1,118 research outputs found
The Potential of Asteroseismology for Hot, Subdwarf B Stars: A New Class of Pulsating Stars?
We present key sample results of a systematic survey of the pulsation
properties of models of hot B subdwarfs. We use equilibrium structures taken
from detailed evolutionary sequences of solar metallicity (Z = 0.02)
supplemented by grids of static envelope models of various metallicities (Z =
0.02, 0.04, 0.06, 0.08, and 0.10). We consider all pulsation modes with l = 0,
1, 2, and 3 in the 80--1500 s period window, the interval currently most
suitable for fast photometric detection techniques. We establish that
significant driving is often present in hot B subdwarfs and is due to an
opacity bump associated with heavy element ionization. We find that models with
Z >= 0.04 show low radial order unstable modes; both radial and nonradial (p,
f, and g) pulsations are excited. The unstable models have Teff > 30,000 K, and
log g > 5.7, depending somewhat on the metallicity. We emphasize that metal
enrichment needs only occur locally in the driving region. On this basis,
combined with the accepted view that local enrichments and depletions of metals
are common place in the envelopes of hot B subdwarfs, we predict that some of
these stars should show luminosity variations resulting from pulsational
instabilities.Comment: 9 pages, AAS LaTeX v4.0 + one postscript figure. Best with times.sty.
Submitted to Astrophysical Journal Letters. Postscript file also available at
http://shemesh.gsfc.nasa.gov/~dorman/Ben.htm
Ultraviolet Imaging of the Globular Cluster 47 Tucanae
We have used the Ultraviolet Imaging Telescope to obtain deep far-UV (1620
Angstrom), 40' diameter images of the prototypical metal-rich globular cluster
47 Tucanae. We find a population of about 20 hot (Teff > 9000 K) objects near
or above the predicted UV luminosity of the hot horizontal branch (HB) and
lying within two half-light radii of the cluster center. We believe these are
normal hot HB or post-HB objects rather than interacting binaries or blue
stragglers. IUE spectra of two are consistent with post-HB phases. These
observations, and recent HST photometry of two other metal-rich clusters,
demonstrate that populations with rich, cool HB's can nonetheless produce hot
HB and post-HB stars. The cluster center also contains an unusual diffuse
far-UV source which is more extended than its V-band light. It is possible that
this is associated with an intracluster medium, for which there was earlier
infrared and X-ray evidence, and is produced by C IV emission or scattered
light from grains.Comment: 13 pages AASLaTeX including one postscript figure and one bitmapped
image, JPEG format. Submitted to the Astronomical Jorunal. Full Postscript
version available at http://www.astro.virginia.edu/~bd4r
Far-Ultraviolet Color Gradients in Early-Type Galaxies
We discuss far-UV (1500 A) surface photometry and FUV-B color profiles for 8
E/S0 galaxies from images taken with the Ultraviolet Imaging Telescope,
primarily during the Astro-2 mission. In three cases, the FUV radial profiles
are more consistent with an exponential than a de Vaucouleurs function, but
there is no other evidence for the presence of a disk or of young, massive
stars. In all cases except M32 the FUV-B color becomes redder at larger radii.
There is a wide range of internal radial FUV-B color gradients. However, we
find no correlation between the FUV-B color gradients and internal metallicity
gradients based on Mg absorption features. We conclude that metallicity is not
the sole parameter controlling the "UV upturn component" in old populations.Comment: 11 pages; tar.gz file includes LaTeX text file, 3 PostScript figures.
Paper to be published in ApJ Letter
UIT Detection of Hot Stars in the Globular Cluster NGC362
We used the Ultraviolet Imaging Telescope during the March 1995 Astro-2
mission to obtain a deep far-UV image of the globular cluster NGC 362, which
was formerly thought to have an almost entirely red horizontal branch (HB). 84
hot (T_eff > 8500 K) stars were detected within a radius of 8'.25 of the
cluster center. Of these, 43 have FUV magnitudes consistent with HB stars in
NGC 362, and at least 34 are cluster members. The number of cluster members is
made uncertain by background contamination from blue stars in the Small
Magellanic Cloud (SMC). There are six candidate supra-HB stars which have
probably evolved from the HB. We discuss the implications of these results for
the production of hot blue stars in stellar populations.Comment: 10 pages AASLaTeX including one postscript figure and one compressed
bitmap, .jpg format. To appear in Ap. J. Letters. Postscript version also
available at http://www.astro.virginia.edu/~bd4r
In vitro techniques for the assessment of neurotoxicity.
