile or curve-of-growth fitting to determine column
densities and doppler parameters. The templates may be downloaded freely from
http://www.pha.jhu.edu/~stephan/h2ools2.htmlComment: 20 pages, 2 tables, 6 figures, submitted to PASP 02-04-2003 Accepted
for publication on 03-05-2003 with revisions, including modified fg1, modifed
fg6 to become fg2 to support improved error discussion. To appear in the June
2003 issue of the PAS
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
Physical Properties and Baryonic Content of Low-Redshift Intergalactic Ly-alpha and O VI Absorption Systems: The PG1116+215 Sight Line
We present HST and FUSE observations of the intergalactic absorption toward
PG1116+215 in the 900-3000 A spectral region. We detect 25 Ly-alpha absorbers
at rest-frame equivalent widths W_r > 30 mA, yielding (dN/dz)_Ly-alpha =
154+/-18 over an unblocked redshift path of 0.162. Two additional weak Ly-alpha
absorbers with W_r ~ 15-20 mA are also present. Eight of the Ly-alpha absorbers
have large line widths (b > 40 km/sec). The detection of narrow OVI in the
broad Ly-alpha absorber at z=0.06244 supports the idea that the Ly-alpha
profile is thermally broadened in gas with T > 10^5 K. We find dN/dz ~ 50 for
broad Ly-alpha absorbers with W_r > 30 mA and b > 40 km/sec. If the broad
Ly-alpha lines are dominated by thermal broadening in hot gas, the amount of
baryonic material in these absorbers is enormous, perhaps as much as half the
baryonic mass in the low-redshift universe. We detect OVI absorption in several
of the Ly-alpha clouds along the sight line. Two detections at z=0.13847 and
z=0.16548 are confirmed by the presence of other ions at these redshifts, while
the detections at z=0.04125, 0.05895, 0.05928, and 0.06244 are based upon the
Ly-alpha and OVI detections alone. The information available for 13
low-redshift OVI absorbers with W_r > 50 mA along 5 sight lines yields
(dN/dz)_OVI ~ 14 and Omega_b(OVI) > 0.0027/h_75, assuming a metallicity of 0.1
solar and an OVI ionization fraction < 0.2. The properties and prevalence of
low-redshift OVI absorbers suggest that they too may be a substantial baryon
repository, perhaps containing as much mass as stars and gas inside galaxies.
The redshifts of the OVI absorbers are highly correlated with the redshifts of
galaxies along the sight line, though few of the absorbers lie closer than
600/h_75 kpc to any single galaxy. [abbreviated]Comment: 99 pages, 30 figures, aastex format, ApJS in pres
Discovery of Enhanced Germanium Abundances in Planetary Nebulae with FUSE
We report the discovery of Ge III 1088.46 in the planetary nebulae
(PNe) SwSt 1, BD+303639, NGC 3132, and IC 4593, observed with the Far
Ultraviolet Spectroscopic Explorer. This is the first astronomical detection of
this line and the first measurement of Ge (Z = 32) in PNe. We estimate Ge
abundances using S and Fe as reference elements, for a range of assumptions
about gas-phase depletions. The results indicate that Ge, which is synthesized
in the initial steps of the s-process and therefore can be self-enriched in
PNe, is enhanced by factors of > 3-10. The strongest evidence for enrichment is
seen for PNe with Wolf-Rayet central stars, which are likely to contain heavily
processed material.Comment: 11 pages, 1 figure, accepted for publication in ApJ Letter
A FUSE survey of high-latitude Galactic molecular hydrogen
Measurements of molecular hydrogen (H_2) column densities are presented for
the first six rotational levels (J=0 to 5) for 73 extragalactic targets
observed with FUSE. All of these have a final signal-to-noise ratio larger than
\snlimit, and are located at galactic latitude |b|>20 deg. The individual
observations were calibrated with the FUSE calibration pipeline CalFUSE version
2.1 or higher, and then carefully aligned in velocity. The final velocity
shifts for all the FUSE segments are listed. H_2 column densities or limits are
determined for the 6 lowest rotational (J) levels for each HI component in the
line of sight, using a curve-of-growth approach at low column densities ~16.5),
and Voigt-profile fitting at higher column densities. Detections include 73
measurements of low-velocity H_2 in the Galactic Disk and lower Halo. Eight
sightlines yield non-detections for Galactic H_2. The measured column densities
range from log N(H_2)=14 to log N(H_2)=20. Strong correlations are found
between log N(H_2) and T_01, the excitation temperature of the H_2, as well as
between log N(H_2) and the level population ratios (log (N(J')/N(J))). The
average fraction of nuclei in molecular hydrogen (f(H_2)) in each sightline is
calculated; however, because there are many HI clouds in each sightline, the
physics of the transition from HI to H_2 can not be studied. Detections also
include H2 in 16 intermediate-velocity clouds in the Galactic Halo (out of 35
IVCs). Molecular hydrogen is seen in one high-velocity cloud (the Leading Arm
of the Magellanic Stream), although 19 high-velocity clouds are intersected;
this strongly suggests that dust is rare or absent in these objects. Finally,
there are five detections of H_2 in external galaxies.Comment: Accepted for ApJ Supplement. Note: figs 7 and 8 not included because
astro-ph rejects them as too bi
Dynamical Expansion of Ionization and Dissociation Front around a Massive Star. II. On the Generality of Triggered Star Formation
We analyze the dynamical expansion of the HII region, photodissociation
region, and the swept-up shell, solving the UV- and FUV-radiative transfer, the
thermal and chemical processes in the time-dependent hydrodynamics code.
