Journal
ASTROPHYSICAL JOURNAL LETTERS
Volume 874, Issue 2, Pages -Publisher
IOP PUBLISHING LTD
DOI: 10.3847/2041-8213/ab0ec0
Keywords
Galaxy: abundances; Galaxy: halo; Galaxy: kinematics and dynamics; Galaxy: stellar content; Galaxy: structure
Categories
Funding
- JSPS [18J11326]
- JSPS KAKENHI [16H02168, 16K05287, 15HP7004]
- Grants-in-Aid for Scientific Research [16H02168, 16K05287, 18J11326] Funding Source: KAKEN
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We report on the very low a-element abundances of a group of metal-poor stars with high orbital energy and with large retrograde motion in the Milky Way halo, whose excess has been reported recently from metallicity and kinematics. We constructed a sample of halo stars with measured abundances and precise kinematics, including similar to 880 stars with [Fe/H] < -0.7, by crossmatching the Stellar Abundances for Galactic Archaeology database to the second data release of Gaia. Three regions in the energy-angular momentum space have been selected: the innermost halo, Gaia Enceladus/Sausage, and the high-energy retrograde halo. While the innermost halo and Gaia Enceladus regions have chemical abundances consistent with high- and low-alpha populations in the halo, respectively, chemical abundances of stars in the high-energy retrograde halo are different from the two populations; their [X/Fe], where X represents Na, Mg, and Ca, are even lower than those in Gaia Enceladus. These abundances, as well as their low mean metallicity, provide a new support for the idea that the retrograde component is dominated by an accreted dwarf galaxy which has a longer star formation timescale and is less massive than Gaia Enceladus/Sausage.
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