Untersuchung offener Sternhaufen mittels SEGUE-Beobachtungen
Zusammenfassung der Projektergebnisse
This project aimed at the detennination of membership and structure of open star clusters observed in the SDSS/SEGUE project. Open clusters are typical respresentatives of the thin disk of our Galaxy, and hence are subject to noticeable interstellar extinction. For this reason the first part of the project was entirely devoted to develop a procedure that allows the determination of the stellar parameters log L, log Te// in the presence of interstellar reddening EB-VThis goal has been achieved: we have developed two different methods for the estimation of the stellar atmospheric parameters (APE) (logg,logre//,[Fe///]) together with the interstellar redenning EB-V from SDSS (u, g, r, i, z) photometry and Johnson BVJHK-photome\ry and combinations thereof. As a prerequisite for both methods of the estimation of atmospheric stellar parameters synthetic colours in the (,u,g,r,i,z) and the Johnson photometric systems have been calculated from the latest Kurucz models of theoretical spectra (Castelli & Kurucz 2006). In practice, we established a fine grid in (log g, log Teff, [Fe/H], on which we calculated extinction independent (so-called) Q colour indices (see, e.g. StraiXys 1977) in the different colour systems. Were the function from (logg, log Te//, [Fe/H] to Q bijective, APE estimation from observed Q would be a simple process. The two methods we developed in this study try to overcome the complications of the estimation process. The first method consists of three steps: 1. interval analysis. We select all theoretical parameters in our grid which satisfy the measured extinction-free colour indices of the star; 2. cluster analysis. We group all possible solutions in atmospheric parameter space and find a set of solutions; 3. selection of the best solution. We consider all possible solutions to select the only one as the most probable solution for the star in process. In the Johnson B,V,J, H and Ks photometric system , and with mean errors of input colors better than 0.01 mag the best results are achieved for main sequence G-K stars (most preferable region). In this case the parameters are accurate to «riogr,// < 0.05, o-iog^ < 0.3 and a-[fe/H] < 0.3. If the accuracy of the colour measurements drops to 0.05 mag, the accuracy of the determined parameters drops to o'logTV// < 0.1,criog^ < 2.0 and (r[Fe/f{] < 0.4 . The definition of "the most preferable regions" depends on the combination of photometric bands, and the precision of the input photometry. The method has been applied to the Johnson system to calibrate the adopted theoretical colours from a specially compiled catalogue of observed colours and spectrocopically determined atmospheric parameters. The method is extremely cpu time comsuming and, by no means, straighforward. The second method searches the point in the parameter grid for which the difference between the observed Q index from the theoretical Q index has its absolute minimum. Extensive Monte-Carlo simulations have been carried out to test the method on the (u, g, r, i, z) and ßVyWÄ'-photometric systems. For SDSS photometry a-phot must be smaller than 0.02 mag in all bands to estimate log T^ff to better than 0.055 for Teff < 5600K, if no a priori information on EB-V is available. The more EB-V is restricted a priori the better get the resulls. Inclusion of infrared observations, e.g. from 2MASS considerably improve the situation. For a-phot > 0.05 mag parameter determination from (u, g, r, i, z) alone without a priori restrictions of the parameter space is not recommended. Both methods allow an estimation of stellar atmospheric parameters (logg, log Tgff, [Fe/H] as well as log L and EB-V under certain constraints, e.g. restrictions on EB-V or to stars on the main sequence. An application of the procedure developed here to the observations of open clusters within the SDSS/SEGUE project could not be carried out within the time frame of the project, mainly due to the lack of manpower.
