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Astron. Astrophys. 351, 212-224 (1999)

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Time series analysis of V 1794 Cygni long-term photometry

L. Jetsu 1,2, J. Pelt 3,4 and I. Tuominen 4

1 Observatory, P.O. Box 14, University of Helsinki, 00014, Finland
2 NORDITA, Blegdamsvej 17, 2100 Copenhagen, Denmark
3 Tartu Observatory, 61 602 Tõravere, Estonia
4 Astronomy Division, University of Oulu, P.O. Box 3000, 90410 Oulu, Finland

Received 8 June 1998 / Accepted 6 August 1999


Standard Johnson UBVRI photometry of V 1794 Cyg (HD199178) between 1975 and 1995 is analysed. Instead of the traditional constant period ephemeris, we determine the seasonal periodicities ([FORMULA]) and the primary and secondary minima epochs ([FORMULA], [FORMULA]) of the normalized UBVRI magnitudes using the three stage period analysis (TSPA) and complementary methods. Our [FORMULA] and [FORMULA] estimates with variable [FORMULA] can adapt easily to both differential rotation and longitudinal activity migration. The seasonal [FORMULA] are utilized in modelling the mean (M) and total amplitude (A) of contemporary light curves in UBVRI. TSPA reveals that the long-term M and A changes of V 1794 Cyg are unpredictable . We search for active longitudes from the [FORMULA] and [FORMULA] series of time points with nonparametric methods. The critical level of 0.0029 for the best [FORMULA] period detected with the Kuiper method is high, but exceeds the 0.001 significance for rejecting the hypothesis that the phases of [FORMULA] and [FORMULA] are randomly distributed. The activity centres in V 1794 Cyg are rarely disrupted, and most probably undergo continuous longitudinal migration, because only one abrupt disruption is observed during 20 years. As for differential rotation , the irregular changes of seasonal [FORMULA] are 7.5%. The surprisingly regular 3.3% changes of yearly [FORMULA] may provide a stellar analogy of the solar "butterfly" diagram.

Key words: stars: individual: V 1794 Cyg – stars: variables: general – stars: activity – techniques: photometric

Send offprint requests to: jetsu@gstar.astro.helsinki.fi

© European Southern Observatory (ESO) 1999

Online publication: November 2, 1999