The R-Process Alliance: Discovery of a Low-α, r-process-enhanced Metal-poor Star in the Galactic Halo
Grebel, Eva K.; Steinmetz, Matthias; Beers, Timothy C.; Placco, Vinicius M.; Bland-Hawthorn, Joss; Wyse, Rosemary F. G.; Kunder, Andrea; Kordopatis, Georges; Chiappini, Cristina; Matijevič, Gal; Helmi, Amina; Gibson, Brad K.; Freeman, Kenneth C.; Seabroke, George; Reid, Warren; Watson, Fred; Venn, Kim A.; Roederer, Ian U.; Ezzeddine, Rana; Sakari, Charli M.; Frebel, Anna; Navarro, Julio; Wallerstein, George; Sneden, Christopher; Cowan, John J.; Hansen, Terese; Farrell, Elizabeth M.
United States, Canada, Germany, Australia, United Kingdom, Netherlands, France
Abstract
A new moderately r-process-enhanced metal-poor star, RAVE J093730.5-062655, has been identified in the Milky Way halo as part of an ongoing survey by the R-Process Alliance. The temperature and surface gravity indicate that J0937-0626 is likely a horizontal branch star. At [Fe/H] = -1.86, J0937-0626 is found to have subsolar [X/Fe] ratios for nearly every light, α, and Fe-peak element. The low [α/Fe] ratios can be explained by an ∼0.6 dex excess of Fe; J0937-0626 is therefore similar to the subclass of “iron-enhanced” metal-poor stars. A comparison with Milky Way field stars at [Fe/H] = -2.5 suggests that J0937-0626 was enriched in material from an event, possibly a Type Ia supernova, that created a significant amount of Cr, Mn, Fe, and Ni and smaller amounts of Ca, Sc, Ti, and Zn. The r-process enhancement of J0937-0626 is likely due to a separate event, which suggests that its birth environment was highly enriched in r-process elements. The kinematics of J0937-0626, based on Gaia DR2 data, indicate a retrograde orbit in the Milky Way halo; J0937-0626 was therefore likely accreted from a dwarf galaxy that had significant r-process enrichment.