Risk assessment is a process often divided into the following steps: a) hazard identification, b) dose-response assessment, c) exposure assessment, and d) risk characterization. Regulatory toxicity studies usually are aimed at providing data for the first two steps. Human case reports, environmental research, and in vitro studies may also be used to identify or to further characterize a toxic hazard. In this report the strengths and limitations of in vitro techniques are discussed in light of their usefulness to identify neurotoxic hazards, as well as for the subsequent dose-response assessment. Because of the complexity of the nervous system, multiple functions of individual cells, and our limited knowledge of biochemical processes involved in neurotoxicity, it is not known how well any in vitro system would recapitulate the in vivo system. Thus, it would be difficult to design an in vitro test battery to replace in vivo test systems. In vitro systems are well suited to the study of biological processes in a more isolated context and have been most successfully used to elucidate mechanisms of toxicity, identify target cells of neurotoxicity, and delineate the development and intricate cellular changes induced by neurotoxicants. Both biochemical and morphological end points can be used, but many of the end points used can be altered by pharmacological actions as well as toxicity. Therefore, for many of these end points it is difficult or impossible to set a criterion that allows one to differentiate between a pharmacological and a neurotoxic effect. For the process of risk assessment such a discrimination is central. Therefore, end points used to determine potential neurotoxicity of a compound have to be carefully selected and evaluated with respect to their potential to discriminate between an adverse neurotoxic effect and a pharmacologic effect. It is obvious that for in vitro neurotoxicity studies the primary end points that can be used are those affected through specific mechanisms of neurotoxicity. For example, in vitro systems may be useful for certain structurally defined compounds and mechanisms of toxicity, such as organophosphorus compounds and delayed neuropathy, for which target cells and the biochemical processes involved in the neurotoxicity are well known. For other compounds and the different types of neurotoxicity, a mechanism of toxicity needs to be identified first. Once identified, by either in vivo or in vitro methods, a system can be developed to detect and to evaluate predictive ability for the type of in vivo neurotoxicity produced. Therefore, in vitro tests have their greatest potential in providing information on basic mechanistic processes in order to refine specific experimental questions to be addressed in the whole animal
Age and Abundance Discrimination in Old Stellar Populations Using Mid-Ultraviolet Colors
The restframe mid-ultraviolet spectral region (2000-3200 A) is important in
analyzing the stellar populations of the "red envelope" systems observed at
high redshifts. Here, we explore the usefulness of the mid-UV for determining
ages and abundances of old populations. A mid-UV to optical/IR wavelength
baseline provides good separation of population components because the main
sequence turnoff dominates the integrated light between 2500 and 4000 A. We
find a six magnitude difference in the mid-UV continuum level over the
metallicity range -1.5 < log Z/Z_o < +0.5 and a comparable difference (per unit
log t) for ages in the range 4-16 Gyr. Logarithmic derivatives of mid-UV colors
with respect to age or metal abundance are 3-10 times larger than for the UBV
region. Most of the spectral information on old populations therefore resides
below 4000 A. We investigate the capability of UBV and mid-UV broad-band colors
to separately determine age and abundance, taking into account precision in the
color measurements. We find that the mid-UV improves resolution in (log t,log
Z) space by about a factor of 3 for a given observational precision.
Contamination by hot horizontal branch phases can seriously affect mid-UV
spectra, reaching over 80% in some cases. However, this is straightforward to
remove as long as far-UV measurements are available. Finally, we show that a 4
Gyr, solar abundance model based on empirical spectra provides an excellent fit
to the mid-UV spectrum of the E galaxy M32. This indicates that the poorer
results obtained from theoretical spectra arise from limitations of the
synthesis models for individual stars. [Condensed]Comment: 38 pages, 8 figures. Accepted by ApJ. Full resolution figures
available at: http://www.astro.virginia.edu/~rwo/miduv-fullresfigs.ps.g
First Kepler results on compact pulsators II: KIC 010139564, a new pulsating subdwarf B (V361 Hya) star with an additional low-frequency mode
We present the discovery of nonradial pulsations in a hot subdwarf B star
based on 30.5 days of nearly continuous time-series photometry using the
\emph{Kepler} spacecraft. KIC 010139564 is found to be a short-period pulsator
of the V361 Hya (EC 14026) class with more than 10 independent pulsation modes
whose periods range from 130 to 190 seconds. It also shows one periodicity at a
period of 3165 seconds. If this periodicity is a high order g-mode, then this
star may be the hottest member of the hybrid DW Lyn stars. In addition to the
resolved pulsation frequencies, additional periodic variations in the light
curve suggest that a significant number of additional pulsation frequencies may
be present. The long duration of the run, the extremely high duty cycle, and
the well-behaved noise properties allow us to explore the stability of the
periodic variations, and to place strong constraints on how many of them are
independent stellar oscillation modes. We find that most of the identified
periodicities are indeed stable in phase and amplitude, suggesting a rotation
period of 2-3 weeks for this star, but further observations are needed to
confirm this suspicion.Comment: 10 pages, accepted for publication in MNRA
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