670 research outputs found
The Distance to the Vela Supernova Remnant
We have obtained high resolution Ca II and Na I absorption line spectra
toward 68 OB stars in the direction of the Vela Supernova Remnant. The stars
lie at distances of 190 -- 2800 pc as determined by Hipparcos and spectroscopic
parallax estimations. The presence of high velocity absorption attributable to
the remnant along some of the sight lines constrains the remnant distance to
250+/-30 pc. This distance is consistent with several recent investigations
that suggest that the canonical remnant distance of 500 pc is too large.Comment: To be published in The Astrophysical Journal Letters Figure 1 y-axis
labels correcte
A Method for Deriving Accurate Gas-Phase Abundances for the Multiphase Interstellar Galactic Halo
We describe a new method for determining total gas-phase abundances for the
Galactic ISM with minimal ionization uncertainties. For sight lines toward
globular clusters containing both UV-bright stars and radio pulsars, one can
measure column densities of HI and several metal ions using UV absorption
measurements and of H II using radio dispersion measurements, thereby
minimizing ionization uncertainties. We apply this method to the globular
cluster Messier 3 sight line using FUSE and HST ultraviolet spectroscopy of the
post-asymptotic giant branch star von Zeipel 1128 and radio observations by
Ransom et al. of millisecond pulsars. Ionized hydrogen is 45+/-5% of the total
along this sight line, the highest measured fraction along a high-latitude
pulsar sight line. We derive total gas-phase abundances log N(S)/N(H) =
-4.87+/-0.03 and log N(Fe)/N(H) = -5.27+/-0.05. Our derived sulfur abundance is
in excellent agreement with recent solar system determinations of Asplund,
Grevesse, & Sauval, but -0.14 dex below the solar system abundance typically
adopted in studies of the ISM. The iron abundance is ~-0.7 dex below the solar
system abundance, consistent with significant depletion. Abundance estimates
derived by simply comparing S II and Fe II to H I are +0.17 and +0.11 dex
higher, respectively, than our measurements. Ionization corrections to the
gas-phase abundances measured in the standard way are, therefore, significant
compared with the measurement uncertainties along this sight line. The
systematic uncertainties associated with the uncertain contribution to the
electron column density from ionized helium could raise these abundances by
<+0.03 dex (+7%). [Abridged]Comment: To appear in the ApJ. 25 pages, including figures and tex
Origin(s) of the Highly Ionized High-Velocity Clouds Based on Their Distances
Previous HST and FUSE observations have revealed highly ionized high-velocity
clouds (HVCs) or more generally low HI column HVCs along extragalactic
sightlines over 70-90% of the sky. The distances of these HVCs have remained
largely unknown hampering to distinguish a "Galactic" origin (e.g., outflow,
inflow) from a "Local Group" origin (e.g., warm-hot intergalactic medium). We
present the first detection of highly ionized HVCs in the Cosmic Origins
Spectrograph (COS) spectrum of the early-type star HS1914+7134 (l = 103, b=+24)
located in the outer region of the Galaxy at 14.9 kpc. Two HVCs are detected in
absorption at v_LSR = -118 and -180 km/s in several species, including CIV,
SiIV, SiIII, CII, SiII, OI, but HI 21-cm emission is only seen at -118 \km.
Within 17 degrees of HS1914+7134, we found HVC absorption of low and high ions
at similar velocities toward 5 extragalactic sight lines, suggesting that these
HVCs are related. The component at -118 km/s is likely associated with the
Outer Arm of the Milky Way. The highly ionized HVC at -180 km/s is an HVC
plunging at high speed onto the thick disk of the Milky Way. This is the second
detection of highly ionized HVCs toward Galactic stars, supporting a "Galactic"
origin for at least some of these clouds.Comment: Submitted to the ApJ Letter
Correlations between O VI Absorbers and Galaxies at Low Redshift
We investigate the relationship between galaxies and metal-line absorption
systems in a large-scale cosmological simulation with galaxy formation. Our
detailed treatment of metal enrichment and non-equilibrium calculation of
oxygen species allow us, for the first time, to carry out quantitative
calculations of the cross-correlations between galaxies and O VI absorbers. We
find the following: (1) The cross-correlation strength depends weakly on the
absorption strength but strongly on the luminosity of the galaxy. (2) The
correlation distance increases monotonically with luminosity from ~0.5-1h^-1
Mpc for 0.1L* galaxies to ~3-5h^-1 Mpc for L* galaxies. (3) The correlation
distance has a complicated dependence on absorber strength, with a
luminosity-dependent peak. (4) Only 15% of O VI absorbers lie near >=Lz*
galaxies. The remaining 85%, then, must arise ``near'' lower-luminosity
galaxies, though, the positions of those galaxies is not well-correlated with
the absorbers. This may point to pollution of intergalactic gas predominantly
by smaller galaxies. (5) There is a subtle trend that for >~0.5Lz* galaxies,
there is a positive correlation between absorber strength and galaxy luminosity
in the sense that stronger absorbers have a slightly higher probability of
finding such a large galaxy at a given projection distance. For less luminous
galaxies, there seems to be a negative correlation between luminosity and
absorber strength.Comment: uses emulateapj, 5 pages including 2 color figures and 2 tables,
accepted for publication in ApJ Letter
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