148 research outputs found
Metallicity Evolution of Damped Lyman-Alpha Galaxies
We have reanalyzed the existing data on Zinc abundances in damped Ly-alpha
(DLA) absorbers to investigate whether their mean metallicity evolves with
time. Most models of cosmic chemical evolution predict that the mass- weighted
mean interstellar metallicity of galaxies should rise with time from a low
value ~ 1/30 solar at z ~ 3 to a nearly solar value at z ~ 0. However, several
previous analyses have suggested that there is little or no evolution in the
global metallicity of DLAs. We have used a variety of statistical techniques to
quantify the global metallicity-redshift relation and its uncertainties, taking
into account both measurement and sampling errors. Three new features of our
analysis are: (a) an unbinned N(H I)-weighted nonlinear chi-square fit to an
exponential relation; (b) survival analysis to treat the large number of limits
in the existing data; and (c) a comparison of the data with several models of
cosmic chemical evolution based on an unbinned N(H I)-weighted chi-square. We
find that a wider range of evolutionary rates is allowed by the present data
than claimed in previous studies. The slope of the exponential fit to the N(H
I)-weighted mean Zn metallicity vs. redshift relation is -0.20 plus minus 0.11
counting limits as detections and -0.27 plus minus 0.12 counting limits as
zeros. Similar results are also obtained if the data are binned in redshift,
and if survival analysis is used. These slopes are marginally consistent with
no evolution, but are also consistent with the rates predicted by several
models of cosmic chemical evolution. Finally, we outline some future
measurements necessary to improve the statistics of the global
metallicity-redshift relation.Comment: 25 pages, 1 figure, accepted for publication in the Astrophysical
Journa
Chandra Detection of X-ray Absorption Associated with a Damped Lyman Alpha System
We have observed three quasars, PKS 1127-145, Q 1331+171 and Q0054+144, with
the ACIS-S aboard the Chandra X-ray Observatory, in order to measure soft X-ray
absorption associated with intervening 21-cm and damped Ly absorbers.
For PKS 1127-145, we detect absorption which, if associated with an intervening
z_{abs}=0.312 absorber, implies a metallicity of 23% solar. If the absorption
is not at z_{abs}=0.312, then the metallicity is still constrained to be less
than 23% solar. The advantage of the X-ray measurement is that the derived
metallicity is insensitive to ionization, inclusion of an atom in a molecule,
or depletion onto grains. The X-ray absorption is mostly due to oxygen, and is
consistent with the oxygen abundance of 30% solar derived from optical nebular
emission lines in a foreground galaxy at the redshift of the absorber.
For Q1331+171 and Q 0054+144, only upper limits were obtained, although the
exposure times were intentionally short, since for these two objects we were
interested primarily in measuring flux levels to plan for future observations.
The imaging results are presented in a companion paper.Comment: 23 pages, 6 figures, accepted for publication in the Astrophysical
Journa
A physical upper limit on the HI column density of gas clouds
An intriguing fact about cosmic gas clouds is that they all appear to have
neutral (atomic) hydrogen column densities smaller than 10^{22} cm^{-2}.
Observations of damped Ly-alpha (DLA) absorption systems further indicate that
the maximum N(HI) decreases with increasing metallicity. It is generally
assumed that this trend is due to a dust-induced selection bias: DLA systems
with high N(HI) and high metallicity contain so much dust that the background
QSO becomes too dim to be included in optically selected surveys. Here, it is
argued that this explanation may not be viable. Instead, it is proposed that
conversion to molecular hydrogen determines the maximum HI column density.
