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

G. Pace

UVES/FLAMES (VLT) high resolution spectra were collected for two solar type stars in  NGC 5822 and three in IC 4756. The large spectral coverage allows us to study both  chromospheric activity, by means of the emission in the core of the chromospheric Ca  II K line, at 3933 Angstroms, and the chemical abundances of elements like iron,  calcium, silicon, titanium, and nickel. 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  Hyades and 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 2 Gyrs),  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 old stars, 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. The dataset presented herewith were collected with this purpose. In addition, it increases the number of open  clusters with metallicity base on high-resolution spectra covering a large wavelength  range, which at present amount to about 35. The clusters mean metallicities are  [Fe/H]=0.01 and 0.05 for IC 4756 and NGC 5822 respectively, with an error of about 0.05 dex. A preliminary analisys shows that their level of CA is intermediate between that of Hyades and Praesepe on one side, and that of the intemediate age and old  stars like the sun, confirming photometric studies that estimate their age to be about 1 Gyr.

XVIII Encontro Nacional de Astronomia e Astrofísica
Évora, Portugal
2008 September

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Faculdade de Ciências da Universidade de Lisboa Universidade do Porto Faculdade de Ciências e Tecnologia da Universidade de Coimbra
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia