Activity cycle of the giant star of Z Andromedae and its spin period

Abstract

We have reanalyzed the long-term optical light curve (LC) of the symbiotic star Z Andromedae, covering 112--yr of mostly visual observations. Two strictly periodic and one quasi-periodic cycles can be identified in this LC. A P1=7550 d quasi periodicity characterizes the repetition time of the outburst episodes of this symbiotic star. Six such events have been recorded so far. During quiescence states of the system, i.e. in time intervals between outbursts, the LC is clearly modulated by a stable coherent period of P2=759.1 d. This is the well known orbital period of the Z And binary system that have been measured also spectroscopically. A third coherent period of P3=658.4 d is modulating the intense fluctuations in the optical brightness of the system during outbursts. We attribute the trigger of the outbursts phenomenon and the clock that drives it, to a solar type magnetic dynamo cycle that operates in the convection and the outer layers of the giant star of the system. We suggest that the intense surface activity of the giant star during maximum phases of its magnetic cycle is especially enhanced in one or two antipode regions, fixed in the atmosphere of the star and rotating with it. Such spots could be active regions around the North and the South poles of a general magnetic dipole field of the star. The P3 periodicity is half the beat of the binary orbital period of the system and the spin period of the giant. The latter is then either 482 or 1790 d. If only one pole is active on the surface of the giant, P3 is the beat period itself, and the spin period is 352 d. It could also be 5000 d if the giant is rotating in retrograde direction. We briefly compare these findings in the LC of Z And to similar modulations that were identified in the LC of two other prototype symbiotics, BF Cyg and YY Her.

arXiv comment9 pages, 4 figures, Accepted for publication in MNRAS

References in corpus (6)

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