Determination of the superoutburst cycle lengths of 206 SU UMa-type dwarf novae

Abstract

ABSTRACT SU UMa stars are characterized by ‘superoutbursts’, which are brighter at maximum light and which last much longer than the more frequent ‘ordinary’ outbursts of these dwarf novae. Although there are now more than 1180 SU UMa-type dwarf novae catalogued, our knowledge on their superoutburst cycle length Cso has hitherto been limited to about 6 ${{\ \rm per\ cent}}$ of the entire sample of known SU UMa stars. Using public data bases we have determined new Cso values for a total of 206 additional SU UMa stars in the range 17 d < Cso < 4590 d (including some ER UMa and WZ Sge-type representants) within total time intervals between 2 and 57 yr, and with an estimated uncertainty of $\pm 11{{\ \rm per\ cent}}$. This way, we are increasing our present knowledge of Cso values by a factor of ∼3.8. Their distribution is characterized by a broad maximum around Cso ≈ 270 d, and slowly decreasing numbers up to Cso ≈ 800 d. The domain Cso > 450 d was unexplored until now; we add here 106 cases ($\sim \! 51{{\ \rm per\ cent}}$ of our total sample) in this range of long cycles, implying a better statistical basis for future studies of their distribution. Our sample contains 16 known WZ Sge stars, and we propose WZ Sge membership for five others hitherto classified as ordinary SU UMa stars. Individual superoutburst timings deviate on average by about $\pm 7{{\ \rm per\ cent}}$ of the cycle length from their overall linear ephemeris, confirming the pronounced quasi-periodic repeatability of superoutbursts. All relevant parameters are listed with their errors, and a table with individual superoutburst epochs of our targets is given, enabling future researchers to combine our results with other (past or future) observations.