19,238 research outputs found
Kinematics of the helium accretor GP Com
We present time-resolved spectra of the double-degenerate binary star, GP
Com. The spectra confirm the presence and period (46.5 min) of the `S'-wave
feature found by Nather, Robinson and Stover. GP Com is erratically variable at
X-ray and UV wavelengths. We have found the equivalent variability in our data,
which, as also seen in UV data, is mostly confined to the emission lines. The
HeII 4686 changes by the largest amount, consistent with X-ray driven
photo-ionisation. The flaring part of the line profiles is broader than the
average, as expected if they are dominated by the inner disc. The HeII 4686
profile is especially remarkable in that its blue-shifted peak is 1400 km/s
from line centre compared to 700 km/s for the HeI lines (the red-shifted peak
is blended with HeI 4713). We deduce that HeII 4686 emission is confined to the
inner 1/4 of the disc. We suggest that the activity of the inner disc indicates
that accretion is significant (and unstable) there, in contrast to quiescent
dwarf novae, in support of models in which GP Com is in a (quasi-)steady-state
of low mass transfer rate. GP Com shows triple-peaked lines profiles which
consist of the usual double-peaked profiles from a disc plus a narrow component
at line centre. We find evidence for both radial velocity and flux variability
in this component, inconsistent with a nebula origin. The radial velocity
amplitude and its phase relative to the `S'-wave are consistent with an origin
on the accreting white dwarf, if the mass ratio, q = M2/M1, is of order 0.02,
as expected on evolutionary grounds. However this explanation is still not
satisfactory as the systemic velocity of the narrow component shows significant
variation from line to line, and we have no explanation for this.Comment: 9 pages Latex, 9 figures. Accepted for publication in Monthly Notice
Detached white-dwarf close-binary stars -- CV's extended family
I review detached binaries consisting of white dwarfs with either other white
dwarfs or low mass main-sequence stars in tight orbits around them. Orbital
periods have been measured for 15 white dwarf/white dwarf systems and 22 white
dwarf/M dwarf systems. While small compared to the number of periods known for
CVs (>300), I argue that each variety of detached system has a space density an
order of magnitude higher that of CVs. While theory matches the observed
distribution of orbital periods of the white dwarf/white dwarf binaries, it
predicts white dwarfs of much lower mass than observed. Amongst both types of
binary are clear examples of helium core white dwarfs, as opposed to the usual
CO composition; similar systems must exist amongst the CVs. White dwarf/M dwarf
binaries suffer from selection effects which diminish the numbers seen at long
and short periods. They are useful for the study of irradiation; I discuss
evidence to suggest that Balmer emission is broadened by optical depth effects
to an extent which limits its usefulness for imaging the secondary stars in
CVs.Comment: 6 pages, to appear in proceedings of Warner symposium on Cataclysmic
Variable star
Period Changes in Ultra-compact Double White Dwarfs
In recent years there has been much interest in the nature of two stars, V407
Vul and RXJ0806+1527, which are widely thought to be binary white dwarfs of
very short orbital period, 570 and 321 seconds respectively. As such they
should be strong sources of gravitational waves and possible ancestors of the
accreting AM CVn stars. Monitoring at X-ray and optical wavelengths has
established that the period of each star is decreasing, at rates compatible
with that expected from gravitational radiation. This has been taken to support
the ``unipolar inductor'' model in which the white dwarfs are detached and the
X-rays produced by the dissipation of magnetically-induced electric currents.
In this paper we show that this interpretation is incorrect because it ignores
associated torques which transfer angular momentum between the spin of the
magnetic white dwarf and the orbit. We show that this torque is 10^5 times
larger than the GR term in the case of V407 Vul, and 10 times larger for
RXJ0806+1527. For V407 Vul, the unipolar inductor model can only survive if the
white dwarf spins 100 times faster than the orbit. Since this could only come
about through accretion, the validity of the unipolar inductor appears
questionable for this star. We also consider whether accretion models can fit
the observed spin-up, concluding that they can, provided that a mechanism
exists for driving the mass transfer rate away from its equilibrium value.Comment: 6 pages, 1 figure, accepted for publication in MNRA
Multiple Pathways to Graduation: New Routes to High School Competition
Examines three approaches -- targeted population, district-wide, and linked learning -- to raising graduation rates, benefits, and challenges; what is required for implementation; and which approaches work well for different types of districts
A double-frequency dwarf nova oscillation
We have detected coherent oscillations (``dwarf nova oscillations'') in
Hubble Space Telescope spectra of the dwarf nova OY Car. The oscillations were
seen towards the end of a superoutburst of OY Car. The oscillations are
extraordinary compared to the many other examples in the literature for two
reasons. First, their amplitude is large, with a peak-to-peak variation of 8 to
20% of the total flux over the range 1100 to 2500A. However, most remarkably we
find that there are two components present simultaneously. Both have periods
close to 18sec (equivalent to 4800 cycles/day) but they are separated by
57.7+/-0.5 cycles/day. The lower frequency component of the pair has a strong
second harmonic while its companion, which has about twice its amplitude, does
not. The oscillation spectra appear hotter than the mean spectrum and
approximately follow the continuum distribution of a black-body with a
temperature in the range 30,000 to 50,000K.
We tentatively suggest that the weaker non-sinusoidal component could
represent the rotation of the white dwarf, although we have been unable to
recover any such signal in quiescent data.Comment: 9 pages, 9 figure
Spectral atlas of dwarf novae in outburst
Up to now, only a very small number of dwarf novae have been studied during
their outburst state (~30 per cent in the Northern hemisphere). In this paper
we present the first comprehensive atlas of outburst spectra of dwarf novae. We
study possible correlations between the emission and absorption lines seen in
the spectra and some fundamental parameters of the binaries. We find that out
of the 48 spectra presented, 12 systems apart from IP Peg show strong HeII in
emission: SS Aur, HL CMa, TU Crt, EM Cyg, SS Cyg, EX Dra, U Gem, HX Peg, GK
Per, KT Per, V893 Sco, IY UMa, and 7 others less prominently: FO And, V542 Cyg,
BI Ori, TY Psc, VZ Pyx, ER UMa, and SS UMi. We conclude that these systems are
good targets for finding spiral structure in their accretion discs during
outburst if models of Smak (2001) and Ogilvie (2001) are correct. This is
confirmed by the fact that hints of spiral asymmetries have already been found
in the discs of SS Cyg, EX Dra and U Gem.Comment: 16 pages, 14 figures. To be published in MNRA
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