318 research outputs found
Spectroscopic confirmation of a white dwarf companion to the B star 16 Dra
Using an Extreme Ultraviolet Explorer (EUVE) spectrum, we confirm the
identification of a white dwarf companion to the B9.5V star 16 Dra (HD150100),
and constrain its surface temperature to lie between 29,000K and 35,000K. This
is the third B star + white dwarf non-interacting Sirius-type binary to be
confirmed, after y Pup (HR2875, HD59635) and theta Hya (HR3665, HD79469). 16
Dra and its white dwarf companion are members of a larger resolved proper
motion system including the B9V star 17 Dra A (HD150117). The white dwarf must
have evolved from a progenitor more massive than this star, i.e. ~3.7 solar
masses. White dwarf companions to B stars are important since they set an
observational limit on the maximum mass for white dwarf progenitors, and can
potentially be used to investigate the high mass ends of the initial-final mass
relation and the white dwarf mass-radius relation.Comment: Accepted for publication in A&A, 22nd May 200
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
HR2875: Spectroscopic discovery of the first B star + white dwarf binary
We report the discovery, in an Extreme Ultraviolet Explorer (EUVE) short
wavelength spectrum, of an unresolved hot white dwarf companion to the
5th-magnitude B5Vp star HR2875. This is the first time that a non-interacting
white dwarf B star binary has been discovered; previously, the the earliest
type star known with a white dwarf companion was Sirius (A1V). Since the white
dwarf must have evolved from a main sequence progenitor with a mass greater
than that of a B5V star (6.0M), this places a lower limit on the
maximum mass for white dwarf progenitors, with important implications for our
knowledge of the initial-final mass relation. Assuming a pure-hydrogen
atmospheric composition, we constrain the temperature of the white dwarf to be
between 39,000K and 49,000K. We also argue that this degenerate star is likely
to have mass significantly greater than the mean mass for white dwarf stars
(0.55M). Finally, we suggest that other bright B stars (e.g.\
Hya) detected in the extreme ultraviolet surveys of the ROSAT Wide
Field Camera and EUVE may also be hiding hot white dwarf companions.Comment: Accepted for MNRAS pink pages; 4 pages, 1 figur
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
A search for hidden white dwarfs in the ROSAT EUV survey
The ROSAT WFC survey has provided us with evidence for the existence of a
previously unidentified sample of hot white dwarfs (WD) in non-interacting
binary systems, through the detection of EUV and soft X-ray emission. These
stars are hidden at optical wavelengths due to their close proximity to much
more luminous main sequence (MS) companions (spectral type K or earlier).
However, for companions of spectral type A5 or later the white dwarfs are
easily visible at far-UV wavelengths, and can be identified in spectra taken by
IUE. Eleven white dwarf binary systems have previously been found in this way
from ROSAT, EUVE and IUE observations (e.g. Barstow et al. 1994). In this paper
we report the discovery of three more such systems through our programmes in
recent episodes of IUE. The new binaries are HD2133, RE J0357+283 (whose
existence was predicted by Jeffries, Burleigh and Robb 1996), and BD+27 1888.
In addition, we have independently identified a fourth new WD+MS binary, RE
J1027+322, which has also been reported in the literature by Genova et al.
(1995), bringing the total number of such systems discovered as a result of the
EUV surveys to fifteen. We also discuss here six stars which were observed as
part of the programme, but where no white dwarf companion was found. Four of
these are coronally active. Finally, we present an analysis of the WD+K0IV
binary HD18131 (Vennes et al. 1995), which includes the ROSAT PSPC X-ray data.Comment: One Latex file containing text, One Tex file containing tables, 15
figures, MNRAS, in pres
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
Comparison of stimulation patterns for FES-cycling using measures of oxygen cost and stimulation cost
<b>Aim</b><p></p>
The energy efficiency of FES-cycling in spinal cord injured subjects is very much lower than that of normal cycling, and efficiency is dependent upon the parameters of muscle stimulation. We investigated measures which can be used to evaluate the effect on cycling performance of changes in stimulation parameters, and which might therefore be used to optimise them. We aimed to determine whether oxygen cost and stimulation cost measurements are sensitive enough to allow discrimination between the efficacy of different activation ranges for stimulation of each muscle group during constant-power cycling. <p></p>
<b>Methods</b><p></p>
We employed a custom FES-cycling ergometer system, with accurate control of cadence and stimulated exercise workrate. Two sets of muscle activation angles (âstimulation patternsâ), denoted âP1â and âP2â, were applied repeatedly (eight times each) during constant-power cycling, in a repeated measures design with a single paraplegic subject. Pulmonary oxygen uptake was measured in real time and used to determine the oxygen cost of the exercise. A new measure of stimulation cost of the exercise is proposed, which represents the total rate of stimulation charge applied to the stimulated muscle groups during cycling. A number of energy-efficiency measures were also estimated. <p></p>
<b>Results</b><p></p>
Average oxygen cost and stimulation cost of P1 were found to be significantly lower than those for P2 (paired <i>t</i>-test, <i>p</i> < 0.05): oxygen costs were 0.56 ± 0.03 l min<sup>â1</sup> and 0.61 ± 0.04 l min<sup>â1</sup>(mean ± S.D.), respectively; stimulation costs were 74.91 ± 12.15 mC min<sup>â1</sup> and 100.30 ± 14.78 mC min<sup>â1</sup> (mean ± S.D.), respectively. Correspondingly, all efficiency estimates for P1 were greater than those for P2. <p></p>
<b>Conclusion</b><p></p>
Oxygen cost and stimulation cost measures both allow discrimination between the efficacy of different muscle activation patterns during constant-power FES-cycling. However, stimulation cost is more easily determined in real time, and responds more rapidly and with greatly improved signal-to-noise properties than the ventilatory oxygen uptake measurements required for estimation of oxygen cost. These measures may find utility in the adjustment of stimulation patterns for achievement of optimal cycling performance. <p></p>
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