243 research outputs found
A photospheric metal line profile analysis of hot DA white dwarfs with circumstellar material
Some hot DA white dwarfs have circumstellar high ion absorption features in
their spectra, in addition to those originating in the photosphere. In many
cases, the line profiles of these absorbing components are unresolved. Given
the importance of the atmospheric composition of white dwarfs to studies of
stellar evolution, extra-solar planetary systems and the interstellar medium,
we examine the effect of including circumstellar line profiles in the abundance
estimates of photospheric metals in six DA stars. The photospheric C and Si
abundances are reduced in five cases where the circumstellar contamination is
strong, though the relative weakness of the circumstellar Si IV absorption
introduces minimal contamination, resulting in a small change in abundance. The
inability of previous, approximate models to reproduce the photospheric line
profiles here demonstrates the need for a technique that accounts for the
physical line profiles of both the circumstellar and photospheric lines when
modelling these blended absorption features.Comment: 7 pages, 5 figues, 3 tables, accepted for publication in MNRA
Towards a standardised line list for G191-B2B, and other DA type objects
We present a comprehensive analysis of the far UV spectrum of G191-B2B over
the range of 900-1700{\AA} using co-added data from the FUSE and STIS archives.
While previous identifications made by Holberg et al. (2003) are reaffirmed in
this work, it is found that many previously unidentified lines can now be
attributed to Fe, Ni, and a few lighter metals. Future work includes extending
this detailed analysis to a wider range of DA objects, in the expectation that
a more complete analysis of their atmospheres can be realised.Comment: 4 pages, 2 figures, 1 table: To appear in the proceedings of the
"18th European White Dwarf Workshop" in Krakow, Poland, 201
Hubble Space Telescope Imaging and Spectroscopy of the Sirius-Like Triple Star System HD 217411
We present Hubble Space Telescope imaging and spectroscopy of HD 217411, a G3
V star associated with the extreme ultraviolet excess source (EUV 2RE
J2300-07.0). This star is revealed to be a triple system with a G 3V primary
(HD 217411 A) separated by ~1.1" from a secondary that is in turn composed of
an unresolved K0 V star (HD 217411 Ba) and a hot DA white dwarf (HD 217411 Bb).
The hot white dwarf dominates the UV flux of the system. However; it is in turn
dominated by the K0 V component beyond 3000 {\AA}. A revised distance of 143 pc
is estimated for the system. A low level photometric modulation having a period
of 0.61 days has also been observed in this system along with a rotational
velocity on the order of 60 km s-1 in the K0 V star. Together both observations
point to a possible wind induced spin up of the K0 V star during the AGB phase
of the white dwarf. The nature of all three components is discussed as are
constraints on the orbits, system age and evolution.Comment: 11 pages, 6 figure
Temporal Changes in Astronauts Muscle and Cardiorespiratory Physiology Pre-, In-, and Post-Spaceflight
NASAs vision for future exploration missions depends on the ability to protect astronauts health and safety for performance of Extravehicular Activity (EVA), and to allow astronauts to safely egress from vehicles in a variety of landing scenarios (e.g. water landing upon return to Earth and undefined planetary/lunar landings). Prolonged exposure to spaceflight results in diminished tolerance to prolonged physical activity, decreased cardiac and sensorimotor function, and loss of bone mineral density, muscle mass, and muscle strength. For over 50 years exercise has been the primary countermeasure against these physiologic decrements during spaceflight, and while the resulting protection is adequate for ISS missions (i.e., Soyuz landing, microgravity EVAs), there is little information regarding time-course changes in muscle and aerobic performance. As spaceflight progresses towards longer exploration missions and vehicles with less robust exercise capabilities compared to ISS, countermeasures will need to be combined and optimized to protect crew health and performance across all organ systems over the course of exploration missions up to 3 years in duration. This will require a more detailed understanding of the dynamic effects of spaceflight on human performance. Thus, the focus of this study is quantifying decrements in physical performance over different mission durations, and to provide detailed information on the physiological rational for why and when observed changes in performance occur. The research proposed will temporally profile changes in astronauts cardiorespiratory fitness, muscle mass, strength, and endurance over spaceflight missions of 2 months, 6 months, and up to 1 year in duration. Additionally, an extrapolation model will provide predictions for changes associated with exploration missions 2-3 years in duration. To accomplish these objectives astronauts will be asked to participate in pre, in, post-flight measurement of muscle performance, muscle size, cardiorespiratory fitness and submaximal performance capabilities, as well as non-invasive assessment of cerebral and muscle oxygenation and perfusion (Table 1). Additionally, ambulatory and in-flight exercise, nutrition, and sleep will be monitored using a variety of commercial technologies and in-flight assessment tools. Significance: Our detailed testing protocol will provide valuable information for describing how and when spaceflight-induced muscle and aerobic based adaptations occur over the course of spaceflight missions up to and beyond 1 year. This information will be vital in the assessment as to whether humans can be physically ready for deep space exploration such as Mars missions with current technology, or if additional mitigation strategies are necessary
A Chandra Search for Coronal X Rays from the Cool White Dwarf GD 356
We report observations with the Chandra X-ray Observatory of the single,
cool, magnetic white dwarf GD 356. For consistent comparison with other X-ray
observations of single white dwarfs, we also re-analyzed archival ROSAT data
for GD 356 (GJ 1205), G 99-47 (GR 290 = V1201 Ori), GD 90, G 195-19 (EG250 = GJ
339.1), and WD 2316+123 and archival Chandra data for LHS 1038 (GJ 1004) and GD
358 (V777 Her). Our Chandra observation detected no X rays from GD 356, setting
the most restrictive upper limit to the X-ray luminosity from any cool white
dwarf -- L_{X} < 6.0 x 10^{25} ergs/s, at 99.7% confidence, for a 1-keV
thermal-bremsstrahlung spectrum. The corresponding limit to the electron
density is n_{0} < 4.4 x 10^{11} cm^{-3}. Our re-analysis of the archival data
confirmed the non-detections reported by the original investigators. We discuss
the implications of our and prior observations on models for coronal emission
from white dwarfs. For magnetic white dwarfs, we emphasize the more stringent
constraints imposed by cyclotron radiation. In addition, we describe (in an
appendix) a statistical methodology for detecting a source and for constraining
the strength of a source, which applies even when the number of source or
background events is small.Comment: 27 pages, 4 figures, submitted to the Astrophysical Journa
Hubble Space Telescope Astrometry of the Procyon System
The nearby star Procyon is a visual binary containing the F5 IV-V subgiant
Procyon A, orbited in a 40.84 yr period by the faint DQZ white dwarf Procyon B.
