A solution to the transition phase in classical novae

One century after the discovery of quasi-periodic oscillations in the optical light curve of Nova GK Per 1901 the cause of the transition phase in a certain part of the nova population is still unknown. Three years ago we suggested a solution for this problem and proposed a possible connection between the transition phase and intermediate polars (IPs). About 10 percent of the cataclysmic variable population are classified as IPs, which is consistent with the rarity (about 15 percent) of the transition phase in novae. Recent observations of three novae seem to support our prediction. The connection is explained as follows: The nova outburst disrupts the accretion disc only in IPs. The recovery of the disc, a few weeks-months after the eruption, causes strong winds that block the radiation from the white dwarf, thus dust is not destroyed. If the winds are very strong as is probably the case in DQ Her (perhaps since its spin period is very short) this leads to a dust minimum.Comment: 6 pages, 1 Latex file, 2 eps figures, 1 .sty file. To appear in the Proceedings of `Classical Nova Explosions', American Inst. of Physics, eds. M. Hernanz and J. Jos

A 6.4-hr positive superhump period in TV Col

Re-examination of photometric data of TV Col (Hellier, 1993) reveals positive superhumps in addition to the negative superhumps previously known. The superhump period is 0.265+/-0.005 day - about 16 percent longer than the orbital period - which obeys the relation between superhump-period excess and orbital period (Stolz and Schoembs 1984). As a confirmed permanent superhumper, the accretion disc of TV Col is naturally thermally stable. Therefore, our result supports the idea of Hellier and Buckley (1993) that the short-term outbursts seen in its light curve are mass transfer events rather than thermal instabilities in the disc. At 5.5-hr, TV Col has a longer orbital period than any known superhumper, and thus a mass ratio which is probably outside the range at which superhumps can occur according to current theory.Comment: 2 pages, Latex file, 2 .eps files. to appear in "Cataclysmic Variables: a 60th Birthday Symposium in Honour of Brian Warner", held in Oxforf, 12-16/4/99, eds. P. Charles, A. King, D. O'Donoghue, New Astronomy Reviews. also available at: ftp://ftp.astro.keele.ac.uk/pub/preprints/Oxford.htm

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

Predicting the Future of Superhumps in Classical Nova Systems

Oscillations observed in the light curve of Nova V1974 Cygni 1992 since summer 1994 have been interpreted as permanent superhumps. From simple calculations based on the Tidal-Disk Instability model of Osaki, and assuming that the accretion disc is the dominant optical source in the binary system, we predict that the nova will evolve to become an SU UMa system as its brightness declines from its present luminosity by another 2-3 magnitudes. Linear extrapolation of its current rate of fading (in magnitude units) puts the time of this phase transition within the next 2-4 years. Alternatively, the brightness decline will stop before the nova reaches that level, and the system will continue to show permanent superhumps in its light curve. It will then be similar to two other old novae, V603 Aql and CP Pup, that still display the permanent superhumps phenomenon 79 and 55 years, respectively, after their eruptions. We suggest that non-magnetic novae with short orbital periods could be progenitors of permanent superhump systems.Comment: 5 pages, 2 eps. figures, Latex, accepted for publication in MNRA

An Irradiation Effect in Nova DN Gem 1912 and the Significance of the Period Gap for Classical Novae

Continuous CCD photometry of the classical nova DN Gem during 52 nights in the years 1992-98 reveals a modulation with a period 0.127844 d. The semi-amplitude is about 0.03 mag. The stability of the variation suggests that it is the orbital period of the binary system. This interpretation makes DN Gem the fourth nova inside the cataclysmic variable (CV) period gap, as defined by Diaz and Bruch (1997), and it bolsters the idea that there is no period gap for classical novae. However, the number of known nova periods is still too small to establish this idea statistically. We eliminate several possible mechanisms for the variation, and propose that the modulation is driven by an irradiation effect. We find that model light curves of an irradiated secondary star, fit the data well. The inclination angle of the system is restricted by this model to 10 deg < i < 65 deg. We also refine a previous estimate of the distance to the binary system, and find d=1.6+/-0.6 kpc.Comment: 7 pages, Latex file, 2 .ps files and 3 .eps files. accepted for publication in MNRAS. also available at: ftp://ftp.astro.keele.ac.uk/pub/preprints/preprints.htm

Interferometric Observations of V838 Monocerotis

We have used long-baseline near-IR interferometry to resolve the peculiar eruptive variable V838 Mon and to provide the first direct measurement of its angular size. Assuming a uniform disk model for the emission we derive an apparent angular diameter at the time of observations (November-December 2004) of $1.83 \pm 0.06$ milli-arcseconds. For a nominal distance of $8\pm2$ kpc, this implies a linear radius of $1570 \pm 400 R_{\odot}$. However, the data are somewhat better fit by elliptical disk or binary component models, and we suggest that the emission may be strongly affected by ejecta from the outburst.Comment: 12 pages, 1 two-part encapsulated postscript figure. Accepted by ApJL. Added a table of observation