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Register a new userThe stunted outbursts of UU Aquarii are likely mass-transfer events
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R. Baptista
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We report a time-lapse eclipse mapping analysis of B-band time-series of the nova-like variable UU Aqr along a typical stunted outburst in 2002 August. Disc asymmetries rotating in the prograde sense in the eclipse maps are interpreted as a precessing elliptical disc with enhanced emission at periastron. From the disc expansion velocity a disc viscosity alpha_{hot}= 0.2 is inferred. The outburst starts with a 10-fold increase in uneclipsed light, probably arising in an enhanced disc wind; the disc response is delayed by 2 d. The results are inconsistent with the disc instability model and suggest that the stunted outburst of UU Aqr are the response of its viscous accretion disc to enhanced mass-transfer events.
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Recent observations of two unusual Z Cam systems, V513 Cas and IW And have shown light curves that seem to contradict the disc-instability model for dwarf novae: outbursts are appearing during standstills of the system when according to the model, the disc is supposed to be in a hot quasi-equilibrium state. We investigate what additional physical processes need to be included in the model to reconcile it with observations of such anomalous Z Cam systems. We used our code for modeling thermal-viscous outbursts of the accretion discs and determined what types of mass-transfer variations reproduce the observed light curves. Outbursts of mass transfer (with a duration of a few days, with a short rise time and an exponential decay) from the stellar companion will account for the observed properties of V513 Cas and IW And, provided they are followed by a short but significant mass-transfer dip. The total mass involved in outbursts is of the order of 10$^{23}$g. We studied the possible origins of these mass transfer outbursts and showed that they most probably result from a giant flare near the secondary star surface, possibly due to the absence of star spots in the $L_1$ region.
We report the analysis of a uniform sample of 31 light curves of the nova-like variable UU Aqr with eclipse mapping techniques. The data were combined to derive eclipse maps of the average steady-light component, the long-term brightness changes, and low- and high-frequency flickering components. The long-term variability responsible for the low and high brightness states is explained in terms of the response of a viscous disk to changes of 20-50 per cent in the mass transfer rate from the donor star. Low- and high-frequency flickering maps are dominated by emission from two asymmetric arcs reminiscent of those seen in the outbursting dwarf nova IP Peg, and are similarly interpreted as manifestation of a tidally-induced spiral shock wave in the outer regions of a large accretion disk. The asymmetric arcs are also seen in the map of the steady-light aside of the broad brightness distribution of a roughly steady-state disk. The arcs account for 25 per cent of the steady-light flux and are a long-lasting feature in the accretion disk of UU Aqr. We infer an opening angle of 10+/-3 degrees for the spiral arcs. The results suggest that the flickering in UU Aqr is caused by turbulence generated after the collision of disk gas with the density-enhanced spiral wave in the accretion disk.
We report simultaneous multicolour observations in 5 bands (UBVRI) of the flickering variability of the cataclysmic variable AE Aqr. Our aim is to estimate the parameters (colours, temperature, size) of the fireballs that produce the optical flares. The observed rise time of the optical flares is in the interval 220 - 440 sec. We estimate the dereddened colours of the fireballs: (U-B)_0 in the range 0.8-1.4, (B-V)_0 ~ 0.03-0.24, (V-I)_0 ~ 0.26-0.78. We find for the fireballs a temperature in the range 10000 - 25000 K, mass (7-90).10^{19} g, size (3-7).10^9 cm (using a distance of d=86 pc). These values refer to the peak of the flares observed in UBVRI bands. The data are available upon request from the authors.
Carbon stars (with C/O> 1) were long assumed to all be giants, because only AGB stars dredge up significant carbon into their atmospheres. The case is nearly iron-clad now that the formerly mysterious dwarf carbon (dC) stars are actually far more common than C giants, and have accreted carbon-rich material from a former AGB companion, yielding a white dwarf and a dC star that has gained both significant mass and angular momentum. Some such dC systems have undergone a planetary nebula phase, and some may evolve to become CH, CEMP, or Ba giants. Recent studies indicate that most dCs are likely from older, metal-poor kinematic populations. Given the well-known anti-correlation of age and activity, dCs would not be expected to show significant X-ray emission related to coronal activity. However, accretion spin-up might be expected to rejuvenate magnetic dynamos in these post mass-transfer binary systems. We describe our Chandra pilot study of six dCs selected from the SDSS for Halpha emission and/or a hot white dwarf companion, to test whether their X-ray emission strength and spectral properties are consistent with a rejuvenated dynamo. We detect all 6 dCs in the sample, which have X-ray luminosities ranging from logLx= 28.5 - 29.7, preliminary evidence that dCs may be active at a level consistent with stars that have short rotation periods of several days or less. More definitive results require a sample of typical dCs with deeper X-ray observations to better constrain their plasma temperatures.
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