Tracing the origin of the Aquarius Stream: I
  We present an abundance analysis of six member stars of the recently
discovered Aquarius stream, in an attempt to ascertain whether this halo stream
is real and, if so, to understand its origin. The mean metallicities of the six
stars have a dispersion of only 0.10 dex, indicating that they are part of a
chemically coherent structure. We then investigate whether the stream
represents the debris of a disrupted dwarf galaxy or a disrupted globular
cluster. The [Ni/Fe] - [Na/Fe] plane provides a good diagnostic: globular
cluster stars and dwarf spheroidal galaxy stars are well separated in this
plane, and the Aquarius stream stars lie unambiguously in the globular cluster
region. The Aquarius stream stars also lie on the distinct [Na/Fe] - [O/Fe] and
[Mg/Fe] - [Al/Fe] relations delineated by Galactic globular cluster stars.
Spectroscopic parameters for the six Aquarius stars show that they are tightly
confined to a 12 Gyr, [Fe/H] = -1.0, alpha-enhanced isochrone, consistent with
their identification as globular cluster debris. We present evidence that the
Aquarius stream may continue through the disk and out into the northern halo.
Our results indicate a globular cluster origin for the Aquarius stream, and
demonstrate the potential for chemical tagging to identify the origins of
Galactic substructures.
