Multiwavelength study of variable stars and star formation in open star

Abstract

The evolution of pre-main-sequence (PMS) stars involves a set of complex physical processes such as evolution of circumstellar disks, accretion processes, bipolar jets and outflows, evolution of angular momentum etc. Because of these processes, the PMS stars show a wide range of variability in their luminosity in almost all wavelengths from Xray to infrared and their variability time scales range from few minutes to years. Several mechanisms are known to induce these variability, for example irregular distribution of cool spots on stellar photosphere, variable hot spots, obscuration from dust, instability in disk, change in accretion rate etc. The evolution of disks and the accretion rates may play a prominent role in the non-periodic variability, whereas due to the presence of cool and hot spots on the photosphere, rotation of the stars may produce periodic/quasi-periodic changes in their light-curve (LC). Variability from accretion is produced as excess continuum and line emission above the photosphere changes. Both short-term (1-5 days, called dippers) and long-term (weeks to years, called f aders) extinction events in Classical T-Tauri stars (CTTSs) stars by dust obscuration and wrap structure in disk have been found in some selected sources. Periodic and quasi-periodic dippers have periodicity distribution consistent with their rotation period. Periods in LCs are direct indicator of the rotational speed and hence are related to the angular momentum. As the PMS stars contract towards the main-sequence (MS), they should increase their rotation speed upto a breakup velocity due to conservation of angular momentum. But numerous studies on PMS stars have revealed that these stars typically rotate at much smaller fraction of the breakup velocity in spite of their significant contraction. Several mechanism for effectively removing angular momentum from PMS stars during the first ∼10 Myrs of their evolution has been proposed including, magnetic star-disk interaction scaled-up solar-type magnetized winds perhaps driven by accretion, and scaled-up solar-type coronal mass ejections, but they are still debated and contribution from the individual components are not clear. The rotating stars with spots on their surface provide an unique tool to determine the rotation period of these stars with precision. Correlating the rotation period with different stellar properties (age, mass, accretion rate, disk mass etc.) may give some clue about the angular momentum evolution in PMS stars. Correlation of variability amplitude with stellar parameters is useful to understand the role of these parameters in brightness variation of young stars. Since spot modulation is one of the main mechanism which in induce variability in PMS stars, any change in size of the spots or asymmetry in the stellar surface will be reflected in amplitude. Class II stars are known to have disk whereas the Class III objects lack it. So, a comparison between Class II and Class III objects in terms of their period and amplitude will provide key information about the role of disk in their variability. Also, studying the LCs and color evolution of PMS variables will tell us the role of different mechanisms such as spots, accretion, obscuration in their variability. Young star clusters (YSCs) are ideal sites to study the variability properties of PMS stars as plenty of them are found in these regions, with broad mass range as well as spread in their ages (few Myrs). It is also easy to constrain the physical parameters such as age and mass of PMS stars through cluster HR diagram. Therefore, we have selected three young open clusters, Sh 2-70, Sh 2-190 and IC 1848 to investigate the angular momentum evolution and variability in PMS stars. In the first chapter we introduce the concepts about star formation and different aspects of variability. The second chapter deals with instrument, data and analysis techniques. Next three chapters are devoted to the results found in these three clusters. Finally we conclude the thesis in the sixth chapter.