Survey of period variations of superhumps in SU UMa-type dwarf novae. VII. The seventh year (2014–2015)

Abstract

Continuing the project described by Kato et al. (2009, PASJ, 61, S395), we collected times of superhump maxima for 102 SU UMa-type dwarf novae, observed mainly during the 2014-2015 season, and characterized these objects. Our project has greatly improved the statistics of the distribution of orbital periods, which is a good approximation of the distribution of cataclysmic variables at the terminal evolutionary stage, and has confirmed the presence of a period minimum at a period of 0.053 d and a period spike just above this period. The number density monotonically decreased toward the longer period and there was no strong indication of a period gap. We detected possible negative superhumps in Z Cha. It is possible that normal outbursts are also suppressed by the presence of a disk tilt in this system. There was no indication of enhanced orbital humps just preceding the superoutburst, and this result favors the thermal-tidal disk instability as the origin of superoutbursts. We detected superhumps in three AM CVn-type dwarf novae. Our observations and recent other detections suggest that 8% of objects showing dwarf nova-type outbursts are AM CVn-type objects. AM CVn-type objects and EI Psc-type objects may be more abundant than previously recognized. OT J213806, a WZ Sge-type object, exhibited remarkably different features between the 2010 and 2014 superoutbursts. Although the 2014 superoutburst was much fainter, the plateau phase was shorter than the 2010 one, and the course of the rebrightening phase was similar. This object indicates that the O − C diagrams of superhumps can indeed be variable, at least in WZ Sge-type objects. Four deeply eclipsing SU UMa-type dwarf novae (ASASSN-13cx, ASASSN-14ag, ASASSN15bu, and NSV 4618) were identified. We studied long-term trends in supercycles in MM Hya and CY UMa and found systematic variations of supercycles of ∼20%.

The eclipsing accreting white dwarf Z chameleontis as seen with TESS

“…A positive superhump has previously been reported in Z Cha by Kato et al (2015) during a 2014 superoutburst of the source. They calculated the mean period of the superhump as 0.07736(8) d during the dominant evolutionary Stage B (see Kato et al 2009).…”

“…QZ Vir and IRXS J0532, Kato et al 2009), but the physical parameters which determine whether Stage A superhumps will occur in a given superoutburst remain unclear. Notably, a previous superoutburst of Z Cha in 2014 did show Stage A superhumps (Kato et al 2015), indicating that the absence of Type A superhumps is a property related to individual superoutbursts rather than to the system. Additionally, using our estimate of the superhump period at its onset, it is possible to obtain an independent estimate for the mass ratio q of the components of the Z Cha system, where q is the mass ratio defined as:…”

“…QZ Vir and IRXS J0532, Kato et al 2009), but the physical parameters which determine whether Stage A superhumps will occur in a given superoutburst remain unclear. Notably, a previous superoutburst of Z Cha in 2014 did show Stage A superhumps (Kato et al 2015), indicating that the absence of Type A superhumps is a property related to individual superoutbursts rather than to the system.…”

“…Previous studies of the superhump period in superoutbursts of Z Cha (e.g. Kato et al 2015) have been unable to determine the period derivative during Stage B using O-C fitting techniques. As such our measurement calculated using the flux-phase diagram is, as far as we are aware, the first superhump period derivative calculated for Z Cha.…”

A Volume-limited Sample of Cataclysmic Variables from Gaia DR2: Space Density and Population Properties

“…While this is roughly in agreement with the accretion rates estimated from observations (Townsley & Gänsicke 2009;Pala et al 2017), the theoretical framework outlined above fails to re-produce a number of observational properties of the Galactic population of CVs: (i) the predicted fractions of CVs above and below the period gap ( 1 per cent and 99 per cent, respectively, de Kool 1992; Kolb 1993;Howell et al 2001) are in clear disagreement with the observations (e.g. 23 per cent and 77 per cent, Gänsicke et al 2009, though the observed samples are typically magnitude-limited, and hence biased towards more luminous CVs); (ii) period bouncers are expected to be the main component ( 40 − 70 per cent) of the present-day Galactic CV population but only a small number of such systems has been identified (Patterson et al 2005;Unda-Sanzana et al 2008;Littlefair et al 2006;Patterson 2011;Kato et al 2015Kato et al , 2016McAllister et al 2017;Neustroev et al 2017;Pala et al 2018); (iii) there are clues of the presence of additional AML mechanisms that are not accounted for by the standard model of CV evolution (Patterson 1998; Knigge et al 2011;Schreiber et al 2016;Pala et al 2017;Zorotovic & Schreiber 2017;Belloni et al 2018;Liu & Li 2019), although the physical origin of this enhanced AML is still unclear.…”