Supervisors info:
Γαζέας Κοσμάς, Λέκτορας, Τμήμα Φυσικής, Εθνικό και Καποδιστριακό Πανεπιστήμιο Αθηνών
Χατζηδημητρίου Δέσποινα, Αναπληρώτρια Καθηγήτρια, Τμήμα Φυσικής, Εθνικό και Καποδιστριακό Πανεπιστήμιο Αθηνών
Καζαντζίδης Στυλιανός, Επίκουρος Καθηγητής, Τμήμα Φυσικής, Εθνικό και Καποδιστριακό Πανεπιστήμιο Αθηνών
Summary:
Stellar clusters are groups of stars, of various masses, held together by mutual gravity for long periods of time. In a first approximation, we assume that all the stars-members of a cluster were formed, almost simultaneously, inside the same interstellar molecular cloud. Therefore, they may share the same age, chemical composition, interstellar absorption and distance from Earth. However, recent studies have revealed that things are much more complicated, as star clusters are not consisted of homogeneous stellar populations (Dotter et al. 2015; Piotto et al. 2015; Gratton et al. 2012; Bekki et al.2011; Sbordone et al. 2011; Vesperini et al. 2011; Piotto et al. 2009 & 2008; D’Antonna et al. 2007). Actually, they may host distinct populations as far as their age, abundances, dynamical state and spatial distribution, concern. In case we detect multiple generations of stars in one cluster, we know that the first generation is made mostly from helium and hydrogen, while the second generation stars include a combination of the initial material, enriched with elements produced during the evolution of the most massive stars of the first starburst. Stellar clusters are valuable laboratories for Astrophysicists as they help us test/complete our knowledge on stellar evolution, N-body interactions as well as the chemical and dynamical history of the galaxies, they are members of.
In the present thesis, we conducted photometric observations of three galactic clusters, open NGC 6705 (M11) and globulars (GC) NGC 6809 (M55), NGC 6838 (M71), in four optical bands (BVRI), using the mirror telescope of University of Athens Observatory. An important product of our work is the homogeneous, calibrated astrometric and photometric catalogue for the thousands of stars in each cluster. This catalogue contributes to the literature, with measurements of R and I magnitudes, which are usually absent from the astronomical databases. By means of the catalog, we build 24 Color-Magnitude Diagrams (CMD), in order to estimate the relative ages of the clusters, using theoretical isochrones of the PARSEC evolutionary code (Bressan et al. 2012, 2013). We also construct 15 Color-Color Diagrams (CCD) that let us estimate reddening of the 6 available color indices. The large number of diagrams gives as the advantage of multiple cross-checking the values we derive, reducing the uncertainties. Knowing the metallicity and the de-reddened color index, (B-V)o of the Red Clump-RC (for open clusters), we get the mean absolute magnitude of the RC, using the empirical law of Bilir et al. 2013. Comparing the last with the corresponding apparent magnitude of our CMD, we derive the distance modulus for the cluster. On the other hand, to get distance modulus of GCs, we apply the empirical law of Demarque et al. (2000), utilizing RR-Lyrae pulsating variables, as a driver. We build the radial density profiles of the clusters and then fitting the appropriate King models (King 1962, 1967, 1995) we estimate some of the structural parameters (i.e. core radius, central and background surface stellar density and concentration parameter). Finally, we examine the spatial (2D) distribution of characteristic, distinct stellar populations searching for gradients that can be explained by mass segregation, second-parameter phenomenon (Horizontal Branch stars) or multiple generations of stars in the same cluster.
