Spectroscopic Studies of Galactic Field Be Stars

Spectroscopic Studies of Galactic Field Be Stars

Be stars provide excellent opportunity to study circumstellar disks. But the disc formation mechanism of classical Be (CBe) stars- the Be phenomenon- is still poorly understood. This can be understood by studying CBe stars in various locations like clusters and fields. Spectra of Be stars show interesting emission lines of different elements like hydrogen, helium, iron, oxygen calcium, etc. These emission lines are valuable indicators in providing information about the circumstellar disks of Be stars. In the past several decades various aspects of Be stars have been studied. But literature review clearly indicates the need of further studies to frame a consolidated picture about Be phenomenon in CBe stars. It is found that especially, the region ?????? 7500 - 8800 ??? is a less studied, and thus poorly understood area in Be star research. But this area shows some interesting features like emission lines calcium, iron, oxygen and Paschen series. So, here we have studied a sample of 118 field CBe stars taken from the catalogue of Jaschek & Egret (1982) and whose medium resolution spectra were obtained in ?????? 3800 ?? 9000 ??? region during December, 2007 to January, 2009 with the 2.1-m Himalayan Chandra Telescope (HCT), located at Hanle, Ladakh, India and operated by the Indian Institute of Astrophysics (IIA), Bangalore. In this thesis, we present three works which investigate the disc properties of our 118 program Be stars by studying their spectral line features, focussing primarily on the less explored ?????? 7500 - 8800 ??? region. Firstly, we have analyzed the less studied Fe II 7712 ??? emission line for our stars to understand the possible Fe II line excitation mechanism in CBe stars. Our work predicts that Ly???fluorescence may be the possible Fe II line excitation mechanism in CBe stars. Secondly, we have studied the Ca II triplet emission lines for our stars and have developed a new technique for deblending Ca II components from their counterpart Paschen lines, thus providing a more efficient way to analyze Ca II lines. Analyzing Ca II lines through this technique, we suggest that the gas producing these lines is optically thick. This leads us to predict that Ca II lines may be an indicator of binarity in Be stars. Lastly, we have estimated the Balmer decrement values, D34 and D54 for 81 of our sample stars to shed light on opacity effects in Be star disks. Our work confirms the disc transient nature of Be stars through epoch-wise D34 and D54 variation study and also suggests that Be star disks are optically thick.

Gourav Banerjee

Physics