Astron. Astrophys. 324, 578-586 (1997)

The onset of photometric variability in red giant stars ^*

A. Jorissen ^{** 1}, N. Mowlavi ², C. Sterken ^{*** 3} and J. Manfroid <sup>**** 4

1</sup> Institut d'Astronomie et d'Astrophysique, Université Libre de Bruxelles, CP 226, Boulevard du Triomphe, B-1050 Bruxelles, Belgium
² Observatoire de Genève, CH-1290 Sauverny, Switzerland
³ Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussel, Belgium
⁴ Institut d'Astrophysique, Université de Liège, Av. de Cointe 5, B-4000 Liège, Belgium

Received 16 July 1996 / Accepted 10 February 1997

Abstract

The onset of variability in red giant stars is studied from the data collected by the 'Long-Term Photometry of Variables' project which operates at ESO (La Silla) since 1982. The differential nature of the observations combined with a multi-night reduction algorithm enables to study the variability of over 50 red giants with an accuracy of the order of 2 to 3 millimagnitudes on time scales ranging from days to years. All red giants with spectral types late G to early K (i.e. ) are stable at the level millimag (where is the standard deviation of the Strömgren y magnitude). Red giants with later spectral types are all variable, and it is found that the minimum variability level increases with increasing , thus defining a minimum-variability boundary. The new data obtained in this paper reveal that the time scale of the associated variability increases with increasing (and thus amplitude), and that the stability of this time scale improves concomitantly. More precisely, irregular variations on time scales of 5 to 10 days (and millimag) characterize red giants with , whereas the variations (with  millimag) become more regular with periods of the order of 50 d for red giants with . A radial-velocity jitter (of the order of 1.5 km s^-1 r.m.s.) is associated with this photometric variability, and suggests that stellar oscillations may be responsible for the observed variations.

Key words: stars: late-type stars: variable: other stars: oscillations

* Based on observations carried out at the European Southern Observatory (ESO, La Silla, Chile), and at the Swiss telescope at the Haute Provence Observatory (France)
** Research Associate, FNRS, Belgium
*** Senior Research Associate, NFWO, Belgium
**** Research Director, FNRS, Belgium

Send offprint requests to: A. Jorissen

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Contents

• 1. Introduction
• 2. The LTPV data set
• 3. The () diagram
• 4. On the absence of stable red stars
  • 4.1. Reddening effects
  • 4.2. Microvariability among red giants
• 5. The period-amplitude relationship
• 6. Radial velocity variations
• 7. Discussion
• 8. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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