First time-series optical photometry from Antarctica

Context. Eradicating the problems associated with the Earth's day-night cycle is mandatory for long and continuous time-series photometry and had been achieved with either large ground-based networks of observatories at different geographic longitudes or when conducted from space. A third possibility is offered by a polar location with astronomically-qualified site characteristics. Aims. We present the first scientific stellar time-series optical photometry from Dome C in Antarctica and analyze approximately 13 000 CCD frames acquired in July 2007. Methods. The optical pilot telescope of the “International Robotic Antarctic Infrared Telescope”, named “small IRAIT” (sIRAIT), and its $\it UBVRI$ CCD photometer were used in $\it BVR$ for a continuous 243 h (10.15 days) with a duty cycle of 98% and a cadence of 155 s. The prime targets were the chromospherically active, spotted binary star V841 Cen and the non-radially pulsating δ-Scuti star V1034 Cen. Results. We confirmed the known 0.2-day fundamental period of V1034 Cen and detected a total of 23 further periods between 2.2 h and 3.5 days. In July 2007, V841 Cen's V amplitude due to spots appeared to be at a record high of 0$\fm$4 in V. We completed a spot-model analysis with a light-curve inversion technique and discovered the star with a spot filling factor of 44% of the visible hemisphere, among the highest ever measured values for active stars, and a temperature-difference photosphere minus spot of 750 ± 100 K. Its odd-numbered (for a single site) rotation period was determined with a higher precision than before (5.8854 ± 0.0026 days), despite our comparably short data set. The rms scatter from a 2.4-h data subset was 3 mmag in V and 4.2 mmag in R. The differential data quality is 3-4 times higher than with the 25 cm Fairborn Automatic Photoelectric Telescope in southern Arizona and is probably due to the exceptionally low scintillation noise at Dome C. Conclusions. We conclude that high-precision CCD photometry with exceptional time coverage and cadence can be acquired at Dome C in Antarctica and be successfully used to complete time-series astrophysics