Cyclicities in the light variations of LBVs I. The multi-periodic behaviour of the LBV candidate ζ1 Sco

Differential Strömgren uvby photometry of ζ1 Sco collected during the time interval 1982-1995 is analysed together with new V BLUW photometry with a complementary discussionof unpublishedJHKLM data.We report the discovery of a∼ 1750-2200 d regular oscillation of amplitude∼ 0. m01 in all bands, and pulsation-like oscillations with at least one frequency f1 = 0.03155 (P ∼ 32 days) and a possible second frequency f2 ∼ 5 4f1 ∼ 0.03891(P ∼ 25.7 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 ζ1 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 ζ1 Sco should be regarded as a candidate Luminous Blue Variable.