4,531 research outputs found
Thermal stability and nova cycles in permanent superhump systems
Archival data on permanent superhump systems are compiled to test the thermal
stability of their accretion discs. We find that their discs are almost
certainly thermally stable as expected. This result confirms Osaki's suggestion
(1996) that permanent superhump systems form a new subclass of cataclysmic
variables (CVs), with relatively short orbital periods and high mass transfer
rates. We note that if the high accretion rates estimated in permanent
superhump systems represent their mean secular values, then their mass transfer
rates cannot be explained by gravitational radiation, therefore, either
magnetic braking should be extrapolated to systems below the period gap or they
must have mass transfer cycles. Alternatively, a new mechanism that removes
angular momentum from CVs below the gap should be invoked.
We suggest applying the nova cycle scenarios offered for systems above the
period gap to the short orbital period CVs. Permanent superhumps have been
observed in the two non-magnetic ex-novae with binary periods below the gap.
Their post-nova magnitudes are brighter than their pre-outburst values. In one
case (V1974 Cyg) it has been demonstrated that the pre-nova should have been a
regular SU UMa system. Thus it is the first nova whose accretion disc was
observed to change its thermal stability. If the superhumps in this system
indicate persistent high mass transfer rates rather than a temporary change
induced by irradiation from the hot post-nova white dwarf, it is the first
direct evidence for mass transfer cycles in CVs. The proposed cycles are driven
by the nova eruption.Comment: 7 pages, 2 eps. figures, Latex, accepted for publication in MNRA
Does TV Col Have the longest Recorded Positive Superhumps?
Re-examination of extensive photometric data of TV Col reveals evidence for a
permanent positive superhump. Its period (6.4 h) is 16 percent longer than the
orbital period and obeys the well known relation between superhump period
excess and binary period. At 5.5-h, TV Col has an orbital period longer than
any known superhumping cataclysmic variable and, therefore, a mass ratio which
might be outside the range at which superhumps can occur according to the
current theory. We suggest several solutions for this problem.Comment: 5 pages, 2 eps. figures, Latex, proceedings of `Evolution of Binary
and Multiple Star Systems', a Meeting in Celebration of Peter Eggleton's 60th
Birthday, Bormio, Italy, ASP Conference Series, eds. Ph. Podsiadlowski et
al., ASP, San Francisc
Low Mass Stars and Brown Dwarfs around Sigma Orionis
We present optical spectroscopy of 71 photometric candidate low-mass members
of the cluster associated with Sigma Orionis. Thirty-five of these are found to
pass the lithium test and hence are confirmed as true cluster members, covering
a mass range of <0.055-0.3M_{sun}, assuming a mean cluster age of <5 Myr. We
find evidence for an age spread on the (I, I-J) colour magnitude diagram,
members appearing to lie in the range 1-7 Myr. There are, however, a
significant fraction of candidates that are non-members, including some
previously identified as members based on photometry alone. We see some
evidence that the ratio of spectroscopically confirmed members to photometric
candidates decreases with brightness and mass. This highlights the importance
of spectroscopy in determining the true initial mass-function.Comment: To appear in the 12th Cambridge Workshop on Cool Stars Stellar
Systems and the Su
Alternatives to Hibernation
I outline the evidence pertinent to the connection between the nova explosion
and mass transfer rates in CVs. I conclude that there is still insufficient
evidence to decide whether or not such a connection exists.Comment: To appear in the proceedings of the Sitges nova conferenc
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