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

We have studied the chromospheric activity in 35 G dwarf stars of 5 open clusters to determine the age-activity rotation relationship. As tracer of chromospheric activity we have used the emission-core flux of the Ca II K line, measured from high-resolution spectra. We have also used published solar data. Our data set constitutes three pairs of coeval objects: the young Hyades and Praesepe (0.6 Gyr), the two intermediate age clusters IC 4651 and NGC 3680 (similar to 1.7 Gyr) and the old M 67 (similar to 4.5 Gyr) and the Sun. In each coeval pair the objects axe indistinguishable, as far as chromospheric activity is concerned. 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 IC4651 age, of about 1 Gyr. Afterwards, activity remains virtually constant for more than 3 Gyr. We have also measured v sin i 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 similar to 2 Gyr. The observed abrupt decline in activity explains very well the Vaughan-Preston gap. What we present here is based on Pace & Pasquini (2004), to which we refer for more details regarding the collecting and processing of the data.