768 research outputs found
Circumstellar features in hot DA white dwarfs
We present a phenomenological study of highly ionised, non-photospheric
absorption features in high spectral resolution vacuum ultraviolet spectra of
23 hot DA white dwarfs. Prior to this study, four of the survey objects (Feige
24, REJ 0457-281, G191-B2B and REJ 1614-085) were known to possess these
features. We find four new objects with multiple components in one or more of
the principal resonance lines: REJ 1738+665, Ton 021, REJ 0558-373 and WD
2218+706. A fifth object, REJ 2156-546 also shows some evidence of multiple
components, though further observations are required to confirm the detection.
We discuss possible origins for these features including ionisation of the
local interstellar environment, the presence of material inside the
gravitational well of the white dwarf, mass loss in a stellar wind, and the
existence of material in an ancient planetary nebula around the star. We
propose ionisation of the local interstellar medium as the origin of these
features in G191-B2B and REJ 1738+665, and demonstrate the need for higher
resolution spectroscopy of the sample, to detect multiple ISM velocity
components and to identify circumstellar features which may lie close to the
photospheric velocity.Comment: 20 figures, 21 page
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Greater V˙O2peak is correlated with greater skeletal muscle deoxygenation amplitude and hemoglobin concentration within individual muscles during ramp-incremental cycle exercise.
It is axiomatic that greater aerobic fitness (V˙O2peak) derives from enhanced perfusive and diffusive O2 conductances across active muscles. However, it remains unknown how these conductances might be reflected by regional differences in fractional O2 extraction (i.e., deoxy [Hb+Mb] and tissue O2 saturation [StO2]) and diffusive O2 potential (i.e., total[Hb+Mb]) among muscles spatially heterogeneous in blood flow, fiber type, and recruitment (vastus lateralis, VL; rectus femoris, RF). Using quantitative time-resolved near-infrared spectroscopy during ramp cycling in 24 young participants (V˙O2peak range: ~37.4-66.4 mL kg-1 min-1), we tested the hypotheses that (1) deoxy[Hb+Mb] and total[Hb+Mb] at V˙O2peak would be positively correlated with V˙O2peak in both VL and RF muscles; (2) the pattern of deoxygenation (the deoxy[Hb+Mb] slopes) during submaximal exercise would not differ among subjects differing in V˙O2peak Peak deoxy [Hb+Mb] and StO2 correlated with V˙O2peak for both VL (r = 0.44 and -0.51) and RF (r = 0.49 and -0.49), whereas for total[Hb+Mb] this was true only for RF (r = 0.45). Baseline deoxy[Hb+Mb] and StO2 correlated with V˙O2peak only for RF (r = -0.50 and 0.54). In addition, the deoxy[Hb+Mb] slopes were not affected by aerobic fitness. In conclusion, while the pattern of deoxygenation (the deoxy[Hb+Mb] slopes) did not differ between fitness groups the capacity to deoxygenate [Hb+Mb] (index of maximal fractional O2 extraction) correlated significantly with V˙O2peak in both RF and VL muscles. However, only in the RF did total[Hb+Mb] (index of diffusive O2 potential) relate to fitness
A comparison of DA white dwarf temperatures and gravities from FUSE Lyman line and ground-based Balmer line observations
Observation of the strengths and profiles of the hydrogen Balmer absorption
series is an established technique for determining the effective temperature
and surface gravity of hot H-rich white dwarf stars. In principle, the Lyman
series lines should be equally useful but, lying in the far-UV, are only
accessible from space. Nevertheless, there are situations (for example, where
the optical white dwarf spectrum is highly contaminated by the presence of a
companion) in which use of the Lyman series may be essential. Therefore, it is
important to establish whether or not the Lyman lines provide an equally valid
means of measurement. We have already made a first attempt to study this
problem, comparing Lyman line measurements from a variety of far-UV instruments
with ground-based Balmer line studies. Within the measurement uncertainties we
found the results from each line series to be broadly in agreement. However, we
noted a number of potential systematic effects that could bias either
measurement. With the availability of the FUSE data archive and observations
from our own Guest Observer programmes, we now have an opportunity of examining
the use of the Lyman series in more detail from observations of 16 DA white
dwarfs. The new results partially reproduce the earlier study, showing that
Balmer and Lyman line determined temperatures are in good agreement up to
\~50000K. However, above this value there is an increasing systematic
difference between the Lyman and Balmer line results, the former yielding the
higher temperature.Comment: Accepted for publication in the Monthly Notices of the Royal
Astronomical Society: 15 pages (inc 10 figures
The Best Brown Dwarf Yet?: A Companion to the Hyades Eclipsing Binary V471 Tau
We have carried out an analysis of about 160 eclipse timings spanning over 30
years of the Hyades eclipsing binary V471 Tauri that shows a long-term
quasi-sinusoidal modulation of its observed eclipse arrival times. The O-Cs
have been analyzed for the ``light-time'' effect that arises from the
gravitational influence of a tertiary companion. The presence of a third body
causes the relative distance of the eclipsing pair to the Earth to change as it
orbits the barycenter of the triple system. The result of the analysis of the
eclipse times yields a light-time semi-amplitude of 137.2+/-12.0 s, an orbital
