Zongyun Li

Superhumps in a Peculiar SU Ursae Majoris-Type Dwarf Nova, ER Ursae Majoris.

We report the photometry of a peculiar SU Ursae Majoris-type dwarf nova, ER Ursae Majoris, for 10 nights during 1998 December and 1999 March, covering a complete rise to the supermaximum and a normal outburst cycle. Superhumps have been found during the rise to the superoutburst. A negative superhump appeared in the December 22 light curve, while the superhump on the next night became positive and had a large-amplitude waveform distinct from that of the previous night. In the normal outbursts we captured, superhumps with larger or smaller amplitudes seem to always exist, although it is not necessarily true for every normal outburst. These results show great resemblance to V1159 Ori. It is more likely that superhumps occasionally exist at essentially all phases of the eruption cycles of ER UMa stars, which should be considered in modeling.

TT Arietis: Spectroscopy and Photometry

We report the photometry and spectroscopy of TT Arietis around the turn of the millennium, when the nova-like variable is in high state. A positive superhump since 1997 still exists. A more precise orbital period than before, 0:13755040+/-1.7x10(-7) days, is established. The intensity of the Balmer emission core is found to modulate at the superhump period rather than the orbital period and 180degrees out of phase with continuum. The intensity of Balmer absorption wings is approximately proportional to the continuum flux. The correlation between Balmer lines and continuum provides a new clue to the study of sources of continuum and emission lines. The mass ratio of the binary is found to be in the range of 0.19+/-0.04, which lends strong support to the proposition that superhumps arise from the inner Lindblad 3: 1 resonance in TT Ari, as in superoutbursts of SU UMa stars.

Spectroscopy of IY Ursae Majoris during Superoutburst

We report time-resolved spectroscopic observations of the recently discovered dwarf nova, IY Ursae Majoris, over 2 days during its 2000 January superoutburst. The spectra show prominent, broad, and double-peaked emission lines of Hα, Hβ, Hγ, Hδ, and He II and weaker He I lines. The radial velocity curve of the peaks of Hα and Hβ has an amplitude of 93 ± 7 km s-1 and a phase offset of 0.12 ± 0.03. From clear evidence for a precessing disk, we obtain a solution to an eccentric outer disk consistent with theoretical works, which demonstrates the validity of the relation between superhumps and tidal effects. The γ velocity of the binary is derived as -4 ± 32 km s-1 according to the solution. The inner part of the disk is also eccentric as evidenced by asymmetric wings in the lines. Therefore, the whole disk is eccentric. Finally, we find that a simple model can explain all asymmetric features of the emission lines.

A Photometry Campaign for IR Geminorum in Quiescence

We report a V band photometry of the SU UMa star IR Gem at quiescence in January 2002. The observations were made with two telescopes spaced ~ 160° apart in longitude. Several photometric modulations have been found. One gives a period of 98.50(13) min, exactly equal to the orbital period determined spectroscopically. Two others occasionally strengthen and seem to be positive and negative superhumps with periods of 103.6(4) and 95.4(4) min, 5.2 % longer and 3.1 % shorter than the orbital period, respectively. A signal at ~ 0.6 c/d in the power spectrum is roughly consistent with the expected period of nodal precession of the disk. There is a puzzling peak at 0.21(3) c/d corresponding to the ~ 4.3 d sine wave seen in the raw light curve. We suspect it to be a beat frequency between the frequencies of apsidal and nodal precessions of the disk. Quasi-periodic cycles with amplitudes 0.15–0.6 mag can be seen in the light curve. The mechanism underlying this modulation is not clear.

Photometry and Spectroscopy of KS Ursae Majoris during Superoutburst

We report photometric and spectroscopic observations of the SU UMa - type dwarf nova KS Ursae Majoris during its 2003 February superoutburst. Modulations with a period of 0. 07017 +/- 0. 00021 days, which is 3.3% larger than the orbital period, have been found during the superoutburst and may be positive superhumps. A maximum trough- to-peak amplitude of around 0.3 mag is determined for this superhump. The spectra show broad absorption- line profiles. The lines display blue and red troughs that alternate in depth. The radial velocity curve of the absorption wings of H beta has an amplitude of 40 +/- 11 km s (-1) and a phase offset of 0. 12 +/- 0. 03. The gamma- velocity of the binary is 3 +/- 9 km s (-1) and varies on the order of 50 km s (-1) from day to day. From other clear evidence for a precessing eccentric disk, we obtain a solution to an eccentric outer disk consistent with theoretical works, which demonstrates the validity of the relation between superhumps and tidal effects. The inner part of the disk is also eccentric, as evidenced by asymmetric and symmetric wings in the lines. Therefore, the whole disk is eccentric, and the variation of the gamma- velocity and the evolutionary asymmetric line profiles could be criteria for a precessing eccentric accretion disk.

Superhumps behavior during normal outbursts in ER Ursae Majoris: Spectroscopy and photometry

We undertook a 4-day spectroscopic observation and conducted a 13-day photometric study of an SU UMa-type dwarf nova, ER Ursae Majoris. The mean K-amplitude for the emission lines is 54 +/- 8 kms(-1) and the gamma velocity is 8 +/- 4 km s(-1) based on our spectroscopic results. A phase shift of 0.22 was also obtained. Our photometric observation confirms superhumps in normal outbursts of ER UMa. and reconciles contradictory observational results obtained by different authors. We find that superhumps possibly develop near each normal outburst maximum and fade out before the next Outburst maximum. If the observed humps were not late superhunips, but ordinary superhumps, much more theoretical work should be done to explain the new phenomena.