The S2 stream: The shreds of a primitive dwarf galaxy

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Aguado, DS 
Myeong, GC 
Evans, NW 
Koposov, SE 

The S2 stream is a kinematically cold stream that is plunging downwards through the Galactic disc. It may be part of a hotter and more diffuse structure called the Helmi stream. We present a multi-instrument chemical analysis of the stars in the metal-poor S2 stream using both high- and low-resolution spectroscopy, complemented with a re-analysis of the archival data to give a total sample of 62 S2 members. Our high-resolution program provides alpha-elements (C, Mg, Si, Ca and Ti), iron-peak elements (V, Cr, Mn, Fe, Ni), n-capture process elements (Sr, Ba) and other elements such as Li, Na, Al, and Sc for a subsample of S2 objects. We report coherent abundance patterns over a large metallicity spread (~1 dex) confirming that the S2 stream was produced by a disrupted dwarf galaxy. The combination of S2's α-elements displays a mildly decreasing trend with increasing metallicity which can be tentatively interpreted as a ``knee'' at [Fe/H]<-2. At the low metallicity end, the n-capture elements in S2 may be dominated by r-process production however several stars are Ba-enhanced, but unusually poor in Sr. Moreover, some of the low-[Fe/H] stars appear to be carbon-enhanced. We interpret the observed abundance patterns with the help of chemical evolution models that demonstrate the need for modest star-formation efficiency and low wind efficiency confirming that the progenitor of S2 was a primitive dwarf galaxy.

stars: abundances, Galaxy: evolution, Galaxy: formation, Galaxy: halo, Galaxy: kinematics and dynamics
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Monthly Notices of the Royal Astronomical Society
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Oxford University Press (OUP)
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Science and Technology Facilities Council (ST/L005174/1)
Science and Technology Facilities Council (ST/N000927/1)
Science and Technology Facilities Council (ST/N005805/1)
Science and Technology Facilities Council (ST/S000623/1)
STFC (ST/T003081/1)