period of P_3 = 30.5+/-1.6 yr and an eccentricity of e_3 = 0.31+/-0.04. The
mass of the tertiary component is M_3 sin i_3 = 0.0393+/-0.0038 Mo when a total
mass of 1.61+/-0.06 Mo for V471 Tau is adopted. For orbital inclinations i_3 >
35 deg, the mass of the third body would be below the stable hydrogen burning
limit of M = 0.07 Mo and it thus would be a brown dwarf. In the next several
years (near maximum elongation), it should be feasible to obtain IR images and
spectra of V471 Tau C that, when combined with the known mass, age, distance,
and [Fe/H], will serve as a benchmark for understanding the physical properties
and evolution of brown dwarfs.Comment: 9 pages, 3 figures, accepted for publication in ApJ Letter
A far-UV survey of three hot, metal-polluted white dwarf stars: WD0455-282, WD0621-376, and WD2211-495
Using newly obtained high-resolution data () from the
\textit{Hubble Space Telescope}, and archival UV data from the \textit{Far
Ultraviolet Spectroscopic Explorer} we have conducted a detailed UV survey of
the three hot, metal-polluted white dwarfs WD0455-282, WD0621-376, and
WD2211-495. Using bespoke model atmospheres we measured , log
, and photospheric abundances for these stars. In conjunction with data from
Gaia we measured masses, radii, and gravitational redshift velocities for our
sample of objects. We compared the measured photospheric abundances with those
predicted by radiative levitation theory, and found that the observed Si
abundances in all three white dwarfs, and the observed Fe abundances in
WD0621-376 and WD2211-495, were larger than those predicted by an order of
magnitude. These findings imply not only an external origin for the metals, but
also ongoing accretion, as the metals not supported by radiative levitation
would sink on extremely short timescales. We measured the radial velocities of
several absorption features along the line of sight to the three objects in our
sample, allowing us to determine the velocities of the photospheric and
interstellar components along the line of sight for each star. Interestingly,
we made detections of circumstellar absorption along the line of sight to
WD0455-282 with three velocity components. To our knowledge, this is the first
such detection of multi-component circumstellar absorption along the line of
sight to a white dwarf.Comment: 19 pages, 23 figures, 8 tables. Accepted for publication in the
Monthly Notices of the Royal Astronomical Societ
Do the constants of nature couple to strong gravitational fields?
Recently, white dwarf stars have found a new use in the fundamental physics
community. Many prospective theories of the fundamental interactions of Nature
allow traditional constants, like the fine structure constant , to vary
in some way. A study by Berengut et al. (2013) used the Fe/Ni V line
measurements made by Preval et al. (2013) from the hot DA white dwarf G191-B2B,
in an attempt to detect any variation in . It was found that the Fe V
lines indicated an increasing alpha, whereas the Ni V lines indicated a
decreasing alpha. Possible explanations for this could be misidentification of
the lines, inaccurate atomic data, or wavelength dependent distortion in the
spectrum. We examine the first two cases by using a high S/N reference spectrum
from the hot sdO BD+284211 to calibrate the Fe/Ni V atomic data. With
this new data, we re-evaluate the work of Berengut et al. (2013) to derive a
new constraint on the variation of alpha in a gravitational field.Comment: 4 pages, 2 figures: To appear in the proceedings of the "19th
European White Dwarf Workshop" in Montreal, Canada, 201
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 (“stimulation patterns”), denoted “P1” and “P2”, 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>−1</sup> and 0.61 ± 0.04 l min<sup>−1</sup>(mean ± S.D.), respectively; stimulation costs were 74.91 ± 12.15 mC min<sup>−1</sup> and 100.30 ± 14.78 mC min<sup>−1</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>
Mass-Radius Relation for Magnetic White Dwarfs
Recently, several white dwarfs with very strong surface magnetic fields have
been observed. In this paper we explore the possibility that such stars could
have sufficiently strong internal fields to alter their structure. We obtain a
revised white dwarf mass-radius relation in the presence of strong internal
magnetic fields. We first derive the equation of state for a fully degenerate
ideal electron gas in a magnetic field using an Euler-MacLaurin expansion. We
use this to obtain the mass-radius relation for magnetic He, C,
and Fe white dwarfs of uniform composition.Comment: 7 pages, 7 figures and 1 table, To appear in Ap
An astronomical search for evidence of new physics: Limits on gravity-induced birefringence from the magnetic white dwarf RE J0317-853
The coupling of the electromagnetic field directly with gravitational gauge
fields leads to new physical effects that can be tested using astronomical
data. Here we consider a particular case for closer scrutiny, a specific
nonminimal coupling of torsion to electromagnetism, which enters into a
metric-affine geometry of space-time. We show that under the assumption of this
nonminimal coupling, spacetime is birefringent in the presence of such a
gravitational field. This leads to the depolarization of light emitted from
extended astrophysical sources. We use polarimetric data of the magnetic white
dwarf to set strong constraints on the essential coupling
constant for this effect, giving k^2 \lsim (19 {m})^2 .Comment: Statements about Moffat's NGT modified. Accepted for publication in
Phys.Rev.
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