Spectroscopic Characterisation of New Accreting Intermediate-Mass Pre-Main Sequence Stars

The thesis presents the spectroscopic study of the stellar parameters, accretion rates and IR excess of Herbig Ae/Be stars. The discs around solar mass stars are expected to evolve differently compared to those around intermediate-mass stars because the stars evolve differently. However, more complete samples of young pre-main sequence stars are not present in literature to quantify the differences. Most current studies in the stellar range of intermediate-mass focuses on the Herbig Ae population, a limited subset of 5-10 Myr old intermediate-mass stars already close to the main sequence. Hence, in this work an unbiased sample of young stars (<5 Myr) in the intermediate-mass range, 1.5-3.5 solar masses are selected to be studied. I use the spectra, optical photometric data and Pre-Main Sequence Evolutionary Tracks to find the parameters like effective temperature, surface gravity, extinction, distance, luminosity, radius, mass and age.

The lines present in the spectra are observed to be correlated with accretion luminosity and the accretion rates are determined. The young nature of the stars are confirmed when they are placed on the HR diagram. It is seen that the younger stars have higher accretion rates compared to the older ones, implying that the accretion rates of the stars decreases as the stars approach the main sequence. This indicates that the accretion rate is an evolutionary property of the Herbig stars. Also, high accretors have significant IR excesses indicating that when discs are young, the stars are young and they have high accretion rates and more excesses; in my sample stars as young as 3 Myr, beyond 3 Myr there is no significant excess and also the accretion rates are low.