Spectroscopic and photometric observations of a Z cam-type dwarf nova, AT Cancri, in standstill

Abstract

We performed quasi-simultaneous spectroscopic and photometric observations of a Z Cam star, AT Cnc, in standstill in 1997 February. V-band time-resolved photometry marginally revealed small-amplitude (∼ 0.01 mag) oscillations with period candidates of 0.132 d, 0.124 d, 0.142 d, and 0.249 d. Low-resolution spectra show Balmer lines in emission and absorption, and Na D absorption. From the radial-velocity variation of Hα, we deduced the system parameters: the orbital period = 0.2011(±0.0006) d, the semi-amplitude = 80(±4) km s-1, the systemic velocity = 11(±3) km s-1 the primary mass = 0.9(±0.5) M(Fish eye sign). The phase of the velocity variation of Hα is very close to that of Na D. There are two possibilities: the Hα emission line radiated (a) from the secondary star, or (b) from the accretion disk. The inclination is estimated to be 17° (± 5°) and 36° (± 12°) based on the respective possibilities. Doppler map drawn under possibility (a) revealed on a broadly extended component in the (-Vx, -Vy) region. In the Doppler map drawn under the possibility (b), the peak exists on the irradiated side of the Doppler position of the secondary and the (+Vx,+Vy) region. We discovered an absorption componet in the blue wing at Hα. The P Cyg profile appeared during a limited period in its orbital phase, which is interpreted to be caused by transient mass ejection episodes.