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


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The onset of photometric variability in red giant stars *

A. Jorissen ** 1, N. Mowlavi 2, C. Sterken *** 3 and J. Manfroid **** 4

1 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. [FORMULA]) are stable at the level [FORMULA] millimag (where [FORMULA] 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 [FORMULA] increases with increasing [FORMULA], 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 [FORMULA] (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 [FORMULA] millimag) characterize red giants with [FORMULA], whereas the variations (with [FORMULA]  millimag) become more regular with periods of the order of 50 d for red giants with [FORMULA]. 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

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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