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Upcoming Seminar/Colloqium

Topic:

Identification and Characterization of Young Stars in Star Forming Regions

Speaker : Piyali Saha
Affiliation : ARIES
Date & Time :
Venue : Auditorium
Abstract :

Stars are born within highly obscured molecular clouds and grow through rapid contractions and violent ejections. In order to unlock the star formation mysteries, we require vigorous use of advanced techniques and the opening of various wavelength regimes, for example, photometry, spectroscopy, astrometry, polarimetry, spectropolarimetry in optical, infrared (IR), millimeter and sub-millimeter wavebands. Based on different methodologies, young stellar objects (YSOs) can be identified and characterized to study the star formation history in the molecular clouds. In this thesis, I identified YSOs based on different methodologies and characterized them based on their properties. I made a detailed study of a cloud complex LDN 1172/1174, located in the Cepheus
Flare which harbors a bright Herbig Be star, HD 200775, illuminating the reflection nebula, NGC 7023, located at LDN 1174. Based on the Gaia DR2 and polarimetry (using 1.04-m Sampurnanand Telescope, ARIES), I estimated the distance to this cloud complex. Based on the distances and proper motions of the known YSOs, I found additional sources comoving with the known YSOs. Near-IR spectroscopy of some of the faint and heavily reddened known YSOs using the NASA IRTF telescope suggests them to be low accreters and hence older than HD 200775 star. It further suggests that star formation was ongoing in the cloud prior to the formation of HD 200775 and presently the star is in the process of dispersing the material surrounding it. Using optical polarimetry, I also made magnetic field geometry of the entire complex and estimated the strength of the projected magnetic field. I estimated a higher star formation rate per unit mass for LDN 1172/1174 which is consistent with the same for the clouds with magnetic field lines oriented parallel with their long axes. The presence of three more Herbig Ae/Be stars, along with other low mass stars were found in the vicinity of HD 200775 which indicates that the star formation is prevalent on a wider region in the clouds LDN 1147/1158, LDN 1172/1174, and LDN 1177. I carried out a high-resolution spectroscopic study of 14 young intermediate-mass stars located in the Perseus OB2 region. Absorption in Balmer lines in all the observed spectra signifies the absence of circumstellar disks surrounding these sources. A majority of these sources are found to be of 2−4 Myr age, which is comparable with the typical disk dispersal timescale of the intermediate-mass stars. I found several diffuse interstellar bands in their spectrum which are characterized as a function of distance and extinction. I made spectropolarimetry of six HAeBe stars using the SALT telescope to know their accretion mechanism. I computed the continuum degree of polarization and position angle of
each star across the Hα and Hβ lines. By combining the results from my work with those from the literature, I made a statistical study to understand the accretion process as a function of spectral type. Using the distance and the proper motion measurements obtained from the Gaia DR2 of the sources identified as YSOs in the vicinity of BRCs, I determined the projected motion of the YSOs and hence the BRCs on the sky plane by assuming that both are kinematically coupled. The relative proper motions of the YSOs associated with the BRCs are found to show a trend of them moving away from the direction of ionizing sources. The results indicate that the YSOs and hence the BRCs are most likely accelerating away from the ionizing source due to the “Rocket Effect”. I also found several sources lying in the vicinity of several BRCs which are kinematically associated with the previously known YSOs and therefore, could be YSO candidates unidentified in earlier studies.

About Speaker :

Piyali Saha, working under Maheswar Gopinathan