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Small-scale morphologic properties of martian gullies: insights from analysis of HiRISE images
Abstract not available
Is the EGRET source 3EG J1621+8203 the radio galaxy NGC 6251?
We discuss the nature of the unidentified EGRET source 3EG J1621+8203. In an
effort to identify the gamma-ray source, we have examined X-ray images of the
field from ROSAT PSPC, ROSAT HRI, and ASCA GIS. Of the several faint X-ray
point sources in the error circle of 3EG J1621+8203, most are stars or faint
radio sources, unlikely to be counterparts to the EGRET source. The most
notable object in the gamma-ray error box is the bright FR I radio galaxy NGC
6251. If 3EG J1621+8203 corresponds to NGC 6251, then it would be the second
radio galaxy to be detected in high energy gamma rays, after Cen A, which
provided the first clear evidence of the detection above 100 MeV of an AGN with
a large-inclination jet. If the detection of more radio galaxies by EGRET has
been limited by its threshold sensitivity, there exists the exciting
possibility that new high energy gamma-ray instruments, with much higher
sensitivity, will detect a larger number of radio galaxies in the future.Comment: 7 pages, 6 figures. Accepted for publication in The Astrophysical
Journal, August 2002 issu
FUSE Observations of Molecular Hydrogen in Translucent Interstellar Clouds: The Line of Sight Toward HD 73882
We report the results of initial FUSE observations of molecular hydrogen (H2)
in translucent clouds. These clouds have greater optical depth than any of the
diffuse clouds previously observed for far-UV H2 absorption, and provide new
insights into the physics and chemistry of such regions. Our initial results
involve observations of HD 73882, a well-studied southern hemisphere star lying
behind substantial interstellar material (E(B-V) = 0.72; A_V = 2.44). We find a
total H2 column density, N(H2) = 1.2 x 10^{21} cm^{-2}, about three times
larger than the values for diffuse clouds previously measured in the far-UV.
The gas kinetic temperature indicated by the ratio N(J=1)/N(J=0) is 58 +/- 10
K. With the aid of ground-based data to calculate an appropriate
multi-component curve of growth, we have determined column densities for all
rotational levels up to J = 7. The J >= 2 states can be reasonably fitted with
a rotational excitation temperature of 307 +/- 23 K. Both the kinetic and
rotational temperatures are similar to those found in previous investigations
of diffuse clouds. The ratios of carbonaceous molecules to hydrogen molecules
are also similar to ratios in diffuse clouds, suggesting a similar chemistry
for this line of sight.Comment: 7 pages, 3 figures, to appear in ApJ Letters (FUSE first-results
issue
Molecular Hydrogen in the FUSE Translucent Lines of Sight: The Full Sample
We report total abundances and related parameters for the full sample of the
FUSE survey of molecular hydrogen in 38 translucent lines of sight. New results
are presented for the "second half" of the survey involving 15 lines of sight
to supplement data for the first 23 lines of sight already published. We assess
the correlations between molecular hydrogen and various extinction parameters
in the full sample, which covers a broader range of conditions than the initial
sample. In particular, we are now able to confirm that many, but not all, lines
of sight with shallow far-UV extinction curves and large values of the
total-to-selective extinction ratio, = / -- characteristic
of larger than average dust grains -- are associated with particularly low
hydrogen molecular fractions (). In the lines of sight with large
, there is in fact a wide range in molecular fractions, despite the
expectation that the larger grains should lead to less H formation.
However, we see specific evidence that the molecular fractions in this
sub-sample are inversely related to the estimated strength of the UV radiation
field and thus the latter factor is more important in this regime. We have
provided an update to previous values of the gas-to-dust ratio, (H)/, based on direct measurements of (H) and (H I).
Although our value is nearly identical to that found with Copernicus data, it
extends the relationship by a factor of 2 in reddening. Finally, as the new
lines of sight generally show low to moderate molecular fractions, we still
find little evidence for single monolithic "translucent clouds" with 1.Comment: 35 pages, 5 tables, 7 figures, accepted for publication in The
Astrophysical Journal Supplements Serie
Intermediate- and High-Velocity Ionized Gas toward zeta Orionis
We combine UV spectra obtained with the HST/GHRS echelle, IMAPS, and
Copernicus to study the abundances and physical conditions in the predominantly
ionized gas seen at high (-105 to -65 km/s) and intermediate velocities (-60 to
-10 km/s) toward zeta Ori. We have high resolution (FWHM ~ 3.3-4.5 km/s) and/or
high S/N spectra for at least two significant ions of C, N, Al, Si, S, and Fe
-- enabling accurate estimates for both the total N(H II) and the elemental
depletions. C, N, and S have essentially solar relative abundances; Al, Si, and
Fe appear to be depleted by about 0.8, 0.3-0.4, and 0.95 dex, respectively.
