Roche tomography of cataclysmic variables - V. A high-latitude starspot on RU Pegasi

Abstract

We present Roche tomograms of the secondary star in the dwarf nova system RU Pegasi derived from blue‐ and red‐arm ISIS data taken on the 4.2‐m William Herschel Telescope. We have applied the entropy landscape technique to determine the system parameters and obtained component masses of M1= 1.06 M⊙, M2= 0.96 M⊙, an orbital inclination angle of i= 43° and an optimal systemic velocity of γ= 7 km s−1. These are in good agreement with previously published values. Our Roche tomograms of the secondary star show prominent irradiation of the inner Lagrangian point due to illumination by the disc and/or bright spot, which may have been enhanced as RU Peg was in outburst at the time of our observations. We find that this irradiation pattern is axisymmetric and confined to regions of the star which have a direct view of the accretion regions. This is in contrast to previous attempts to map RU Peg which suggested that the irradiation pattern was non‐symmetric and extended beyond the terminator. We also detect additional inhomogeneities in the surface distribution of stellar atomic absorption that we ascribe to the presence of a large starspot. This spot is centred at a latitude of ∼82° and covers approximately 4 per cent of the total surface area of the secondary. In keeping with the high‐latitude spots mapped on the cataclysmic variables (CVs) AE Aqr and BV Cen, the spot on RU Peg also appears slightly shifted towards the trailing hemisphere of the star. Finally, we speculate that early mapping attempts which indicated non‐symmetric irradiation patterns which extended beyond the terminator of CV donors could possibly be explained by a superposition of symmetric heating and a large spot.

Roche tomography of cataclysmic variables – VIII. The irradiated and spotted dwarf nova, SS Cygni

“…However, given that these features are reproduced independently in both nights' maps, and given that features with a similar morphology have also been found in AE Aqr (where there was very dense phase sampling; Hill et al 2016), we interpret these as groups of starspots that have been smeared together in the reconstruction. Most prominently on the leading-inner hemisphere of SS Cyg, we find a large region affected by irradiation, as is also prominently seen in IP Peg (Watson et al 2003) and RU Peg (Dunford et al 2012). This brightness asymmetry (stemming from the ionization of atoms, reducing absorption line depth) is clearly seen in Figure 10, where the fractional spot coverage peaks around 170 • (i.e.…”

The activity of the dwarf nova RU Pegasi with rapidly changing outburst types

“…The secondary (donor) of RU Peg is highly magnetically active, similarly as in CVs BV Cen and AE Aqr. The Roche tomograms show a near-polar spot on this component (Dunford et al 2012). These tomograms of RU Peg also show prominent irradiation of the secondary star's hemisphere confined to regions that directly view the accretion regions near the peak magnitude of outburst (Dunford et al 2012).…”

“…This disc overflow, varying with time, could also contribute to the change of τ R and P. Doppler tomography of RU Peg in one outburst (Dunford et al 2012) shows a highly asymmetric extension of the width of the mass stream. Such tomograms obtained in various outbursts and quiescences could help assess how and at which places the collision of the inflowing matter and the disc occurs at various times.…”

“…It is also possible to assess the values of r s of the future events by comparing models of relative delays between the optical and ultraviolet outburst light curves because outbursts close to the A-type have the UV delay considerably more prominent than the B-types (Smak 1998). Also, the changes in Doppler tomograms can shed more light; such a tomogram exists only for one outburst (Dunford et al 2012). Although the investigation of the evolution of the orbital modulation during the start of various outbursts of RU Peg might pinpoint the location of the trigger of the instability, i ≈ 33 • (Friend et al 1990) indicates its low amplitude and the absence of eclipses.…”

Roche tomography of CVs

“…Type Orbital period (h) V magnitude References AE Aquarii NL 9.88 10.7 [7], [8], [9], [4] BV Centauri DN, UG 14.7 12.6 [9], [10] V426 Ophiuchi DN, ZCam, IP? 6.85 13.5 [9], [11] RU Pegasi DN, UG 8.99 12.5 [12] SS Cygni DN, UG 6.6 11.2 Table 1: Systems with LSD maps published or being analysed…”