PASJ: Publ. Astron. Soc. Japan 50, 333-342 (1998)

The of the SU Ursae Majoris Star EK Trianguli Australis and Evidence for Ring-Like Accretion Disks in Long-Supercycle Length SU Ursae Majoris Stars

Ronald E. MENNICKENT* and Jose ARENAS Departmento de Fisica, Facultad de Ciencias, Fisicas y Matemdticas, Universidad de Conception, Casilla 4009, Conception, Chile E-mail(RM): rmennick@stars. cfm. udec. cl Downloaded from https://academic.oup.com/pasj/article/50/3/333/2949021 by guest on 01 October 2021 (Received 1997 November 26; accepted 1998 April 27) Abstract An orbital period of 0.06288(5) d has been found from a study of the Ha emission line. In addition, we have detected an extra line emitting source located « 80° apart from the vector joining the secondary-primary centers, as measured in the opposite sense to the binary rotational motion. This is not the expected location for the hotspot in dwarf novae. This anomaly could be removed by assuming a line emission lagging behind the binary motion. In addition, we have estimated line emissivity a (oc r~ ) and disk radius (R = rin/rout) for 8 SU UMa stars. Most stars fit a = 1.8 ± 0.1 but AK Cnc and WZ Sge strongly deviate from the mean; their emission line shapes can be explained assuming a post- outburst mostly emitting close to the white dwarf (AK Cnc) and a ring-like disk (WZ Sge). In addition, we have found a tendency of long-super cycle length SU UMa stars to show very compact (large R] probably ring-like) accretion disks. If the supercycle length were basically controlled by the mass transfer rate (M), the inner disk radius would be a function of M. A white dwarf magnetic field ~ 5000 G is required to fit the truncation radius with the magnetosphere radius of SU UMa stars. Key words: Accretion — Evolution — Stars: cataclysmic — Stars: dwarf novae — Stars: variable — Stars: individual (WX Ceti, AK Cncri, AQ Eridani, RZ Leonis, TU Mensae, EK TrianguU Australis, CU Velorum, WZ Sagittae)

1. Introduction bursts recorded by Bateson through 20- observations, the basic cycle length seems to be larger than 120 days. Vogt and Semeniuk (1980) established the SU UMa na- The quoted range of visual magnitudes for EK TrA is ture of this finding with period 10.4-16.6: (Downes, Shara 1993). Ps = 0.0649(1) d on the 1979 June superoutburst (for EK TrA is listed in Ritter's catalogue (Ritter, Kolb a review of SU UMa stars see Warner 1995a,b). Large 1993) witn an orbital period PQ = 0.0636 d. However, amplitude superhumps were later observed by Hassall this is an estimate based on the measured (1985) being interpreted as probably linked to the disk's period. No weU established value for the orbital period, hotspot. She also reported a changing line spectrum as weu ^ for the stellar masses, has been given up to going from absorption to emission during decline and now. a P Cyg feature in the ClV (1550 A) line. An IUE post-outburst spectrum revealed a white-dwarf flux con- 0 ~, ,. , ^ , ^ , ,. ., r r.rw , m -™™ ^r /^.. . , 2. Observations and Data Reduction tribution of ~ 25% and a Twd « 18000 K (Gansicke et al. 1997). These authors also derived a distance of We observed EK TrA during three observing runs at « 180 pc. The star undergoes intervals of comparative Las Campanas Observatory, Chile. On 1991 August 3, inactivity, and there is some evidence indicating transient a low-resolution spectrum was obtained with the 2.5 m fluctuations around an intermediate magnitude, resem- Du pont telescope using the Modular Spectrograph and bling Z Cam variables (Bateson 1976). This could indi- the CRAF CCD chip (1024 pixels square,