Chromospheric activity and lithium abundance in the NGC2516 and NGC3766 open clusters

Abstract

The aim of this thesis is the analysis of chromospheric activity and lithium abundance of late type stars in open clusters of different ages. The study of open cluster is very important for our understanding of stellar evolution and atmospheres, since all cluster members have almost the same age and chemical composition and this allows to infer their properties more easily than individual stars. In this work two open clusters have been considered, NGC2516 and NGC3766, using spectra acquired by the ESO spectrograph FLAMES at VLT, in the LiI and CaII H and K region. Most of our knowledge of young stars comes from the observations of general properties of open clusters, since they represents a good tool for testing the stellar evolution models, and the age measurement' methods. In fact, stellar ages can be determined only by comparison with theoretical models, and open clusters are widely used to calibrate methods of age determination, since their observation allows to determine average properties of a group of stars approximatly coeval. One of the methods used to determine the age of young stars is the lithium depletion, based on the determination of the lithium abundance of a cluster's star. This method has been used in this work to determine the age of both observed clusters. Their lithium depletion has been derived, confirming that, for the cluster NGC2516, the age is approximatly around 150Myr, as observed previously by other authors. Assuming solar metallicity for NGC3766, direct comparison of Li equivalent with other clusters indicates an age close to that of the Pleiades or even older, in disagreement with previous age determination. A metallicity higher than solar may explain at least partially this disagreement. Chromospheric activity in NGC2516 has also been studied using Ca II H & K lines VLT/FLAMES-GIRAFFE observations. Since no VLT/FLAMES-GIRAFFE observations of Mt. Wilson standards are available, I applied the method of Linsky et al.,1979, ApJs,41,47, and Strassmeier et al., 2000,A&AS, 142, 275. A comparison with the standard Mt. Wilson method has been done using FEROS spectra. A study of astrophysical parameter determination using the spectral region between 3990 and 4040 A has also been performed. This work is organised as follows: the problem of age determination in open clusters is briefly discussed in Chapter 1; Chapter 2 contains a review of chromospheric activity indicators; Chapter 3 contains a description of the cluster studied; Chapter 4 describes the reduction of VLT/FLAMES-GIRAFFE data; Chapter 5 describes the method used to derive stellar physical parameters. Finally, in Chapter 6 the chromospheric activity analysis for NGC2516 and the lithium abundance analysis in both NGC2516 and NGC3766 are presented.