59 research outputs found
The Very Highly Ionized Broad Absorption Line System of the QSO SBS1542+541
We have analyzed the broad absorption line system of the bright (V=16.5)
high-redshift (z=2.361) QSO SBS1542+541 using UV spectra from the HST FOS along
with optical data from the MMT and the Steward Observatory 2.3m telescope.
These spectra give continuous wavelength coverage from 1200 to 8000 Angstroms,
corresponding to 340 to 2480 Angstroms in the QSO rest frame. This object
therefore offers a rare opportunity to study broad absorption lines in the
rest-frame extreme UV. We find that the absorption system is dominated by very
high-ionization species, including O VI, NeVIII, and SiXII. We also identify
apparently saturated broad Lyman-series lines of order Ly-gamma and higher.
There is strong evidence for partial occultation of the QSO emission source,
particularly from the higher-order Lyman lines which indicate a covered
fraction less than 0.2. Overall, the data suggest a correlation between a
larger covered fraction and a higher state of ionization. We suggest that the
different covered fractions can be explained by either a special line of sight
through a disk-like geometry or by the existence of density fluctuations of a
factor >2 in the BAL gas. Our photoionization models of the system indicate a
large column density and high ionization state similar to that found in X-ray
``warm absorbers''.Comment: 31 pages, 13 figures, to be published in Ap
A Composite HST Spectrum of Quasars
We construct a composite quasar spectrum from 284 HST FOS spectra of 101
quasars with redshifts z > 0.33. The spectrum covers the wavelengths between
350 and 3000 A in the rest frame. There is a significant steepening of the
continuum slope around 1050 A. The continuum between 1050 and 2200 A can be
modeled as a power law with alpha = -0.99. For the full sample the power-law
index in the extreme ultraviolet (EUV) between 350 and 1050 A is alpha = -1.96.
The continuum flux in the wavelengths near the Lyman limit shows a depression
of about 10 percent. The break in the power-law index and the slight depression
of the continuum near the Lyman limit are features expected in Comptonized
accretion-disk spectra.Comment: 10 figures To appear in the February 1, 1997, issue of the Ap.
The Rest-Frame Extreme Ultraviolet Spectral Properties of QSOs
We use a sample of 332 Hubble Space Telescope spectra of 184 QSOs with z >
0.33 to study the typical ultraviolet spectral properties of QSOs, with
emphasis on the ionizing continuum. Our sample is nearly twice as large as that
of Zheng et al. (1997) and provides much better spectral coverage in the
extreme ultraviolet (EUV). The overall composite continuum can be described by
a power law with index alpha_EUV = -1.76 +/- 0.12 (f_nu ~ nu^alpha) between 500
and 1200 Angstroms. The corresponding results for subsamples of radio-quiet and
radio-loud QSOs are alpha_EUV = -1.57 +/- 0.17 and alpha_EUV = -1.96 +/- 0.12,
respectively. We also derive alpha_EUV for as many individual objects in our
sample as possible, totaling 39 radio-quiet and 40 radio-loud QSOs. The typical
individually measured values of alpha_EUV are in good agreement with the
composites. We find no evidence for evolution of alpha_EUV with redshift for
either radio-loud or radio-quiet QSOs. However, we do find marginal evidence
for a trend towards harder EUV spectra with increasing luminosity for
radio-loud objects. An extrapolation of our radio-quiet QSO spectrum is
consistent with existing X-ray data, suggesting that the ionizing continuum may
be represented by a single power law. The resulting spectrum is roughly in
agreement with models of the intergalactic medium photoionized by the
integrated radiation from QSOs.Comment: 14 pages using emulateapj, 15 figures, accepted for publication in
Ap
Far-ultraviolet Spectroscopy of Venus and Mars at 4 A Resolution with the Hopkins Ultraviolet Telescope on Astro-2
Far-ultraviolet spectra of Venus and Mars in the range 820-1840 A at 4 A
resolution were obtained on 13 and 12 March 1995, respectively, by the Hopkins
Ultraviolet Telescope (HUT), which was part of the Astro-2 observatory on the
Space Shuttle Endeavour. Longward of 1250 A, the spectra of both planets are
