125 research outputs found
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
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
Detection of Ne VIII in the Low-Redshift Warm-Hot IGM
High resolution FUSE and STIS observations of the bright QSO HE 0226-4110
(zem = 0.495) reveal the presence of a multi-phase absorption line system at
zabs(O VI) = 0.20701 containing absorption from H I (Ly alpha to Ly theta), C
III, O III, O IV, O VI, N III, Ne VIII, Si III, S VI and possibly S V. Single
component fits to the Ne VIII and O VI absorption doublets yield logN(Ne VIII)
= 13.89+/-0.11 and logN(O VI) = 14.37+/-0.03. The Ne VIII and O VI doublets are
detected at 3.9 sigma and 16 sigma significance levels, respectively. This
represents the first detection of intergalactic Ne VIII, a diagnostic of gas
with temperature in the range from 5x10(5) to 1x10(6) K. The O VI and Ne VIII
are not likely created in a low density medium photoionized solely by the
extragalactic background at z = 0.2 since the required path length of ~11 Mpc
implies the Hubble flow absorption line broadening would be ~10 times greater
than the observed line widths. A collisional ionization origin is therefore
more likely. Assuming [Ne/H] and [O/H] = -0.5, the value N(Ne VIII)/N(O VI) =
0.33+/-0.10 is consistent with gas in collisional ionization equilibrium near
T=5.4x10(5) K with logN(H)= 19.9 and N(H)/N(H I) = 1.7x10(6). The observations
support the basic idea that a substantial fraction of the baryonic matter at
low redshift exists in hot very highly ionized gaseous structures.Comment: 32 pages text and 9 pages of figures. Accepted by the Astrophysical
Journa
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