Chemical-abundance patterns of disrupted dwarf galaxies

Abstract

Within this ongoing FAPESP-funded PhD (Proc. 2021/10429-0), we are studying the chemical and dynamical (or simply "chemodynamical") structure of the ancient Galactic halo and the properties of its stellar populations, providing clues about the sequence of merging events that took place throughout the history of the Milky Way. Here, we propose a one-year Research Internship Abroad (RIA) at the University of Chicago/USA under the supervision of Prof. Dr. Alexander P. Ji. We propose to investigate the abundance patterns of very metal-poor ([Fe/H] < -2.0) stars associated with a pair of halo substructures, namely the Helmi stream and Sequoia. These are the debris of dwarf galaxies accreted by the Milky Way in the past and, hence, provide a window into the properties of such low-mass systems that existed at high redshift (z > 1), prior to their infall into the Galactic halo. Large-scale multi-element spectroscopic surveys do not cover the VMP regime and this RIA project is designed to be complementary to such endeavors. Over the course of 2021, we (P.I. Limberg) collected high-resolution (R = 40,000) spectra of 22 dynamically vetted (Limberg et al. 2021a) members of Helmi stream and Sequoia (11 stars each) with the GRACES instrument at Gemini North (8.1 m) with available [Fe/H] values (i.e., confirmed to be VMP; Limberg et al. 2021b). This newly acquired data will be utilized in the determination of precise abundances of both light (e.g., alpha and iron-peak) and heavy (neutron-capture) elements. Our specific goals are (i) to estimate the true level of ± enrichment of VMP stars in Helmi stream, (ii) to test whether or not Sequoia is chemically distinguishable from the bulk of the Milky Way's halo, (iii) to measure other light elements useful for separating accreted from in situ populations, and (iv) to verify if VMP stars from these disrupted dwarf galaxies show enhancements in neutron-capture elements. We expect this RIA period to result in at least two papers to be submitted to high-impact peer-reviewed journals. (AU)