Study of negative and positive superhumps in ER Ursae Majoris

Abstract

We carried out photometric observations of the SU UMa-type dwarf nova ER UMa during 2011 and 2012, which showed the existence of persistent negative superhumps even during the superoutburst. We performed a two-dimensional period analysis of its light curves by using a method called “least absolute shrinkage and selection operator” (Lasso) and the “phase dispersion minimization” (PDM) analysis, and found that the period of negative superhumps systematically changed between a superoutburst and the next superoutburst. The trend of the period change can be interpreted as a reflection of the change of the disk radius. This change is in agreement with the one predicted by the thermal tidal instability model. The normal outburst during a supercycle showed a general trend that the rising rate to its maximum becomes slower as the next superoutburst is approaching. The change can be interpreted as the consequence of the increased gas-stream flow into the inner region of the disk as a result of the tilted disk. Some of superoutbursts were found to be triggered by a precursor normal outburst when the positive superhump appeared to develop. The positive and negative superhumps coexisted during the superoutburst. Positive superhumps were prominent only for four or five days after the supermaximum, while the signal of negative superhumps became stronger after the middle phase of the superoutburst plateau. A simple combination of the positive and negative superhumps was found to be insufficient for reproduction of the complex profile variation. We were able to detect the developing phase of positive superhumps (stage A superhumps) for the first time in ER UMa-type dwarf novae. Using the period of stage A superhumps, we obtained a mass ratio of 0.100(15), which indicates that ER UMa is on the ordinary evolutional track of cataclysmic variable stars.

On the colour variations of negative superhumps

“…A part of R c light curve of ER UMa in our campaign. The names of the outbursts are taken from those designated in Ohshima et al (2014). We named each quiescence (abbreviated as Q), normal outburst (abbreviated as N), and superoutburst (abbreviated as S) as described here.…”

“…The errors of the PDM method were estimated using the Lafter-Kinmann class method developed by Fernie (1989) and Kato et al (2010). Also, a hint of negative superhumps was observed near the end of S2, as pointed out by Ohshima et al (2014). Figure 2 shows the phase-averaged R c light curve and g ′ − I c colour variations, folded with the periods obtained above.…”

“…Figure 1 shows the representative R c light curve of our observations, in which two superoutbursts and six normal outbursts are visible. The names of each outburst use the same description as those in Ohshima et al (2014). Here we define the names of quiescence as those described in their figure.…”

“…In figure 3, we also present phase-averaged R c and g ′ − I c colour variations of positive superhumps observed on BJD 2455627.92−28.12 folded with 0.06592(5) d, corresponding to the strongest signal in R c band during the stage B of S2 using the PDM method. 3 Ohshima et al (2014) reported that positive and negative superhumps coexisted during S2 except for the early stage of the superoutburst. However, we were unable to detect a hint of negative superhumps on BJD 2455627.92−28.12, possibly due to the short coverage of our data and a much weaker signal of negative superhumps than positive ones.…”

“…ER UMa-type dwarf novae are a subclass of SU UMatype dwarf novae and their main characteristics are that their supercycle is unusually short, typically less than 50 d (Kato, Kunjaya 1995). ER UMa-type dwarf novae are also known as the systems exhibiting negative superhumps in their light curves (Patterson et al 1995;Ohshima et al 2014), and are considered to be a result of retrograde precession of a tilted disk (Wood, Burke 2007). During appearance of negative superhumps, the importance of gas stream overflow was discussed by Barrett et al (1988), in which they suggested that the physical source of the negative superhump period was the sweeping of the accretion stream hot spot across the face of a tilted disk.…”