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Astron. Astrophys. 326, 640-646

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Cyclicities in the light variations of LBVs*

I. The multi-periodic behaviour of the LBV candidate FORMULA Sco

C. Sterken1,**, M. de Groot2, and A.M. van Genderen3

1University of Brussels (VUB), Pleinlaan 2, B-1050 Brussels, Belgium
2Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland
3Leiden Observatory, Postbus 9513, 2300RA Leiden, The Netherlands

Received 2 June 1996 / Accepted 14 May 1997


Differential Strömgren uvby photometry of FORMULA Sco collected during the time interval 1982-1995 is analysed together with new VBLUW photometry with a complementary discussion of unpublished JHKLM data. We report the discovery of a ~ 1750-2200 d regular oscillation of amplitude FORMULA in all bands, and pulsation-like oscillations with at least one frequency FORMULA ( FORMULA days) and a possible second frequency FORMULA days). Most likely, all such stars exhibit multi-periodic light variability. A strong noise component remaining in the prewhitened signal leads us to postulate that the driver for LBV-like eruptions is stochastic resonance. This hypothesis is supported by the historic light curve of FORMULA Sco which shows that two centuries ago the star was about 2 magnitudes brighter, while a millenium ago it was only 1 magnitude brighter than today, an indication that FORMULA Sco should be regarded as a candidate Luminous Blue Variable.

Key words: stars: variables - stars: oscillations - stars: LBVs - stars: individual: FORMULA Sco - techniques: photometric

*Based on observations obtained at the European Southern Observatory at La Silla, Chile
**Belgian Fund for Scientific Research (FWO)

Send offprint requests to: C. Sterken

© European Southern Observatory (ESO) 1997

Online publication: September 18, 1997
Last change: October 15, 1997