Chemical Analysis of the Ultrafaint Dwarf Galaxy Grus II. Signature of High-mass Stellar Nucleosynthesis
Hansen, TT, Marshall, JL, Simon, JD, Li, TS, Bernstein, RA, Pace, AB, Ferguson, P, Nagasawa, DQ, Kuehn, K, Carollo, D, Geha, M, James, D, Walker, A, Diehl, HT, Aguena, M, Allam, S, Avila, S, Bertin, E, Brooks, D, Buckley-Geer, E, Burke, DL, Carnero Rosell, A, Carrasco Kind, M, Carretero, J, Costanzi, M, Da Costa, LN, Desai, S, De Vicente, J, Doel, P, Eckert, K, Eifler, TF, Everett, S, Ferrero, I, Frieman, J, Garcia-Bellido, J, Gaztanaga, E, Gerdes, DW, Gruen, D, Gruendl, RA, Gschwend, J, Gutierrez, G, Hinton, SR, Hollowood, DL, Honscheid, K, Kuropatkin, N, Maia, MAG, March, M, Miquel, R, Palmese, A, Paz-Chinchon, F, Plazas, AA, Sanchez, E, Santiago, B, Scarpine, V, Serrano, S, Smith, M, Soares-Santos, M, Suchyta, E, Swanson, MEC, Tarle, G, Varga, TN, Wilkinson, R
Abstract
We present a detailed abundance analysis of the three brightest member stars at the top of the giant branch of the \nultrafaint dwarf (UFD) galaxy Grus II. All stars exhibit a higher than expected [Mg/Ca] ratio compared to metalpoor stars in other UFD galaxies and in the Milky Way (MW) halo. Nucleosynthesis in high-mass ( 20 Me) corecollapse supernovae has been shown to create this signature. The abundances of this small sample (three) stars \nsuggests the chemical enrichment of Grus II could have occurred through substantial high-mass stellar evolution, \nand is consistent with the framework of a top-heavy initial mass function. However, with only three stars it cannot \nbe ruled out that the abundance pattern is the result of a stochastic chemical enrichment at early times in the galaxy. \nThe most metal-rich of the three stars also possesses a small enhancement in rapid neutron-capture (r-process) \nelements. The abundance pattern of the r-process elements in this star matches the scaled r-process pattern of the \nsolar system and r-process enhanced stars in other dwarf galaxies and in the MW halo, hinting at a common origin \nfor these elements across a range of environments. All current proposed astrophysical sites of r-process element \nproduction are associated with high-mass stars, thus the possible top-heavy initial mass function of Grus II would \nincrease the likelihood of any of these events occurring. The time delay between the α and r-process element \nenrichment of the galaxy favors a neutron star merger as the origin of the r-process elements in Grus II.