Molecular hydrogen forms on the surface of dust grains and is destroyed by
photodissociation. Therefore, the molecular fraction correlates with both the
dust content and, because of self-shielding, the total hydrogen column density,
and anticorrelates with the intensity of the incident UV radiation. It is shown
that the first relation can account for the observed anticorrelation between
the maximum N(HI) and metallicity.Comment: Accepted for publication in the Astrophysical Journal Letters. 4
pages, 2 figures. Minor change
Metals and Dust in Intermediate-redshift Damped Ly-alpha Galaxies
We report spectroscopic observations with the Multiple Mirror Telescope for
11 damped Lyman-alpha absorbers (DLAs) or strong DLA candidates at 0.1 < z <
1.5, including several absorbers discovered in the Sloan Digital Sky Survey. In
particular, we have measured absorption lines of Zn II, Cr II, Ni II, Fe II, Mn
II, Ti II, Ca II, and Si II. These measurements have doubled the sample of Zn
and Cr measurements at z < 1. The average relative abundance patterns in these
objects are very similar to those found for high-redshift DLAs reported in the
literature. Our observations suggest that the dust content, as determined by
[Cr/Zn], does not show much change with redshift. We also examine the sample
for correlation of [Cr/Zn] with estimates of the quasar reddening. Our data
suggest that the global mean metallicity of DLAs, as measured by the gas phase
abundance of Zn, at best shows a weak evolution with redshift over the range
0.4 < z <3.9.Comment: aastex52, 47 pages including 11 postscript figures. Accepted for
publication in The Astrophysical Journa
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
Hubble Space Telescope Observations of Element Abundances in Low-redshift Damped Lyman-alpha Galaxies and Implications for the Global Metallicity-Redshift Relation
Most models of cosmic chemical evolution predict that the mass-weighted mean
interstellar metallicity of galaxies should rise with time from a low value
solar at to a nearly solar value at . In the
absence of any selection effects, the damped Lyman-alpha absorbers (DLAs) in
quasar spectra are expected to show such a rise in global metallicity. However,
it has been difficult to determine whether or not DLAs show this effect,
primarily because of the very small number of DLA metallicity measurements at
low redshifts. In an attempt to put tighter constraints on the low-redshift end
of the DLA metallicity-redshift relation, we have observed Zn II and Cr II
lines in four DLAs at , using the Space Telescope Imaging
Spectrograph (STIS) onboard the Hubble Space Telescope (HST). These
observations have provided the first constraints on Zn abundances in DLAs with
. In all the three DLAs for which our observations offer meaningful
constraints on the metallicity, the data suggest that the metallicities are
much lower than the solar value. These results are consistent with recent
imaging studies indicating that these DLAs may be associated with dwarf or low
surface brightness galaxies. We combine our results with higher redshift data
from the literature to estimate the global mean metallicity-redshift relation
for DLAs. We find that the global mean metallicity shows at most a slow
increase with decreasing redshift. ...(Please see the paper for the complete
abstract).Comment: 56 pages, including 13 figures. Accepted for publication in the
Astrophysical Journa
The SOPHIE search for northern extrasolar planets. V. Follow-up of ELODIE candidates: Jupiter-analogs around Sun-like stars
We present radial-velocity measurements obtained in a programs underway to
search for extrasolar planets with the spectrograph SOPHIE at the 1.93-m
telescope of the Haute-Provence Observatory. Targets were selected from
catalogs observed with ELODIE, mounted previously at the telescope, in order to
detect long-period planets with an extended database close to 15 years. Two new
Jupiter-analog candidates are reported to orbit the bright stars HD150706 and
HD222155 in 16.1 and 10.9 yr at 6.7 (+4.0,-1.4) and 5.1(+0.6,-0.7) AU and to
have minimum masses of 2.71 (+1.44,-0.66) and 1.90 (+0.67,-0.53) M_Jup,
respectively. Using the measurements from ELODIE and SOPHIE, we refine the
parameters of the long-period planets HD154345b and HD89307b, and publish the
first reliable orbit for HD24040b. This last companion has a minimum mass of
4.01 +/- 0.49 M_Jup orbiting its star in 10.0 yr at 4.92 +/- 0.38 AU. Moreover,