Following our previous paper, we investigate the time evolutions with various
ambient number densities and central stars. Our calculations show that basic
evolution is qualitatively similar among our models with different parameters.
The molecular gas is finally accumulated in the shell, and the gravitational
fragmentation of the shell is generally expected. The quantitative differences
among models are well understood with analytic scaling relations. The detailed
physical and chemical structure of the shell is mainly determined by the
incident FUV flux and the column density of the shell, which also follow the
scaling relations. The time of shell-fragmentation, and the mass of the
gathered molecular gas are sensitive tothe ambient number density. In the case
of the lower number density, the shell-fragmentation occurs over a longer
timescale, and the accumulated molecular gas is more massive. The variations
with different central stars are more moderate. The time of the
shell-fragmentation differs by a factor of several with the various stars of
M_* = 12-101 M_sun. According to our numerical results, we conclude that the
expanding HII region should be an efficient trigger for star formation in
molecular clouds if the mass of the ambient molecular material is large enough.Comment: 49 pages, including 17 figures ; Accepted for publication in Ap
The Radiative Feedback of the First Cosmological Objects
In hierarchical models of structure formation, an early cosmic UV background
(UVB) is produced by the small (T_vir < 10^4 K) halos that collapse before
reionization. The UVB at energies below 13.6eV suppresses the formation of
stars or black holes inside small halos, by photo-dissociating their only
cooling agent, molecular H2. We self-consistently compute the buildup of the
early UVB in Press-Schechter models, coupled with H2 photo-dissociation both in
the intergalactic medium (IGM), and inside virialized halos. We find that the
intergalactic H2 has a negligible effect on the UVB, both because its initial
optical depth is small (tau<0.1), and because it is photo-dissociated at an
early stage. If the UV sources in the first collapsed halos are stars, then
their UV flux suppresses further star-formation inside small halos. This
results in a pause in the buildup of the UVB, and reionization is delayed until
larger halos (T_vir> 10^4 K) collapse. If the small halos host mini-quasars
with hard spectra extending to approximately 1 keV, then their X-rays balance
the effects of the UVB, the negative feedback does not occur, and reionization
can be caused by the small halos.Comment: 16 pages, 16 figures included, uses emulateapj.sty. Submitted to Ap
Are Giant Planets Forming Around HR 4796A?
We have obtained FUSE and HST STIS spectra of HR 4796A, a nearby 8 Myr old
main sequence star that possesses a dusty circumstellar disk whose inclination
has been constrained from high resolution near-infrared observations to be ~17
deg from edge-on. We searched for circumstellar absorption in the ground states
of C II at 1036.3 A, O I at 1039.2 A, Zn II at 2026.1 A, Lyman series H2, and
CO (A-X) and failed to detect any of these species. We place upper limits on
the column densities and infer upper limits on the gas masses assuming that the
gas is in hydrostatic equilibrium, is well-mixed, and has a temperature, Tgas ~
65 K. Our measurements suggest that this system possesses very little molecular
gas. Therefore, we infer an upper limit for the gas:dust ratio (<4.0) assuming
that the gas is atomic. We measure less gas in this system than is required to
form the envelope of Jupiter.Comment: 10 pages, 3 figures (including 1 color figure), accepted for
publication in Ap
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
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