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Upcoming Seminar/Colloqium


Study of the stellar relics and their origin in the MW halo

Speaker : Avrajit Bandyopadhyay
Affiliation : ARIES, Nainital
Date & Time :
Venue : Zoom and auditorium
Abstract :

We present a study on the detailed elemental abundances and kinematics of the metal-poor stars and stellar systems in the halo. The bright halo stars were selected for high-resolution spectroscopic follow-up through HESP. We discuss the abundances detected for several stars in the survey, which include main-sequence stars, subgiants, and red giants. The stars show typical alpha-element enhancements and the odd-even nucleosynthesis pattern for the light elements. The Fe-peak elements mostly track the iron content; the observed trends are consistent with other metal-poor stars. The depletion of Li is observed for the metal-poor stars as they ascend the giant branch. The trends for depletion in Li with temperature could be quantified; we have derived a scaling relation for the depletion of Li as a function of temperature for giants and main-sequence stars. The slope of the Spite plateau at the metal-poor end was also found to vanish when the Li abundance was studied as a function of distance from the galactic plane, indicating that the Li-rich stars are not preferentially located at larger distances from the Galactic plane. We have also used astrometric parameters from Gaia-EDR3 to complement our study, and derived kinematics to differentiate between the motions of the stars; those formed in situ and accreted. The orbits for the program stars have also been derived and studied backwards in time to trace the origin. Overall, the different classes of stars are found to exhibit very similar distributions of Li, which points towards a common origin and mixed stellar populations in the halo. 

Globular clusters are expected to lose a large amount of stellar mass during their migration and evolution in the Halo. We are using Gaia-EDR3 to study the loss of stars from the Galactic GCs and quantify the contribution of GCs to the formation of the halo. Along with the standard procedure of tagging GC stars in the halo, we have also initiated a study to identify and study stars of GC origin in the annular space outside the individual GCs. We select candidates in the outskirts of the cluster using proper motions, CMDs and distances as key parameters to determine the likely targets for GC escapees. The selected annular region in the sky just outside the cluster radius is expected to show an over-density of the GC stars than the normal halo field. When ejected from the cluster, these stars could be tagged using spectroscopy. RR Lyrae (RRL) are another group of old, pulsating variable stars found in the horizontal branch of old stellar systems like dwarf satellite galaxies and globular clusters. They are extremely important to understand the accretion history and Galaxy formation models. Here we discuss the study of RRL stars in the Ursa Minor dwarf satellite galaxy. For the variables within the proposed test fields, we have performed PSF photometry along with aperture correction, saturation correction, and zero-point magnitude to get the final magnitudes of the detected stars. We cross-match the data with Gaia-EDR3 to remove the field contamination using proper motion and parallax criteria. In the process, we could detect several new variable stars. The detailed census would provide valuable insights into the characterization of these stellar systems and the formation history of the halo.

About Speaker :

Dr. Avrajit Bandyopadhyay is currently a postdoc at ARIES. This seminar is part of his annual progress evaluation.