Using images obtained over two decades with the Hubble Space Telescope, and
historical measurements back to the 19th century, we have determined precise
orbital elements. Combined with measurements of the parallax and the motion of
the A component, these elements yield dynamical masses of 1.478 +/- 0.012 Msun
and 0.592 +/- 0.006 Msun for A and B, respectively.
The mass of Procyon A agrees well with theoretical predictions based on
asteroseismology and its temperature and luminosity. Use of a standard
core-overshoot model agrees best for a surprisingly high amount of core
overshoot. Under these modeling assumptions, Procyon A's age is ~2.7 Gyr.
Procyon B's location in the H-R diagram is in excellent agreement with
theoretical cooling tracks for white dwarfs of its dynamical mass. Its position
in the mass-radius plane is also consistent with theory, assuming a
carbon-oxygen core and a helium-dominated atmosphere. Its progenitor's mass was
1.9-2.2 Msun, depending on its amount of core overshoot.
Several astrophysical puzzles remain. In the progenitor system, the stars at
periastron were separated by only ~5 AU, which might have led to tidal
interactions and even mass transfer; yet there is no direct evidence that these
have occurred. Moreover the orbital eccentricity has remained high (~0.40). The
mass of Procyon B is somewhat lower than anticipated from the
initial-to-final-mass relation seen in open clusters. The presence of heavy
elements in its atmosphere requires ongoing accretion, but the place of origin
is uncertain.Comment: Accepted by Astrophysical Journa
X-ray Evidence of the Common Envelope Phase of V471 Tauri
Chandra Low Energy Transmission Grating Spectrograph observations of the
pre-cataclysmic binary V471 Tau have been used to estimate the C/N abundance
ratio of the K dwarf component for the first time. While the white dwarf
component dominates the spectrum longward of 50 AA, at shorter wavelengths the
observed X-ray emission is entirely due to coronal emission from the K dwarf.
The H-like resonance lines of C and N yield an estimate of their logarithmic
abundance ratio relative to the Sun of [C/N]=-0.38+/-0.15 - half of the
currently accepted solar value. We interpret this result as the first clear
observational evidence for the presumed common envelope phase of this system,
during which the surface of the K dwarf was contaminated by CN-cycle processed
material dredged up into the red giant envelope. We use the measured C/N ratio
to deduce that 0.015-0.04 Msun was accreted by the K dwarf while engulfed, and
show that this is consistent with a recent tentative detection of 13C in the K
dwarf photosphere, and with the measured Li abundance in the scenario where the
red giant companion was Li-rich during the common envelope phase.Comment: 6 pages, 2 figures, ApJL accepte
FUSE and HST/STIS far-ultraviolet observations of AM Herculis in an extended low state
We have obtained FUSE and HST/STIS time-resolved spectroscopy of the polar AM
Herculis during a deep low state. The spectra are entirely dominated by the
emission of the white dwarf. Both the far-ultraviolet (FUV) flux as well as the
spectral shape vary substantially over the orbital period, with maximum flux
occurring at the same phase as during the high state. The variations are due to
the presence of a hot spot on the white dwarf, which we model quantitatively.
The white dwarf parameters can be determined from a spectral fit to the faint
phase data, when the hot spot is self-eclipsed. Adopting the distance of
79+8-6pc determined by Thorstensen, we find an effective temperature of
19800+-700K and a mass of Mwd=0.78+0.12-0.17Msun. The hot spot has a lower
temperature than during the high state, ~34000-40000K, but covers a similar
area, ~10% of the white dwarf surface. Low state FUSE and STIS spectra taken
during four different epochs in 2002/3 show no variation of the FUV flux level
or spectral shape, implying that the white dwarf temperature and the hot spot
temperature, size, and location do not depend on the amount of time the system
has spent in the low state. Possible explanations are ongoing accretion at a
low level, or deep heating, both alternatives have some weaknesses that we
discuss. No photospheric metal absorption lines are detected in the FUSE and
STIS spectra, suggesting that the average metal abundances in the white dwarf
atmosphere are lower than 1e-3 times their solar values.Comment: ApJ in press, 12 pages, 11 figure
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