Issue A&A Volume 426, Number 3, November II 2004 Page(s) 1021 - 1034 Section Stellar atmospheres DOI http://dx.doi.org/10.1051/0004-6361:20040568

A&A 426, 1021-1034 (2004)
DOI: 10.1051/0004-6361:20040568

The age-activity-rotation relationship in solar-type stars

G. Pace^{1, 2} and L. Pasquini<sup>1

1</sup>  European Southern Observatory, Karl Schwarzschild Strasse 2, 85748, Garching bei München, Germany
    e-mail: gpace@eso.org
²  Dipartimento di Astronomia, Università di Trieste, via GB Tiepolo 11, 34131 Trieste, Italy

(Received 31 March 2004 / Accepted 27 June 2004 )

Abstract
We present Ca  K line chromospheric fluxes measured from high-resolution spectra in 35 G dwarf stars of 5 open clusters to determine the age-activity-rotation relationship from the young Hyades and Praesepe (0.6 Gyr) to the old M 67 (~4.5 Gyr) through the two intermediate age clusters IC 4651 and NGC 3680 (~1.7 Gyr). The full amplitude of the activity index within a cluster is slightly above 60 % for all clusters but one, NGC 3680, in which only two stars were observed. As a comparison, the same Solar Ca index varies by ~ during a solar cycle. Four of our clusters (Hyades and Praesepe, IC 4651 and NGC 3680) are pairs of twins as far as age is concerned; the Hyades have the same chromospheric-activity level as Praesepe, at odds with early claims based on X-ray observations. Both stars in NGC 3680 are indistinguishable, as far as chromospheric activity is concerned, from those in the coeval IC 4651. This is a validation of the existence of an age-activity relationship. On the other hand, the two intermediate age clusters have the same activity level as the much older M 67 and the Sun. Our data therefore shows that a dramatic decrease in chromospheric activity takes place in solar stars between the Hyades and the IC 4651 age, of about 1 Gyr. Afterwards, activity remains virtually constant for more than 3 Gyr. We have also measured for all of our stars and the average rotational velocity shows the same trend as the chromospheric-activity index. We briefly investigate the impact of this result on the age determinations of field G dwarfs in the solar neighborhood; the two main conclusions are that a consistent group of "young" stars (i.e. as active as Hyades stars) is present, and that it is virtually impossible to give accurate chromospheric ages for stars older than ~2 Gyr. The observed abrupt decline in activity explains very well the Vaughan-Preston gap.

Key words: stars: late-type -- stars: activity -- stars: chromosphere -- stars: rotation

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