902 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
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
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
A new constraint on cosmological variability of the proton-to-electron mass ratio
Exotic cosmologies predict variability of the fundamental physical constants
over the cosmic time. Using the VLT/UVES high resolution spectra of the quasar
Q0347-3819 and unblended electronic - vibrational - rotational lines of the H2
molecule identified at z = 3.025 we test possible changes in the proton - to -
electron mass ratio mu_0 = m_p/m_e over the period of 11 Gyr. We obtained a new
constraint on the time - averaged variation rate of mu_0 of |d mu /d t /mu_0| <
5 10^{-15} yr^{-1} (1 sigma c.l.). The estimated 1 sigma uncertainty interval
of the |Delta mu/mu_0| ratio of about 0.004% implies that since the time when
the H2 spectrum was formed at z = 3.025, mu_0 has not changed by more than a
few thousands of a percent.Comment: 5 pages, 3 figures, a revised version accepted by MNRA
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
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
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
Thermal stability of cold clouds in galaxy halos
We consider the thermal properties of cold, dense clouds of molecular
hydrogen and atomic helium. For cloud masses below 10^-1.7 Msun, the internal
pressure is sufficient to permit the existence of particles of solid or liquid
hydrogen at temperatures above the microwave background temperature. Optically
thin thermal continuum emission by these particles can balance cosmic-ray
heating of the cloud, leading to equilibria which are thermally stable even
though the heating rate is independent of cloud temperature. For the Galaxy,
the known heating rate in the disk sets a minimum mass of order 10^-6 Msun
necessary for survival. Clouds of this type may in principle comprise most of
the dark matter in the Galactic halo. However, we caution that the equilibria
do not exist at redshifts z > 1 when the temperature of the microwave
background was substantially larger than its current value; the formation and
survival of such clouds to the present epoch therefore remain open questions.Comment: 5 pp incl 2 figs, LaTeX, emulateapj.sty; ApJ Letters in press.
Significant revisions, results qualitatively unchange
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
On the discrete equation model for compressible multiphase fluid flows
The modeling of multi-phase flow is very challenging, given the range of scales as well as the diversity of flow regimes that one encounters in this context. We revisit the discrete equation method (DEM) for two-phase flow in the absence of heat conduction and mass transfer. We analyze the resulting probability coefficients and prove their local convexity, rigorously establishing that our version of DEM can model different flow regimes ranging from the disperse to stratified (or separated) flow. Moreover, we reformulate the underlying mesoscopic model in terms of an one-parameter family of PDEs that interpolates between different flow regimes. We also propose two sets of procedures to enforce relaxation to equilibrium. We perform several numerical tests to show the flexibility of the proposed formulation, as well as to interpret different model components. The one-parameter family of PDEs provides an unified framework for modeling mean quantities for a multiphase flow, while at the same time identifying two key parameters that model the inherent uncertainty in terms of the underlying microstructure
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