While various ion ratios would be consistent with collisional ionization
equilibrium (CIE) for T ~ 25,000-80,000 K, the widths of individual
high-velocity absorption components indicate that T ~ 9000 K -- so the gas is
not in CIE. Analysis of the C II fine-structure excitation equilibrium yields
estimated densities (n_e ~ n_H ~ 0.1-0.2 cm^{-3}), thermal pressures (2 n_H T ~
2000-4000 cm^{-3}K), and thicknesses (0.5-2.7 pc) for the individual clouds. We
compare the abundances and physical properties derived for these clouds with
those found for gas at similar velocities toward 23 Ori and tau CMa, and also
with several models for shocked gas. While the shock models can reproduce some
features of the observed line profiles and some of the observed ion ratios,
there are also significant differences. The measured depletions suggest that
\~10% of the Al, Si, and Fe originally locked in dust in the pre-shock medium
may have been returned to the gas phase, consistent with predictions for the
destruction of silicate dust in a 100 km/s shock. The near-solar gas phase
abundance of carbon, however, seems inconsistent with the predicted longer time
scales for the destruction of graphite grains.Comment: 50 pages, 9 figures; aastex; accepted by Ap
9.7 micrometer Silicate Absorption in a Damped Lyman-alpha Absorber at z=0.52
We report a detection of the 9.7 micrometer silicate absorption feature in a
damped Lyman-alpha (DLA) system at z_{abs} = 0.524 toward AO0235+164, using the
Infrared Spectrograph (IRS) onboard the Spitzer Space Telescope. The feature
shows a broad shallow profile over about 8-12 micrometers in the absorber rest
frame and appears to be > 15 sigma significant in equivalent width. The feature
is fit reasonably well by the silicate absorption profiles for laboratory
amorphous olivine or diffuse Galactic interstellar clouds. To our knowledge,
this is the first indication of 9.7 micrometer silicate absorption in a DLA. We
discuss potential implications of this finding for the nature of the dust in
quasar absorbers. Although the feature is relatively shallow (tau_{9.7} =
0.08-0.09), it is about 2 times deeper than expected from extrapolation of the
tau_{9.7} vs. E(B-V) relation known for diffuse Galactic interstellar clouds.
Further studies of the 9.7 micrometer silicate feature in quasar absorbers will
open a new window on the dust in distant galaxies.Comment: 4 pages, 2 figures, submitted to Astrophysical Journal Letters
(revised version including referee's comments
Exclusion limits on the WIMP-nucleon cross-section from the Cryogenic Dark Matter Search
The Cryogenic Dark Matter Search (CDMS) employs low-temperature Ge and Si
detectors to search for Weakly Interacting Massive Particles (WIMPs) via their
elastic-scattering interactions with nuclei while discriminating against
interactions of background particles. For recoil energies above 10 keV, events
due to background photons are rejected with >99.9% efficiency, and surface
events are rejected with >95% efficiency. The estimate of the background due to
neutrons is based primarily on the observation of multiple-scatter events that
should all be neutrons. Data selection is determined primarily by examining
calibration data and vetoed events. Resulting efficiencies should be accurate
to about 10%. Results of CDMS data from 1998 and 1999 with a relaxed
fiducial-volume cut (resulting in 15.8 kg-days exposure on Ge) are consistent
with an earlier analysis with a more restrictive fiducial-volume cut.
Twenty-three WIMP candidate events are observed, but these events are
consistent with a background from neutrons in all ways tested. Resulting limits
on the spin-independent WIMP-nucleon elastic-scattering cross-section exclude
unexplored parameter space for WIMPs with masses between 10-70 GeV c^{-2}.
These limits border, but do not exclude, parameter space allowed by
supersymmetry models and accelerator constraints. Results are compatible with
some regions reported as allowed at 3-sigma by the annual-modulation
measurement of the DAMA collaboration. However, under the assumptions of
standard WIMP interactions and a standard halo, the results are incompatible
with the DAMA most likely value at >99.9% CL, and are incompatible with the
model-independent annual-modulation signal of DAMA at 99.99% CL in the
asymptotic limit.Comment: 40 pages, 49 figures (4 in color), submitted to Phys. Rev. D;
v.2:clarified conclusions, added content and references based on referee's
and readers' comments; v.3: clarified introductory sections, added figure
based on referee's comment
Relative abundance pattern along the profile of high redshift Damped Lyman-alpha systems
We investigated abundance ratios along the profiles of six high-redshift
Damped Lyman-alpha systems, three of them associated with H2 absorption, and
derived optical depths in each velocity pixel. The variations of the pixel
abundance ratios were found to be remarkably small and usually smaller than a
factor of two within a profile. This result holds even when considering
independent sub-clumps in the same system. The depletion factor is
significantly enhanced only in those components where H2 is detected. There is
a strong correlation between [Fe/S] and [Si/S] abundances ratios, showing that
the abundance ratio patterns are definitely related to the presence of dust.
The depletion pattern is usually close to the one seen in the warm halo gas of
our Galaxy.Comment: 14 page
Evidence for the presence of dust in intervening QSO absorbers from the Sloan Digital Sky Survey
We find evidence for dust in the intervening QSO absorbers from the spectra
of QSOs in the Sloan Digital Sky Survey Data Release 1. No evidence is found
for the 2175 A feature which is present in the Milky Way dust extinction curve.
The extinction curve resembles the SMC extinction curve. The observed
Delta(g-i) excess for QSOs with strong absorption systems appears to be a
result of the reddening due to dust in the intervening absorbers.Comment: Poster paper presented at the IAU Colloquium #199 on "Probing
Galaxies through Quasar Absorption Lines" held in Shanghai, China from March
14th to 18th, 200
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