25 research outputs found
The Cosmic Origins Spectrograph and the Future of Ultraviolet Astronomy
I describe the capabilities of the Cosmic Origins Spectrograph, scheduled for
May 2009 installation on the Hubble Space Telescope. With a factor-of-ten
increase in far-UV throughput for moderate resolution spectroscopy, COS will
enable a range of scientific programs that study hot stars, AGN, and gas in the
interstellar medium, intergalactic medium, and galactic halos. We also plan a
large-scale HST Spectroscopic Legacy Project for QSO absorption lines, galactic
halos, and AGN outflows. Studies of next-generation telescopes for UV/O
astronomy are now underway, including small, medium, and large missions to fill
the imminent ten-year gap between the end of Hubble and a plausible launch of
the next large mission. Selecting a strategy for achieving these goals will
involve hard choices and tradeoffs in aperture, wavelength, and capability.Comment: To appear in Future Directions in Ultraviolet Astronomy (AIP Conf
Proc
Highly Ionized Envelopes of High Velocity Clouds
We present recent results on highly ionized gas in Galactic High-Velocity
Clouds (HVCs), originally surveyed in OVI (Sembach et al. 2003). In a new
FUSE/HST survey of SiII/III/IV (Shull et al. 2009) toward 37 AGN, we detected
SiIII (lambda 1206.500 A) absorption with a sky coverage fraction 81 +/- 5% (61
HVCs along 30 of 37 high-latitude sight lines). The SiIII (lambda 1206.500 A)
line is typically 4-5 times stronger than OVI (lambda 1031.926 A). The mean HVC
column density of perhaps 10^19 cm^-2 of low-metallicity (0.1 - 0.2 Z_sun)
ionized gas in the low halo. Recent determinations of HVC distances allow us to
estimate a total reservoir of ~10^8 M_sun. Estimates of infall velocities
indicate an infall rate of around 1 M_sun yr^-1, comparable to the
replenishment rate for star formation in the disk. HVCs appear to be sheathed
by intermediate-temperature gas (10^4.0 - 10^4.5 K) detectable in SiIII and
SiIV, as well as hotter gas seen in OVI and other high ions. To prepare for HST
observations of 10 HVC-selected sight lines with the Cosmic Origins
Spectrograph (COS), we compile FUSE/STIS spectra of these ions, plus FeIII,
CIII, CIV, and SIV. Better constraints on the physical properties of HVC
envelopes and careful treatment of HVC kinematics and infall rates should come
from high-quality (S/N ~ 30-40) COS data.Comment: 3 pages, 1 figure, published in Future Directions in Ultraviolet
Spectroscopy, Proceedings of the AIP Conference held October 20-22, 2008 in
Annapolis, Marylan
The Magellanic System: What have we learnt from FUSE?
I review some of the findings on the Magellanic System produced by the Far
Ultraviolet Spectroscopic Explorer (FUSE) during and after its eight years of
service. The Magellanic System with its high-velocity complexes provides a
nearby laboratory that can be used to characterize phenomena that involve
interaction between galaxies, infall and outflow of gas and metals in galaxies.
These processes are crucial for understanding the evolution of galaxies and the
intergalactic medium. Among the FUSE successes I highlight are the coronal gas
about the LMC and SMC, and beyond in the Stream, the outflows from these
galaxies, the discovery of molecules in the diffuse gas of the Stream and the
Bridge, an extremely sub-solar and sub-SMC metallicity of the Bridge, and a
high-velocity complex between the Milky Way and the Clouds.Comment: A contributed paper to the FUSE Annapolis Conference "Future
Directions in Ultraviolet Spectroscopy.", 5 pages. To appear as an AIP
Conference Proceedin
Tracking The Post-BBN Evolution Of Deuterium
The primordial abundance of deuterium produced during Big Bang
Nucleosynthesis (BBN) depends sensitively on the universal ratio of baryons to
photons, an important cosmological parameter probed independently by the Cosmic
Microwave Background (CMB) radiation. Observations of deuterium in
high-redshift, low-metallicity QSO Absorption Line Systems (QSOALS) provide a
key baryometer, determining the baryon abundance at the time of BBN to a
precision of 5%. Alternatively, if the CMB-determined baryon to photon ratio is
used in the BBN calculation of the primordial abundances, the BBN-predicted
deuterium abundance may be compared with the primordial value inferred from the
QSOALS, testing the standard cosmological model. In the post-BBN universe, as
gas is cycled through stars, deuterium is only destroyed so that its abundance
measured anytime, anywhere in the Universe, bounds the primordial abundance
from below. Constraints on models of post-BBN Galactic chemical evolution
follow from a comparison of the relic deuterium abundance with the
FUSE-inferred deuterium abundances in the chemically enriched, stellar
processed material of the local ISM.Comment: 8 pages, 5 figures, to appear in the Proceedings of the Future
Directions in Ultraviolet Spectroscopy Conferenc
Intergalactic Baryons in the Local Universe
Simulations predict that shocks from large-scale structure formation and
galactic winds have reduced the fraction of baryons in the warm, photoionized
phase (the Lya forest) from nearly 100% in the early universe to less than 50%
today. Some of the remaining baryons are predicted to lie in the warm-hot
ionized medium (WHIM) phase at T=10^5-10^7 K, but the quantity remains a highly
tunable parameter of the models. Modern UV spectrographs have provided
unprecedented access to both the Lya forest and potential WHIM tracers at z~0,
and several independent groups have constructed large catalogs of far-UV IGM
absorbers along ~30 AGN sight lines. There is general agreement between the
surveys that the warm, photoionized phase makes up ~30% of the baryon budget at
z~0. Another ~10% can be accounted for in collapsed structures (stars,
galaxies, etc.). However, interpretation of the ~100 high-ion (OVI, etc)
absorbers at z<0.5 is more controversial. These species are readily created in
the shocks expected to exist in the IGM, but they can also be created by
photoionization and thus not represent WHIM material. Given several pieces of
observational evidence and theoretical expectations, I argue that most of the
observed OVI absorbers represent shocked gas at T~300,000 K rather than
photoionized gas at T<30,000 K, and they are consequently valid tracers of the
WHIM phase. Under this assumption, enriched gas at T=10^5-10^6 K can account
for ~10% of the baryon budget at z<0.5, but this value may increase when bias
and incompleteness are taken into account and help close the gap on the 50% of
the baryons still "missing".Comment: Invited review to appear in "Future Directions in Ultraviolet
Spectroscopy", Oct 20-22, 2008, Annapolis, MD, M. E. Van Steenberg, ed.