dominated by emission of the CO Fourth Positive band system and strong OI and
CI multiplets. In addition, CO Hopfield-Birge bands, B - X (0,0) at 1151 A and
C - X (0,0) at 1088 A, are detected for the first time, and there is a weak
indication of the E - X (0,0) band at 1076 A in the spectrum of Venus. The B -
X band is blended with emission from OI 1152. Modeling the relative intensities
of these bands suggests that resonance fluorescence of CO is the dominant
source of the emission, as it is for the Fourth Positive system. Shortward of
Lyman-alpha, other emission features detected include OII 834, OI lambda 989,
HI Lyman-beta, and NI 1134 and 1200. For Venus, the derived disk brightnesses
of the OI, OII, and HI features are about one-half of those reported by Hord et
al. (1991) from Galileo EUV measurements made in February 1990. This result is
consistent with the expected variation from solar maximum to solar minimum. The
ArI 1048, 1066 doublet is detected only in the spectrum of Mars and the derived
mixing ratio of Ar is of the order of 2%, consistent with previous
determinations.Comment: 8 pages, 5 figures, accepted for publication in ApJ, July 20, 200
Faint High Latitude Carbon Stars Discovered by the Sloan Digital Sky Survey: Methods and Initial Results
We report the discovery of 39 Faint High Latitude Carbon Stars (FHLCs) from
Sloan Digital Sky Survey commissioning data. The objects, each selected
photometrically and verified spectroscopically, range over 16.6 < r* < 20.0,
and show a diversity of temperatures as judged by both colors and NaD line
strengths. At the completion of the Sloan Survey, there will be many hundred
homogeneously selected and observed FHLCs in this sample. We present proper
motion measures for each object, indicating that the sample is a mixture of
extremely distant (>100 kpc) halo giant stars, useful for constraining halo
dynamics, plus members of the recently-recognized exotic class of very nearby
dwarf carbon (dC) stars. Motions, and thus dC classification, are inferred for
40-50 percent of the sample, depending on the level of statistical significance
invoked. The new list of dC stars presented here, although selected from only a
small fraction of the final SDSS, doubles the number of such objects found by
all previous methods. (Abstract abridged).Comment: Accepted for publication in The Astronomical Journal, Vol. 124, Sep.
2002, 40 pages, 7 figures, AASTeX v5.
Five High-Redshift Quasars Discovered in Commissioning Imaging Data of the Sloan Digital Sky Survey
We report the discovery of five quasars with redshifts of 4.67 - 5.27 and
z'-band magnitudes of 19.5-20.7 M_B ~ -27. All were originally selected as
distant quasar candidates in optical/near-infrared photometry from the Sloan
Digital Sky Survey (SDSS), and most were confirmed as probable high-redshift
quasars by supplementing the SDSS data with J and K measurements. The quasars
possess strong, broad Lyman-alpha emission lines, with the characteristic sharp
cutoff on the blue side produced by Lyman-alpha forest absorption. Three
quasars contain strong, broad absorption features, and one of them exhibits
very strong N V emission. The amount of absorption produced by the Lyman-alpha
forest increases toward higher redshift, and that in the z=5.27 object (D_A ~
0.7) is consistent with a smooth extrapolation of the absorption seen in lower
redshift quasars. The high luminosity of these objects relative to most other
known objects at z >~ 5 makes them potentially valuable as probes of early
quasar properties and of the intervening intergalactic medium.Comment: 13 pages in LaTex format, two postscirpt figures. Submitted to the
Astronomical Journa
The Discovery of a Second Field Methane Brown Dwarf from Sloan Digital Sky Survey Commissioning Data
We report the discovery of a second field methane brown dwarf from the
commissioning data of the Sloan Digital Sky Survey (SDSS). The object, SDSS
J134646.45-003150.4 (SDSS 1346-00), was selected because of its very red color
and stellar appearance. Its spectrum between 0.8-2.5 mic is dominated by strong
absorption bands of H_2O and CH_4 and closely mimics those of Gliese 229B and
SDSS 162414.37+002915.6 (SDSS 1624+00), two other known methane brown dwarfs.
