22 research outputs found
The first confirmed superoutburst of the dwarf nova GALEX J215818.5+241924
In 2011 October an optical transient was reported in Pegasus as a possible
nova. The object had an ultraviolet counterpart, GALEX J215818.5+241924. In
this paper we present follow-up photometry of the object which revealed the
presence of superhumps, with peak-to-peak amplitude of up to 0.22 magnitudes,
diagnostic of it being a member of the SU UMa family of dwarf novae. The
outburst amplitude was 4.6 magnitudes and it lasted at least 10 days, with a
maximum brightness of magnitude 14.3. We determined the mean superhump period
from our first 5 nights of observations as Psh = 0.06728(21) d. However
analysis of the O-C residuals showed a dramatic evolution in Psh during the
outburst. During the first part of the plateau phase the period increased with
dPsh/dt = +2.67(15) x 10-4. There was then an abrupt change following which the
period decreased with dPsh/dt = -2.08(9)x 10-4. We found a signal in the power
spectrum of the photometry which we tentatively interpret as the orbital signal
with Porb = 0.06606(35) d. Thus the superhump period excess was epsilon =
0.020(8), such value being consistent with other SU UMa systems of similar
orbital period.Comment: Accepted for publication in the Journal of the British Astronomical
Association. 10 pages, 3 figures. arXiv admin note: text overlap with
arXiv:1005.5378. Have corrected outburst amplitude, reworded the first 2
sentences of the Abstract for clarity and solved some typo
BK Lyncis: The Oldest Old Nova?... And a Bellwether for Cataclysmic-Variable Evolution
We summarize the results of a 20-year campaign to study the light curves of
BK Lyncis, a nova-like star strangely located below the 2-3 hour orbital period
gap in the family of cataclysmic variables. Two apparent "superhumps" dominate
the nightly light curves - with periods 4.6% longer, and 3.0% shorter, than
P_orb. The first appears to be associated with the star's brighter states
(V~14), while the second appears to be present throughout and becomes very
dominant in the low state (V~15.7).
Starting in the year 2005, the star's light curve became indistinguishable
from that of a dwarf nova - in particular, that of the ER UMa subclass.
Reviewing all the star's oddities, we speculate: (a) BK Lyn is the remnant of
the probable nova on 30 December 101, and (b) it has been fading ever since,
but has taken ~2000 years for the accretion rate to drop sufficiently to permit
dwarf-nova eruptions. If such behavior is common, it can explain other puzzles
of CV evolution. One: why the ER UMa class even exists (because all members can
be remnants of recent novae). Two: why ER UMa stars and short-period novalikes
are rare (because their lifetimes, which are essentially cooling times, are
short). Three: why short-period novae all decline to luminosity states far
above their true quiescence (because they're just getting started in their
postnova cooling). Four: why the orbital periods, accretion rates, and
white-dwarf temperatures of short-period CVs are somewhat too large to arise
purely from the effects of gravitational radiation (because the unexpectedly
long interval of enhanced postnova brightness boosts the mean mass-transfer
rate). These are substantial rewards in return for one investment of
hypothesis: that the second parameter in CV evolution, besides P_orb, is time
since the last classical-nova eruption.Comment: PDF, 46 pages, 4 tables, 10 figures; in preparation; more info at
http://cbastro.org
IM Normae: The Death Spiral of a Cataclysmic Variable?
We present a study of the orbital light curves of the recurrent nova IM
Normae since its 2002 outburst. The broad "eclipses" recur with a 2.46 hour
period, which increases on a timescale of 1.28(16)x10^6 years. Under the
assumption of conservative mass-transfer, this suggests a rate near 10^-7
M_sol/year, and this agrees with the estimated /accretion/ rate of the
postnova, based on our estimate of luminosity. IM Nor appears to be a close
match to the famous recurrent nova T Pyxidis. Both stars appear to have very
high accretion rates, sufficient to drive the recurrent-nova events. Both have
quiescent light curves which suggest strong heating of the low-mass secondary,
and very wide orbital minima which suggest obscuration of a large "corona"
around the primary. And both have very rapid orbital period increases, as
expected from a short-period binary with high mass transfer from the low-mass
component. These two stars may represent a final stage of nova -- and
cataclysmic-variable -- evolution, in which irradiation-driven winds drive a
high rate of mass transfer, thereby evaporating the donor star in a paroxysm of
nova outbursts.Comment: PDF, 30 pages, 3 tables, 6 figures; accepted, in press, ApJ; more
info at http://cbastro.org