(April 2009). 8 pages, five figure
Testing Binary Population Synthesis Models with Hot Subdwarfs
Models of binary star interactions have been successful in explaining the
origin of field hot subdwarf (sdB) stars in short period systems, but
longer-period systems that formed via Roche-lobe overflow (RLOF) mass transfer
from the present sdB to its companion have received less attention. We map sets
of initial binaries into present-day binaries that include sdBs and
main-sequence stars, distinguishing "observable" sdBs from "hidden" ones. We
aim to find out whether (1) the existing catalogues of sdBs are sufficiently
fair samples of all the kinds of sdB binaries that theory predicts; or instead
whether (2) large predicted hidden populations mandate the construction of new
catalogues, perhaps using wide-field imaging surveys such as 2MASS, SDSS, and
Galex. We also report on a pilot study to identify hidden subdwarfs, using
2MASS and GALEX data.Comment: 3 pages with 2 figures. Uses AIP style files. To appear in Future
Directions in Ultraviolet Astronomy, ed. Michael E. VanSteenberg (AIP Conf
Proc
Metallicity measurements in AGNs
Measuring metallicity in the nuclear regions of AGNs is difficult because
only a few lines are observed and ionization correction becomes a major
problem. Nitrogen to carbon ratio has been widely used as an indicator for
metallicity, but precise measurements have been lacking. We made such
measurements for the first time using a wide baseline of ionization states with
observations from FUSE, HST and Chandra. OVI observations with FUSE were
crucial in this effort. We measured super-solar metallicities in two AGNs and
found that N/C does not scale with metallicity. This suggests that chemical
enrichment scenario in nuclear regions of galaxies may be different from
traditional models of metal enrichment.Comment: To appear in Future Directions in Ultraviolet Astronomy (AIP Conf
Proc), Ed. Michael E. Van Steenber
Recent FUSE Observations of Diffuse O VI Emission from the Interstellar Medium
We present new results from our survey of diffuse O VI-emitting gas in the
interstellar medium with the Far Ultraviolet Spectroscopic Explorer (FUSE).
Background observations obtained since 2005 have yielded eleven new O VI
detections of 3-sigma significance, and archival searches have revealed two
more. An additional 15 sight lines yield interesting upper limits. Combined
with previous results, these observations reveal the large-scale structure of
the O VI-bearing gas in the quadrant of the sky centered on the Magellanic
Clouds. The most prominent feature is a layer of low-velocity O VI emission
extending more than 70 degrees from the Galactic plane. At low latitudes (|b| <
30 degrees), the emission comes from narrow, high-density conductive interfaces
in the local ISM. At high latitudes, the emission is from extended, low-density
regions in the Galactic halo. We also detect O VI emission from the interface
region of the Magellanic System, a structure recently identified from H I
observations. These are the first detections of emission from high-ionization
species in the Magellanic System outside of the Clouds themselves.Comment: 18 pages, 11 Postscript figures. To appear in Ap
Modeling H2 Fluorescence in Planetary Atmospheres with Partial Frequency Redistribution
We present the modeling of partial frequency redistribution (PRD) effects for
the fluorescent emission lines of molecular hydrogen, the general computational
approximations, and the applications to planetary atmospheres, as well as
interstellar medium. Our model is applied to FUSE observations of Jupiter,
Saturn, and reflection nebulae, allowing an independent confirmation of the H2
abundance and the structure of planetary atmospheres.Comment: To appear in Proc. Future Directions in Ultraviolet Spectroscopy,
Annapolis, Oct. 20-22 200
OVI Absorption in the Milky Way Disk, and Future Prospects for Studying Absorption at the Galaxy-IGM Interface
We present a brief summary of results from our FUSE program designed to study
OVI absorption in the disk of the Milky Way. As a full analysis of our data has
now been published, we focus on the improvements that FUSE afforded us compared
to Copernicus data published thirty years ago. We discuss FUSE's limitations in
studying OVI absorption from nearby galaxies using background QSOs, but present
FUSE spectra of two probes which indicate the absence of OVI (but the presence
of Lyman-beta) absorption 8 and 63 kpc from a foreground galaxy. Finally, we
discuss the need for a more sensitive UV spectrograph to map out the physical
conditions of baryons around galaxies.Comment: Invited review to appear in the proceedings of "Future Directions in
Ultraviolet Spectroscopy" meeting, held Oct 20-22, 2008, Annapolis, MD. To
appear as an AIP Conference Proceedin