11,451 research outputs found
Projected rotational velocities of WD1614+136 and WD1353+409 - implications for the rate of galactic Type Ia supernovae
The white dwarf stars WD1614+136 and WD1353+409 are not sufficiently massive
to have formed through single star evolution. However, observations to date
have not yet found any evidence for binarity. It has therefore been suggested
that these stars are the result of a merger. In this paper we place an upper
limit of approximately 50kms on the projected rotational velocities of both
stars. This suggests that, if these stars are the results of a merger,
efficient angular momentum loss with accompanying mass loss must have occurred.
If the same process occurs following the merging of more massive white dwarf
stars, the predicted rate of Type Ia supernovae due to merging white dwarfs may
have been greatly over-estimated. Further observations to determine binarity in
WD1614+136 and WD1353+409 are therefore encouraged.Comment: 3 pages. 1 figur
WD1953-011 - a magnetic white dwarf with peculiar field structure
We present H-alpha spectra of the magnetic white dwarf star WD1953-011 which
confirm the presence of the broad Zeeman components corresponding to a field
strength of about 500kG found by Maxted & Marsh (1999). We also find that the
line profile is variable over a timescale of a day or less. The core of the
H-alpha line also shows a narrow Zeeman triplet corresponding to a field
strength of of about 100kG which appears to be almost constant in shape. These
observations suggest that the magnetic field on WD1953-011 has a complex
structure and that the star has a rotational period of hours or days which
causes the observed variability of the spectra. We argue that neither an offset
dipole model nor a double-dipole model are sufficient to explain our
observations. Instead, we propose a two component model consisting of a high
field region of magnetic field strength of about 500kG covering about 10% of
the surface area of the star superimposed on an underlying dipolar field of
mean field strength of about 70kG. Radial velocity measurements of the narrow
Zeeman triplet show that the radial velocity is constant to within a few km/s
so this star is unlikely to be a close binary.Comment: Accpeted for publication in MNRAS. 4 pages, 2 figure
KPD1930+2752 - a candidate Type Ia supernova progenitor
We present spectra of the pulsating sdB star KPD1930+2752 which confirm that
this star is a binary. The radial velocities measured from the H-alpha and
HeI6678 spectral lines vary sinusoidally with the same period (2h 17m) as the
ellipsoidal variability seen by Billeres et al. (2000). The amplitude of the
orbital motion (349.3+-2.7 km/s) combined with the canonical mass for sdB stars
(0.5 solar masses) implies a total mass for the binary of 1.47+-0.01 solar
masses The unseen companion star is almost certainly a white dwarf star. The
binary will merge within about 200 million years due to gravitational wave
radiation. The accretion of helium and other elements heavier than hydrogen
onto the white dwarf which then exceeds the Chandrasekhar mass (1.4 solar
masses) is a viable model for the cause of Type Ia supernovae. KPD1930+2752 is
the first star to be discovered which is a good candidate for the progenitor of
a Type Ia supernova of this type which will merge on an astrophysically
interesting timescale.Comment: Accepted for publication in MNRAS. 4 pages, 2 figures. Added
Institutio
The mass ratio distribution of short period double degenerate stars
Short period double degenerates (DDs) are close white dwarf - white dwarf
binary stars which are the result of the evolution of interacting binary stars.
We present the first definitive measurements of the mass ratio for two DDs,
WD0136+768 and WD1204+450, and an improved measurement of the mass ratio for
WD0957-666. We compare the properties of the 6 known DDs with measured mass
ratios to the predictions of various theoretical models. We confirm the result
that standard models for the formation of DDs do not predict sufficient DDs
with mass ratios near 1. We also show that the observed difference in cooling
ages between white dwarfs in DDs is a useful constraint on the initial mass
ratio of the binary. A more careful analysis of the properties of the white
dwarf pair WD1704+481.2 leads us to conclude that the brighter white dwarf is
older than its fainter companion. This is the opposite of the usual case for
DDs and is caused by the more massive white dwarf being smaller and cooling
faster. The mass ratio in the sense (mass of younger star)/(mass of older star)
is then 1.43+-0.06 rather than the value 0.70+-0.03 given previously.Comment: Accepted for publication in MNRA
Association of spin-labeled local anesthetics at the hydrophobic surface of acetylcholine receptor in native membranes from Torpedo marmorata.
The triple degenerate star WD1704+481
WD1704+481 is a visual binary in which both components are white dwarfs. We
present spectra of the H-alpha line of both stars which show that one component
(WD1704+481.2 = Sanduleak B = GR 577) is a close binary with two white dwarf
components. Thus, WD1704+481 is the first known triple degenerate star. From
radial velocity measurements of the close binary we find an orbital period of
0.1448d, a mass ratio, q=Mbright/Mfaint of q=0.70+-0.03 and a difference in the
gravitational redshifts of 11.5+-2.3km/s. The masses of the close pair of white
dwarfs predicted by the mass ratio and gravitational redshift difference
combined with theoretical cooling curves are 0.39+-0.05 solar mass and
0.56+-0.07 solar masses. WD1704+481 is therefore also likely to be the first
example of a double degenerate in which the less massive white dwarf is
composed of helium and the other white dwarf is composed of carbon and oxygen.Comment: 5 pages, 4 figure
Orbital periods of the binary sdB stars PG0940+068 and PG1247+554
We have used the radial velocity variations of two sdB stars previously
reported to be binaries to establish their orbital periods. They are
PG0940+068, (P=8.33d) and PG1247+554 (P=0.599d). The minimum masses of the
unseen companions, assuming a mass of 0.5 solar masses for the sdB stars, are
0.090 +/- 0.003 solar masses for PG1247+554 and 0.63 +/- 0.02 solar masses for
PG0940+068. The nature of the companions is not constrained further by our
data.Comment: 5 pages, 2 figure
The mass and radius of the M dwarf companion to GD 448
We present spectroscopy and photometry of GD 448, a detached white dwarf - M
dwarf binary with a period of 2.47h. We find that the NaI 8200A feature is
composed of narrow emission lines due to irradiation of the M dwarf by the
white dwarf within broad absorption lines that are essentially unaffected by
heating. Combined with an improved spectroscopic orbit and gravitational red
shift measurement from spectra of the H-alpha line, we are able to derive
masses for the white dwarf and M dwarf directly (0.41 +/- 0.01 solar masses and
0.096 +/- 0.004 solar masses, respectively). We use a simple model of the CaII
emission lines to establish the radius of the M dwarf assuming the emission
from its surface to be proportional to the incident flux per unit area from the
white dwarf. The radius derived is 0.125 +/- 0.020 solar radii. The M dwarf
appears to be a normal main-sequence star in terms of its mass and radius and
is less than half the size of its Roche lobe. The thermal timescale of the M
dwarf is much longer than the cooling age of the white dwarf so we conclude
that the M dwarf was unaffected by the common-envelope phase. The anomalous
width of the H-alpha emission from the M dwarf remains to be explained, but the
strengh of the line may be due to X-ray heating of the M dwarf due to accretion
onto the white dwarf from the M dwarf wind.Comment: 8 pages, 8 figure
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