SDSS 1346-00 is approximately 1.5 mag fainter than Gliese 229B, suggesting that
it lies about 11 pc from the sun. The ratio of flux at 2.1 mic to that at 1.27
mic is larger for SDSS 1346-00 than for Gliese 229B and SDSS 1624+00, which
suggests that SDSS 1346-00 has a slightly higher effective temperature than the
others. Based on a search area of 130 sq. deg. and a detection limit of z* =
19.8, we estimate a space density of 0.05 pc^-3 for methane brown dwarfs with
T_eff ~ 1000 K in the 40 pc^3 volume of our search. This estimate is based on
small-sample statistics and should be treated with appropriate caution.Comment: 9 pages, 3 figures, AASTeX, to appear in ApJ Letters, authors list
update
Extreme Ultraviolet Absorption Lines in LyA Forest Absorbers and the Oxygen Abundance in the Intergalactic Medium
We create stacked composite absorption spectra from Hubble Space Telescope
Faint Object Spectrograph data from four quasi-stellar objects to search for
absorption lines in the extreme ultraviolet wavelength region associated with
LyA forest absorbers in the redshift range 1.6 < z < 2.9. We successfully
detect O V 630 in LyA absorbers throughout the 10^13 to 10^16.2 cm^-2 column
density range. For a sample of absorbers with 10^13.2 < N(H I) < 10^14.2 cm^-2,
corresponding to gas densities ranging from around the universal mean to
overdensities of a few, we measure an O V 630 equivalent width of 10.9 +/- 3.7
mA. We estimate the detection is real with at least 99% confidence. We only
detect O IV 788, O IV 554, O III 833, and HeI 584 in absorbers with LyA
equivalent widths > 0.6 A, which are likely associated with traditional
metal-line systems. We find no evidence in any subsamples for absorption from N
IV 765, NeV 568, NeVI 559, NeVIII 770, 780, or MgX 610, 625. The measured
equivalent widths of O V suggest values of in the range -1.7 to
-0.6 for 10^13.2 < N(H I) < 10^15 cm^-2. The lack of detectable O IV absorption
except in the strongest absorption systems suggests a hard ionizing background
similar to the standard Haardt & Madau spectrum. Using photoionization models,
we estimate that the oxygen abundance in the intergalactic medium with respect
to the solar value is [O / H] around -2.2 to -1.3. Comparing to studies of C
IV, we estimate [O / C] around 0.3 to 1.2. The overabundance of oxygen relative
to carbon agrees with other low-metallicity abundance measurements and suggests
enrichment of the intergalactic medium by Type II supernovae.Comment: Accepted for publication in Nov 10, 2002 Ap
Hopkins Ultraviolet Telescope Observations of Her X-1
We have obtained a far-ultraviolet spectrum of the X-ray binary Hercules
X-1/HZ Herculis using the Hopkins Ultraviolet Telescope aboard the Astro-1
space shuttle mission in 1990 December. This is the first spectrum of Her X-1
that extends down to the Lyman limit at 912 A. We observed emission lines of O
VI, N V, and C IV, and the far UV continuum extending to the Lyman limit. We
examine the conditions of the emitting gas through line strengths, line ratios,
and doublet ratios. The UV flux is lower by about a factor of 2 than expected
at the orbital phase of the observation. We model the UV continuum with a
simple power-law and with a detailed model of an X-ray-illuminated accretion
disk and companion star. The power-law provides a superior fit, as the detailed
model predicts too little flux below 1200 A. We note, however, that there are
uncertainties in the interstellar reddening, in the background airglow
spectrum, and in the long-term phase of the accretion disk. We have searched
the data for UV line and continuum pulsations near the neutron star spin period
but found none at a detectable level.Comment: 19 pages, 5 figures, accepted by the Astrophysical Journa
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