Structure of an initial n1ain sequence star of 1nixed opacity with negligible abundance of heavy ele1nents and studies of so1ne young clusters

Abstract

In this thesis we have investigated some aspects of stellar structure and evolution. The thesis is divided into two sections. Section I deals with the structure of a star while section 2 deals with stu.J.y .. of some young galactic clusters. In chapters I and 2, in section 1, necessary theoretical aspects of stellar structure have been discussed. In chapter 3 we have determined the structure of a star of 2.5 solar mass in which the opacity is mixed, due to both electron scattering and free-free transition, and the abundance of heavy elements is negligible. Section 2 A contains a brief outline of current views of star formation and investigation of three young galactic clusters, namely, NGC 1976, NGC 2264, and the Orion Nebula cluster following constant mass evolution of pre-main sequence stars. By analysing the observational data we have detem1ined the mass functions, formation rates, and the mass-age correlation of stars in the clusters. In section 2 B we have tested the theoretical prediction of an accretion model of star formation with observation of seven galactic clusters of young and intermediate ages, namely, NGC 1976, NGC 2264, NGC 6383, NGC 8611, NGC 6716, NGC 1245, and the Orion Nebula cluster. The predictions include the existence of a mass function, a formation rate of stars, and an explanation for the observed spread in the HR diagram of young clusters. To facilitate comparison we have deduced stellar masses of the first three clusters by polytropic method while for the remaining clusters we have used masses as deduced by other authors by using more reliable theoretical evolutionary tracks. NGC 2264 has been considered in both cases.