On Echo Outbursts and ER UMa Supercycles in SU UMa-type Cataclysmic Variables

I present a variation on Osaki's tidal-thermal-instability model for SU UMa behavior. I suggest that in systems with the lowest mass ratios, the angular-momentum dissipation in an eccentric disk is unable to sustain the disk on the hot side of the thermal instability. This decoupling of the tidal and thermal instabilities in systems with q < 0.07 allows a better explanation of the `echo' outbursts of EG Cnc and the short supercycles of RZ LMi and DI UMa. The idea might also apply to the soft X-ray transients.Comment: To appear in PASP, April 2001 (6 pages, 4 figs

Curious Variables Experiment (CURVE). CCD photometry of active dwarf nova DI UMa

We report an analysis of photometric behaviour of DI UMa, an extremely active dwarf nova. The observational campaign (completed in 2007) covers five superoutbursts and four normal outbursts. We examined principal parameters of the system to understand peculiarities of DI UMa, and other active cataclysmic variables. Based on precise photometric measurements, temporal light curve behaviour, O-C analysis, and power spectrum analysis, we investigated physical parameters of the system. We found that the period of the supercycle now equals 31.45 +/-0.3 days. Observations during superoutbursts infer that the period of superhumps equals P_sh = 0.055318(11) days (79.66 +/- 0.02 min). During quiescence, the light curve reveals a modulation of period P_orb = 0.054579(6) days (78.59 +/- 0.01 min), which we interpret as the orbital period of the binary system. The values obtained allowed us to determine a fractional period excess of 1.35% +/- 0.02%, which is surprisingly small compared to the usual value for dwarf novae (2%-5%). A detailed O-C analysis was performed for two superoutbursts with the most comprehensive coverage. In both cases, we detected an increase in the superhump period with a mean rate of dot_P/P_sh = 4.4(1.0)*10^{-5}. Based on these measurements, we confirm that DI UMa is probably a period bouncer, an old system that reached its period minimum a long time ago, has a secondary that became a degenerate brown dwarf, the entire system evolving now toward longer periods. DI UMa is an extremely interesting object because we know only one more active ER UMa star with similar characteristics (IX Dra).Comment: Accepted for publication in Astronomy & Astrophysic

Hydrodynamic simulations of irradiated secondaries in dwarf novae

We investigate numerically the surface flow on the secondary star during outbursts. We use a simple model for the irradiation and the geometry of the secondary star: the irradiation temperature is treated as a free parameter and the secondary is replaced by a spherical star with a space-dependent Coriolis force that mimics the effect of the Roche geometry. The Euler equations are solved in spherical coordinates with the TVD-MacCormack scheme. We show that the Coriolis force leads to the formation of a circulation flow from high latitude region to the close vicinity of the $L_1$ point. However no heat can be efficiently transported to the $L_1$ region due to the rapid radiative cooling of the hot material as it enters the equatorial belt shadowed from irradiation. Under the assumption of hydrostatic equilibrium, the Coriolis force could lead to a moderate increase of the mass transfer rate by pushing the gas in the vertical direction at the $L_1$ point, but only during the initial phases of the outburst (about 15 -- 20 orbital periods). We conclude that the Coriolis force does not prevent a flow from the heated regions of the secondary towards the $L_1$ region, at least during the initial phase of an outburst, but the resulting increase of the mass transfer rate is moderate, and it is unlikely to be able to account for the duration of long outbursts.Comment: 11 pages, 11 figures, accepted for publication in A&

Permanent Superhumps in V1974 Cyg

We present results of 32 nights of CCD photometry of V1974 Cygni, from the years 1994 and 1995. We verify the presence of two distinct periodicities in the light curve: 0.0812585 day~1.95 hours and 0.0849767 d~2.04 hr. We establish that the shorter periodicity is the orbital period of the underlying binary system. The longer period oscillates with an average value of |dot(P)| ~ 3x10^(7)--typical to permanent superhumps. The two periods obey the linear relation between the orbital and superhump periods that holds among members of the SU Ursae Majoris class of dwarf novae. A third periodicity of 0.083204 d~2.00 hr appeared in 1994 but not in 1995. It may be related to the recently discovered anti-superhump phenomenon. These results suggest a linkage between the classical nova V1974 Cyg and the SU UMa stars, and indicate the existence of an accretion disk and permanent superhumps in the system no later than 30 months after the nova outburst. From the precessing disk model of the superhump phenomenon we estimate that the mass ratio in the binary system is between 2.2 and 3.6. Combined with previous results this implies a white dwarf mass of 0.75-1.07 M sun.Comment: 11 pages, 10 eps. figures, Latex, accepted for publication in MNRA

Superoutbursts, superhumps and the tidal-thermal instability model

We include the tidal instability due to the 3:1 resonance in the disc instability model developed by Hameury et al. (1998) and modified by Buat-Menard et al. (2001). We confirm earlier results by Osaki (1989) that the tidal instability can account for the SU UMa light curves. We show that in ultra-low mass ratio systems such as ER UMa stars and WZ Sge stars, the superoutburst ends while the disc is still eccentric, as proposed by Hellier (2001). However, since the disc shrinks rapidly once a cooling wave has started, the eccentricity should stop shortly after the end of a superoutburst. This result disagrees with the suggestion by Hellier that decoupling the thermal and tidal instability in the TTI model can account for late superhumps and echo outbursts in ultra-low mass ratio systems. We propose instead that ER UMa short supercycles can be explained either by the alternation of narrow and wide outbursts similar to those occurring in SS Cyg, or by the effects of irradiation (Hameury et al. 2000). In both cases, we predict that superhumps should be permanent, which is suggested by observations (Gao et al. 1999). We can also reproduce light curves similar to those of EG Cnc, varying the mass transfer rate in a TTI model including both irradiation and the presence of an inner hole in the disc.Comment: Astronomy and Astrophysics, submitted. 8 pages, 5 figure

