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

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The Orbital Period of the SU Ursae Majoris Star EK Trianguli Australis and Evidence for Ring-Like Accretion Disks in Long-Supercycle Length SU Ursae Majoris Stars PASJ: Publ. Astron. Soc. Japan 50, 333-342 (1998) The Orbital Period 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 radial velocity 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 white dwarf 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 accretion disk 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-year 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 dwarf nova finding superhumps 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 superhump (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, 12 /mi width) 1 cate that EK TrA resembles in some aspects WW Cet, a and a grating of 300 lines mm" tilted by 22°. A pro­ long period-peculiar-dwarf nova wandering in quiescence jected slit width of 1" gave a wavelength range from by well over 1 mag (Ringwald et al. 1996). From the out- 4340-9OOO A and a resolution * 10 A. Two nights later, * Visiting Astronomer at Las Campanas Observatory. we obtained 12 spectra at higher resolution; this time we © Astronomical Society of Japan • Provided by the NASA Astrophysics Data System R.E. Mennickent and J. Arenas [Vol. 50y used grating 821 tilted an angle of 34° and a slit width of Table 1. Journal of spectroscopic (above) and 0."7, yielding a resolution of 3 A, and useful wavelength photometric (below) observations. range from 6400-8800 A. In addition, 66 spectra were taken on 1995 March 20-24, with a spectral resolution Date (UT) N AX R HJDbegin T (s) of 4.5 A and useful spectral range from 5000-6880 A. This time the CCD TEK # 1 (1024 square pixels, 24 //m 08/91/03 1 4340-9000 10 8471.0784 900 width) was used with grating 600 tilted by 37° (at second 08/91/04 12 6400-8800 3.0 8473.4923 500 order). 03/95/21 11 5000-6880 4.5 9797.8750 300 Direct images were obtained at the 1 m Swope tele­ 03/95/22 34 5000-6880 4.5 9798.7795 300 03/95/23 21 5000-6880 4.5 9799.7865 600 scope on 1995 March 12, 14, and 16, using the V filter. The CCD TEK # 2 (1024 square pixels, 21 /xm width), 03/95/12 36 — — 9788.7608 300 was used, yielding a 10.'2 wide square field. Differen­ 03/95/14 24 — — 9790.7385 300 Downloaded from https://academic.oup.com/pasj/article/50/3/333/2949021 by guest on 01 October 2021 tial magnitudes were calculated using the "phot" aper­ 03/95/16 2 — — 9792.8959 300 ture photometry package of IRAF. The optimum aper­ ture radius defined by Howell (1992) was chosen. This Note. N is the number of science frames, T the exposure time, radius matches the HWHM of the point spread function AA the wavelength range, and R the resolution in A. The zero (psf), minimizing the noise contribution due to sky pixels point of the heliocentric Julian day is 2440000. and readout noise. In addition, this small aperture (ra­ dius ~ 1.5-2 pixels, always chosen equal to the HWHM dition, the full width at half maximum (FWHM) was of the psf) was especially useful for EK TrA, due to the measured by fitting a simple Gaussian to the emission presence of several nearby optical companions. The near­ profiles. The equivalent widths were measured integrat­ est object is about 2 mag fainter than EK TrA and lo­ ing the line flux between the points where the line wings cated about 5" to the south-west. We neglected any intersect the continuum level. At the same intensity, the light contribution from the companions since they always full width at zero intensity (FWZI) was measured. All were located more than 5a apart from EK TrA (a is the these measures were made interactively in the computer standard deviation of a Gaussian type point spread func­ using the "d," "k," "e," and cursor routines in the "splot" tion). Differential magnitudes were determinated using IRAF package. Sources of errors are the profile S/N ra­ a comparison star (CI) located 145."5 to the North and tio, continuum normalization, and A calibration. For the 23."8 to the East from the variable. In addition, a check equivalent widths, they are of order of 10%. star (C2), of magnitude very similar to EK TrA, located Radial velocities, referred to the Local Standard of 128."9 to the North and 61." 1 to the East, was also mea­ h Rest (Vsun = 20 km s"\ asun = 18 , and S8un = +30°), sured. An estimate of the photometric error, derived were measured using the double Gaussian method first from the the standard deviation of the C2 — Cl differ­ proposed by Schneider and Young (1980) and refined by ences gave 0.03 mag. Shafter (1983) and Home et al. (1986). This method All CCD images were reduced with standard IRAF provides a robust diagnostic test for investigating the routines, correcting them by bias level and detector re­ behavior of different profile sections during the orbital sponse. One-dimensional spectra were extracted and cal­ cycle. The method consists of to simultaneously shift ibrated in wavelength using comparison spectra with typ­ two Gaussians of standard deviation ag (or alternatively ically 20 He-Ar-Ne lines. The rms of the calibration func­ full width at half maximum FWHMg) and separation _1 tion was typically lower than 0.3 A (14 km s at Ha). A along the emission profile until a velocity is found for We tested the stability of the CCD to shifts in wave­ which the convolved flux in both is the same. Chang­ length by cross-correlating comparison lamps taken the ing A and FWHMg we can probe different velocity sec­ same night. In all cases, the mean shift was less than tions of every profile. Too few comparison fines around 0.1 pixel. Details of the observations are given in table 1. He I 5876 precluded accurate wavelength calibration for We realized several measures in the calibrated spec­ this fine in 1995. For this run, we only measured Ha tra. The peak separation was measured after deblend- radial velocities. ing the emission profiles with two Gaussians of variable For all photometric and spectroscopic measures a pe­ sizes. In order to search for changes in the asymmetry riod search was carried out using two complementary of the emission profiles, we also measured the V/R ra­ methods: the "pdm" algorithm which is incorporated in the IRAF reduction program (Stellingwerf 1978) and the tio of these Gaussians, defined as -y—p, being 7C,/V, •*r — J-c analysis of variance periodogram (Schwarzenberg-Czerny and if the values of the intensities measured at the con­ 1989) which is included in the MIDAS software.
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