Secular Variation in the Interval of Outbursts in Z Cam-type Dwarf Novae

The secular variation in the interval of outbursts in the following six Z Cam-type dwarf novae (including the subtype IW And-type) is investigated: Z Cam, RX And, AH Her, HL CMa, SY Cnc, and WW Cet. An analysis using the $O-C$ diagram shows that the interval of outbursts is not steady in one system. The outburst properties before standstill are the decrease in outburst interval, enhancement of the magnitude in quiescence, and disappearance of the long outburst. Meanwhile, several objects have at least two typical intervals of outbursts. These characteristics are difficult to be explained only by the variation in mass transfer from the secondary.Comment: 15 pages, 12 figures,accepted to Stars and Galaxies No.5 id.

Discovery of Negative Superhumps during a Superoutburst of January 2011 in ER Ursae Majoris

We report on a discovery of "negative" superhumps during the 2011 January superoutburst of ER UMa. During the superoutburst which started on 2011 January 16, we detected negative superhumps having a period of 0.062242(9) d, shorter than the orbital period by 2.2%. No evidence of positive superhumps was detected during this observation. This finding indicates that the disk exhibited retrograde precession during this superoutburst, contrary to all other known cases of superoutbursts. The duration of this superoutburst was shorter than those of ordinary superoutbursts and the intervals of normal outbursts were longer than ordinary ones. We suggest a possibility that such unusual outburst properties are likely a result of the disk tilt, which is supposed to be a cause of negative superhumps: the tilted disk could prevent the disk from being filled with materials in the outmost region which is supposed to be responsible for long-duration superoutbursts in ER UMa-type dwarf novae. The discovery signifies the importance of the classical prograde precession in sustaining long-duration superoutbursts. Furthermore, the presence of pronounced negative superhumps in this system with a high mass-transfer rate favors the hypothesis that hydrodynamical lift is the cause of the disk tilt.Comment: 8 pages, 3 figures, Accepted for publication in PASJ Lette

Discovery of a Long-duration Superflare on a Young Solar-type Star EK Draconis with Nearly Similar Time Evolution for H alpha and White-light Emissions

Young solar-type stars are known to show frequent "superflares, " which may severely influence the habitable worlds on young planets via intense radiation and coronal mass ejections. Here we report an optical spectroscopic and photometric observation of a long-duration superflare on the young solar-type star EK Draconis (50-120 Myr age) with the Seimei telescope and Transiting Exoplanet Survey Satellite. The flare energy 2.6 x 10³⁴ erg and white-light flare duration 2.2 hr are much larger than those of the largest solar flares, and this is the largest superflare on a solar-type star ever detected by optical spectroscopy. The H alpha emission profile shows no significant line asymmetry, meaning no signature of a filament eruption, unlike the only previous detection of a superflare on this star. Also, it did not show significant line broadening, indicating that the nonthermal heating at the flare footpoints is not essential or that the footpoints are behind the limb. The time evolution and duration of the H alpha flare are surprisingly almost the same as those of the white-light flare, which is different from general M-dwarf (super-)flares and solar flares. This unexpected time evolution may suggest that different radiation mechanisms than general solar flares are predominant, such as: (1) radiation from (off-limb) flare loops and (2) re-radiation via radiative back-warming, in both of which the cooling timescales of flare loops could determine the timescales of H alpha and white light