the data provide evidence of a third bound object in the HD24040 system. With a
surrounding dust debris disk, HD150706 is an active G0 dwarf for which we
partially corrected the effect of the stellar spot on the SOPHIE
radial-velocities. HD222155 is an inactive G2V star. On the basis of the
previous findings of Lovis and collaborators and since no significant
correlation between the radial-velocity variations and the activity index are
found in the SOPHIE data, these variations are not expected to be only due to
stellar magnetic cycles. Finally, we discuss the main properties of this new
population of long-period Jupiter-mass planets, which for the moment, consists
of fewer than 20 candidates. These stars are preferential targets either for
direct-imaging or astrometry follow-up to constrain the system parameters and
for higher precision radial-velocity to search for lower mass planets, aiming
to find a Solar System twin.Comment: accepted for publication in Astronomy & Astrophysic
HAT-P-49b: A 1.7 M_J Planet Transiting a Bright 1.5 M_S F-Star
We report the discovery of the transiting extrasolar planet HAT-P-49b. The
planet transits the bright (V = 10.3) slightly evolved F-star HD 340099 with a
mass of 1.54M_S and a radius of 1.83 R_S. HAT-P-49b is orbiting one of the 25
brightest stars to host a transiting planet which makes this a favorable
candidate for detailed follow-up. This system is an especially strong target
for Rossiter- McLaughlin follow-up due to the fast rotation of the host star,
16 km/s. The planetary companion has a period of 2.6915 d, mass of 1.73 M_J and
radius of 1.41 R_J. The planetary characteristics are consistent with that of a
classical hot Jupiter but we note that this is the fourth most massive star to
host a transiting planet with both M_p and R_p well determined.Comment: Accepted to the Astronomical Journa
Super-Solar Metallicity in Weak Mg II Absorption Systems at z ~ 1.7
Through photoionization modeling, constraints on the physical conditions of
three z ~ 1.7 single-cloud weak Mg II systems (W_r(2796) < 0.3A) are derived.
Constraints are provided by high resolution R = 45,000, high signal-to-noise
spectra of the three quasars HE0141-3932, HE0429-4091, and HE2243-6031 which we
have obtained from the ESO archive of VLT/UVES. Results are as follows:
(1) The single-cloud weak Mg II absorption in the three z ~ 1.7 systems is
produced by clouds with ionization parameters of -3.8 < logU < -2.0 and sizes
of 1-100 pc.
(2) In addition to the low-ionization phase Mg II clouds, all systems need an
additional 1-3 high-ionization phase C IV clouds within 100 km/s of the Mg II
component. The ionization parameters of the C IV phases range from -1.9 < logU
< -1.0, with sizes of tens of parsecs to kiloparsecs.
(3) Two of the three single-cloud weak Mg II absorbers have near-solar or
super-solar metallicities, if we assume a solar abundance pattern. Although
such large metallicities have been found for z < 1 weak Mg II absorbers, these
are the first high metallicities derived for such systems at higher redshifts.
(4) Two of the three weak Mg II systems also need additional low-metallicity,
broad Lya absorption lines, offset in velocity from the metal-line absorption,
in order to reproduce the full Lya profile.
(5) Metallicity in single-cloud weak Mg II systems are more than an order of
magnitude larger than those in Damped Lya systems at z ~ 1.7. In fact, there
appears to be a gradual decrease in metallicity with increasing N(HI), from
these, the most metal-rich Lya forest clouds, to Lyman limit systems, to
sub-DLAs, and finally to the DLAs.Comment: 25 pages, 7 figures, accepted by the Astrophysical Journa
HAT-P-55b: A Hot Jupiter Transiting a Sun-like Star
We report the discovery of a new transiting extrasolar planet, HAT-P-55b. The
planet orbits a V = 13.207 +/- 0.039 sun-like star with a mass of 1.013 +/-
0.037 solar masses, a radius of 1.011 +/- 0.036 solar radii and a metallicity
of -0.03 +/- 0.08. The planet itself is a typical hot Jupiter with a period of
3.5852467 +/- 0.0000064 days, a mass of 0.582 +/- 0.056 Jupiter masses and a
radius of 1.182 +/- 0.055 Jupiter radii. This discovery adds to the increasing
sample of transiting planets with measured bulk densities, which is needed to
put constraints on models of planetary structure and formation theories.Comment: 7 pages, 4 figures, accepted for publication in PAS
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