Identification and characterization of young stars in star forming regions

Abstract

Star formation is a chain of processes in which molecular clouds get converted to a compact stellar systems, which are the potential platforms of life. Although, numerous observations and simulations have been performed over few decades to understand these processes and evolutionary sequences, many questions remain unanswered. Several factors are found to regulate the star formation processes, e.g. magnetic field and turbulence of the molecular clouds, supernova explosions, high stellar winds and so on. The formation and interaction of low and high mass stars can be constructive as well as destructive in small and large scale environments. In the very early stage of its formation, the young stellar object (YSO) shows emission in millimeter and sub-millimeter. Accretion of circumstellar material and outflow are the signatures of interaction of YSOs with the surroundings. Being associated with the circumstellar disks, YSOs are often found to have infrared (IR) excess. Due to presence of accretion, YSOs show emission in Hα, Hβ, Bracketγ, Paschenβ etc. in their spectra. Due to accretion shocks, YSOs can be associated with X-ray and ultraviolet emission also. Being formed together, YSOs share similar kinematic properties as they form together in molecular clouds. Based on these ways, YSOs can be identified to study the star formation history in the molecular clouds. In this thesis, we identified and characterized young stars in star forming regions. We made a detailed study of an elongated cloud named LDN 1172/1174, showing “head-tail” structure which is located in the Cepheus constellation. LDN 1174 (NGC 7023) is illuminated by a Herbig Be star HD 200775. At first we made an attempt to estimate the distance to this cloud as it is not well estimated in previous literature. Based on the latest Gaia DR2 distances of the YSOs associated with LDN 1172/1174 we computed its distance as 335 ± 11 pc. We further confirm the estimated distance to this cloud complex based on the polarization measurements of the stars projected background to the cloud. In the polarizaion vs. distance (obtained from Gaia DR2) plot of these observed stars we found a hike in polarization at ∼ 335 pc. Now, based on the median absolute deviation of the distance and proper motion values of the YSOs we found 20 additional sources having similar kinematics as the YSOs. These sources could possibly be YSOs missed in the earlier studies. They are having little or no near-IR excess and some of them are associated with X-ray emission. Optical spectroscopic observations of four of them show signature of Hα in emission. We classify them based on their IR spectral index. We found that these comoving sources are of 1 − 2 Myr in age. Finally, we made a census of YSOs towards HD 20075, which is a Herbig Be star responsible to ionize LDN 1174. We also made near-IR spectoscopic study towards 10 YSOs to obtain a detailed knowledge of star formation in LDN 1172/1174. We estimated their physical properties and found that star formation in this cloud complex started prior to the birth of the central ionzing source HD 200775. Magnetic field plays a significant role in evolution of molecular clouds and regulation of star formation in them. Therefore, we estimated the orientation and strength of the projected magnetic field BPOS towards LDN 1172/1174 complex by optical R-band polarimetric observations. We found that BPOS follows the cloud structure. We compared our findings with the Planck results also and found that the inferred (BPOS) is well corelated in optical R-band and Planck observations. We extend the work done in LDN 1172/1174 on a wider scale in the Cepheus Flare. There are two more Herbig AeBe stars (BD+68◦1118 and PV Cep) within 1.5 ◦ radius of HD 200775 which share similar kinematics. We found an age sequence in the stellar associations from BD+68◦1118 (∼ 10 Myr), HD 200775 (∼ 1 Myr) and PV Cep (∼ 0.1 Myr). Based on the distance and proper motion distribution of the previously known YSOs from Gaia DR2, we found a loosely bound cluster surrounding BD+68◦1118 which contains relatively older cluster members whereas PV Cep is not associated with any cluster. The comoving sources are mainly low-mass M-type sources and have lack of IR excess which is consistent with their ages. The young intermediate-mass sources located in the Perseus OB2 region were observed spectroscopically with high resolution for investigating their spectral types with more accuracy. These sources don’t have nearIR excess and emission in Balmer lines. Most of their ages are lying within 2 − 4 Myr and yet they are having lack of circumstellar disks. The typical disk dispersal timescale for intermediate-mass stars (3 − 7 Myr) is comparable with their ages due to which it is suspected that these sources have aleady shreaded their surrounding disks. We present temperature, extinction, mass, age, and other properties of the observed sources. We also explain their kinematic properties using the latest Gaia DR2 and have found these sources are widely scattered across the region. Some of these sources are kinematically asscoiated with the low-mass YSOs. The intermediate-mass Herbig Ae and Be (HAeBe) stars bridge the gap between low-mass stars, whose formation is fairly well understood and high mass stars, whose formation still poses challenges. Lower mass stars are formed through magnetospheric accretion, whereas high mass stars are thought to form through disk accretion. Linear spectropolarimetry is an excellent tool to probe the circumstellar disks around stars on scales of order stellar radii, i.e. the accretion region of young stars. Now, in case of HBe stars one can observe a depolarization indicaing disk accretion while in HAe stars line polarization can be seen signifying magnetospheric accretion. We acquired spectropolarimetric data of six HAeBe stars and investigated about their accretion mechansims. Based on the optical polarimetric study of the YSOs associated with BRC 18, we found that polarization measurements of these sources are quite random with respect to the cloud’s projected magnetic field. This finding indicates that the circumstellar disks of the YSOs are also oriented arbitrarily. Based on the kinematic properties of the associated YSOs using the latest Gaia DR2, we showed that BRC 18 and most of the other BRCs are moving away from the direction of ionization, which is a signature of the “Rocket Effect”. We have found a strong correlation between the angle of ionizing source with the IRAS source and the proper motion of the YSOs with respect to the celestial north-south axis. In addition, we searched for sources having projected motion similar to the YSOs and could possibly be young sources not detected in earlier surveys. A fraction of them show near-IR excess and similar age distribution as the known YSOs and candidates.