The S2 Stream: the shreds of a primitive dwarf galaxy
  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 $\alpha$-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.
