Astron. Astrophys. 326, 640-646 (1997)

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Cyclicities in the light variations of LBVs ^*

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

small.htm C. Sterken1; ^**, M. de Groot ² and A.M. van Genderen <sup>3

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

Received 2 June 1996 / Accepted 14 May 1997

Abstract

Differential Strömgren uvby photometry of  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during the time interval 1982-1995 is analysed together with new VBLUW photometry with a complementary discussion of unpublished JHKLM data.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 in all bands, and pulsation-like oscillations with at least one frequency ( days) and a possible second frequency 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  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  Sco should be regarded as a candidate Luminous Blue Variable.

Key words: stars: variables stars: oscillations stars: LBVs stars: individual:  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

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Contents

• 1. Introduction
• 2. The data
  • 2.1. uvby photometry, ESO
  • 2.2. VBLUW photometry, ESO
  • 2.3. LTPV uvby photometry, ESO
• 3. The light curves
• 4. Search for periodicities
• 5. Discussion
  • 5.1. The day oscillation
  • 5.2. The regular light variations of  Sco on a time scale of several weeks to one month
  • 5.3. The stochastic variability
  • 5.4.  Sco as a candidate LBV
• 6. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 1997

Online publication: October 15, 1997
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