Curious Variables Experiment (CURVE). IX Dra - a Clue for Understanding Evolution of Cataclysmic Variable Stars

摘要

We report extensive photometry of frequently outbursting dwarf nova IX Dra. During five months of observations the star went into three superoutbursts and seven ordinary outbursts. This allowed us to determine its supcrcycle and cycle lengths as equal to 54±1 and 3.1±0.1 days, respectively. During the September 2003 superoutburst, which had the best observational coverage, IX Dra displayed clear superhumps with a period of P_(sh) = 0.066968(17) days (96.43±0.02 min). This period was constant during the whole superoutburst. Another period, which was clearly present in the light curve of IX Dra in superoutburst, had a value of 0.06646(6) days (95.70±0.09 min) and we interpret it as the orbital period of the binary. Thus IX Dra is the first SU UMa star showing orbital modulation during the entire superoutburst. The beat between these two periods is the main cause of an unusual phase reversal of superhumps - a phenomenon which was previously observed in ER UMa. If our interpretation of the second periodicity is correct, IX Dra has an extremely low period excess E equal to only 0.76% ± 0.03%. This implies very low mass ratio q = 0.035 ± 0.003, which strongly suggests that the system contains a brown dwarf-like degenerate secondary of mass ≈ 0.03M_⊙ and that IX Dra is the most evolved dwarf nova known. Such a very low mass ratio results in the outer edge of the accretion disk reaching 80% of the distance between the components of the system. In turn, this allows the disk particles to enter a 2:1 resonance and leads to the appearance of the orbital period in the light curve of the entire superoutburst. The high level of activity and brightness of IX Dra indicate that very old cataclysmic variables go through episodes of increased activity leading to loss of angular momentum through mass loss from the system. Modulations with the orbital period are also detectable during normal outbursts and in quiescence.