The nature of dwarf nova outbursts

We show that if the dwarf-nova disc instability model includes the effects of heating by stream impact and tidal torque dissipation in the outer disc, the calculated properties of dwarf-nova outbursts change considerably, and several notorious deficiencies of this model are repaired. In particular: (1) outside-in outbursts occur for mass transfer rates lower than in the standard model as required by observations; (2) the presence of long (wide) and short (narrow) outbursts with similar peak luminosities is a natural property of the model. Mass-transfer fluctuations by factors ~ 2 can explain the occurrence of both long and short outbursts above the cataclysmic variable period gap, whereas below 2 hr only short normal outbursts are expected (in addition to superoutbursts which are not dealt with in this article). With additional heating by the stream and tidal torques, such fluctuations can also explain the occurrence of both outside-in and inside-out outbursts in SS Cyg and similar systems. The occurrence of outside-in outbursts in short orbital-period, low mass-transfer-rate systems requires the disc to be much smaller than the tidal-truncation radius. In this case the recurrence time of both inside-out and outside-in outbursts have a similar dependence on the mass-transfer rate.Comment: 11 pages, 8 figures Submitted to Astronomy and Astrophysic

The fractional Keller-Segel model

The Keller-Segel model is a system of partial differential equations modelling chemotactic aggregation in cellular systems. This model has blowing up solutions for large enough initial conditions in dimensions d >= 2, but all the solutions are regular in one dimension; a mathematical fact that crucially affects the patterns that can form in the biological system. One of the strongest assumptions of the Keller-Segel model is the diffusive character of the cellular motion, known to be false in many situations. We extend this model to such situations in which the cellular dispersal is better modelled by a fractional operator. We analyze this fractional Keller-Segel model and find that all solutions are again globally bounded in time in one dimension. This fact shows the robustness of the main biological conclusions obtained from the Keller-Segel model

SDSS J210014.12+004446.0: A New Dwarf Nova with Quiescent Superhumps?

We report follow-up observations of the Sloan Digital Sky Survey Cataclysmic Variable SDSS J210014.12+004446.0 (hereafter SDSS J2100). We obtained photometry and spectroscopy in both outburst and quiescent states, providing the first quiescent spectrum of this source. In both states, non-sinusoidal photometric modulations are apparent, suggestive of superhumps, placing SDSS J2100 in the SU UMa subclass of dwarf novae. However, the periods during outburst and quiescence differ significantly, being 2.099 plus or minus 0.002 hr and 1.96 plus or minus 0.02 hr respectively. Our phase-resolved spectroscopy during outburst yielded an estimate of about 2 hr for the orbital period, consistent with the photometry. The presence of the shorter period modulation at quiescence is unusual, but not unique. Another atypical feature is the relative weakness of the Balmer emission lines in quiescence. Overall, we find a close similarity between SDSS J2100 and the well-studied superhump cataclysmic Variable V503 Cygni. By analogy, we suggest that the quiescent modulation is due to a tilted accretion disk -- producing negative superhumps -- and the modulation in outburst is due to positive superhumps from the precession of an elliptical disk.Comment: 6 pages, 5 eps figures, accepted by PASP Dec. 16th, 200

High spectral resolution time-resolved optical spectroscopy of V893 Sco

We present high resolution time-resolved optical spectra of the high inclination short orbital period dwarf nova V893 Sco. We performed spectral analysis through radial velocity measurements, Doppler mapping, and ratioed Doppler maps. Our results indicate that V893 Sco's accretion disk is dissimilar to WZ Sge's accretion disk, and does not fit any of the current accretion disk models. We derive the system parameters M1 and i, and present evidence for V893 Sco as a very young cataclysmic variable and an ER UMa star. We advance the hypothesis that all ER UMa stars may be newly formed cataclysmic variables.Comment: 23 pages (total), 8 figures, accepted by Ap

Contact Discontinuities in Models of Contact Binaries Undergoing Thermal Relaxation Oscillations

In this paper we pursue the suggestion by Shu, Lubow & Anderson (1979) and Wang (1995) that contact discontinuity (DSC) may exist in the secondary in the expansion TRO (thermal relaxation oscillation) state. It is demonstrated that there is a mass exchange instability in some range of mass ratio for the two components. We show that the assumption of {\it constant} volume of the secondary should be relaxed in DSC model. For {\it all} mass ratio the secondary alway satisfies the condition that no mass flow returns to the primary through the inner Lagrangian point. The secondary will expand in order to equilibrate the interaction between the common convective envelope and the secondary. The contact discontinuity in contact binary undergoing thermal relaxation does not violate the second law of thermodynamics. The maintaining condition of contact discontinuity is derived in the time-dependent model. It is desired to improve the TRO model with the advanced contact discontinuity layer in future detailed calculations.Comment: 5 pages in emulateapj, 1 figur