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Chemical abundances and chromospheric activity in solar type stars of the open clusters NGC 5822 and IC 4756

Giancarlo Pace

Chromospheric activity (CA) in stars is powered by rotation in the presence of  a deep convective envelope, it therefore fades as the magnetical braking  mechanism  spins down the star. This makes CA  a potential age indicator: the  higher the CA, the younger the age. Previous study showed that, while Hyadesand  Praesepe are chromospherically active as normal for their young age, CA in two  intermediate age clusters (NGC 3680 and IC 4651, between 1.5 and 2Gyrs), have already dropped to the level of  the much older Sun and M 67.
This, on the one hand means that CA is not a good age indicator for oldstars, and on the other hand stresses the importance of CA open cluster data at about  1 Gyr in order to have a good calibration of the CA fading law, which can be  then applied to study field stars, for which independent age determinations are  very uncertain. To this aim, high resolution spectra are being analysed. The  sames were collected for two solar type stars in NGC5822 and three in IC 4756,  which are open clusters about 1 Gyr old. Thisspectra also allow us to increase  the number of open clusters with precise chemical abundance determination,  which at the moment amount to about 35.

2008 September 24, 13:30

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