882 research outputs found
Near-infrared spectroscopy of the very low mass companion to the hot DA white dwarf PG1234+482
We present a near-infrared spectrum of the hot (
55,000 K) DA white dwarf PG 1234+482. We confirm that a very low mass companion
is responsible for the previously recognised infrared photometric excess. We
compare spectra of M and L dwarfs, combined with an appropriate white dwarf
model, to the data to constrain the spectral type of the secondary. We find
that uncertainties in the 2MASS photometry of the white dwarf prevent us
from distinguishing whether the secondary is stellar or substellar, and assign
a spectral type of L01 (M9-L1).Therefore, this is the hottest and youngest
( yr) DA white dwarf with a possible brown dwarf companion.Comment: 5 pages, 2 figures, accepted by MNRA
Organic syntheses employing supercritical carbon dioxide as a reaction solvent
Chemical reactions are readily carried out using supercritical carbon dioxide as the reaction medium. Supercritical carbon dioxide is of special value as a reaction medium in reactions for synthesizing polypeptides, for sequencing polypeptides, or for amino acid analysis
Organic syntheses employing supercritical carbon dioxide as a reaction solvent
Chemical reactions are readily carried out using supercritical carbon dioxide as the reaction medium. Supercritical carbon dioxide is of special value as a reaction medium in reactions for synthesizing polypeptides, for sequencing polypeptides, or for amino acid analysis
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
Hubble Space Telescope Spectroscopy of the Balmer lines in Sirius B
Sirius B is the nearest and brightest of all white dwarfs, but it is very
difficult to observe at visible wavelengths due to the overwhelming scattered
light contribution from Sirius A. However, from space we can take advantage of
the superb spatial resolution of the Hubble Space Telescope to resolve the A
and B components. Since the closest approach in 1993, the separation between
the two stars has become increasingly favourable and we have recently been able
to obtain a spectrum of the complete Balmer line series for Sirius B using
HST?s Space Telescope Imaging Spectrograph (STIS). The quality of the STIS
spectra greatly exceed that of previous ground-based spectra, and can be used
to provide an important determination of the stellar temperature (Teff =
25193K) and gravity (log g = 8.556). In addition we have obtained a new, more
accurate, gravitational red-shift of 80.42 +/- 4.83 km s-1 for Sirius B.
Combining these results with the photometric data and the Hipparcos parallax we
obtain new determinations of the stellar mass for comparison with the
theoretical mass-radius relation. However, there are some disparities between
the results obtained independently from log g and the gravitational redshift
which may arise from flux losses in the narrow 50x0.2arcsec slit. Combining our
measurements of Teff and log g with the Wood (1995) evolutionary mass-radius
relation we get a best estimate for the white dwarf mass of 0.978 M. Within the
overall uncertainties, this is in agreement with a mass of 1.02 M obtained by
matching our new gravitational red-shift to the theoretical M/R relation.Comment: 11 pages, 6 figures, accepted for publication in the Monthly Notices
of the Royal Astronomical Societ
High-resolution extreme ultraviolet spectroscopy of G191-B2B: structure of the stellar photosphere and the surrounding interstellar medium
We have continued our detailed analysis of the high-resolution (R= 4000) spectroscopic observation of the DA white dwarf G191-B2B, obtained by the Joint Astrophysical Plasmadynamic Experiment (J-PEX) normal incidence sounding rocket-borne telescope, comparing the observed data with theoretical predictions for both homogeneous and stratified atmosphere structures. We find that the former models give the best agreement over the narrow waveband covered by J-PEX, in conflict with what is expected from previous studies of the lower resolution but broader wavelength coverage Extreme Ultraviolet Explorer spectra. We discuss the possible limitations of the atomic data and our understanding of the stellar atmospheres that might give rise to this inconsistency. In our earlier study, we obtained an unusually high ionization fraction for the ionized He ii present along the line of sight to the star. In the present paper, we obtain a better fit when we assume, as suggested by Space Telescope Imaging Spectrograph results, that this He ii resides in two separate components. When one of these is assigned to the local interstellar cloud, the implied He ionization fraction is consistent with measurements along other lines of sight. However, the resolving power and signal-to-noise available from the instrument configuration used in this first successful J-PEX flight are not sufficient to clearly identify and prove the existence of the two components
Unaltered V̇O2 kinetics despite greater muscle oxygenation during heavy-intensity two-legged knee extension versus cycle exercise in humans
Relative perfusion of active muscles is greater during knee extension ergometry (KE) than cycle ergometry (CE). This provides the opportunity to investigate the effects of increased O₂ delivery (Q̇O₂) on deoxygenation heterogeneity among quadriceps muscles and pulmonary V̇O₂ kinetics. Using time-resolved near-infrared spectroscopy, we hypothesized that compared with CE the superficial vastus lateralis (VL), superficial rectus femoris and deep VL in KE would have 1) a smaller amplitude of the exercise-induced increase in deoxy[Hb+Mb] (related to the balance between V̇O₂ and Q̇O₂); 2) a greater amplitude of total[Hb+Mb] (related to the diffusive O₂ conductance); 3) a greater homogeneity of regional muscle deoxy[Hb+Mb]; and 4) no difference in pulmonary V̇O₂ kinetics. Eight participants performed square-wave KE and CE exercise from 20 W to heavy work rates. Deoxy[Hb+Mb] amplitude was less for all muscle regions in KE (P<0.05: superficial, KE 17-24 vs. CE 19-40; deep, KE 19 vs. CE 26 μM). Further, the amplitude of total[Hb+Mb] was greater for KE than CE at all muscle sites (P<0.05: superficial, KE 7-21 vs. CE 1-16; deep, KE 11 vs. CE -3 μM). Although the amplitude and heterogeneity of deoxy[Hb+Mb] was significantly lower in KE than CE during the first minute of exercise, the pulmonary V̇O₂ kinetics was not different for KE and CE. These data show that the microvascular Q̇O₂ to V̇O₂ ratio, and thus tissue oxygenation, was greater in KE than CE. This suggests that pulmonary and muscle V̇O₂ kinetics in young healthy humans are not limited by Q̇O₂ during heavy-intensity cycling
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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