Chemical Fingerprinting and Chemical Analysis of Galactic Halo Substructure

Dr Mei-Yin Chou (Institute of Astronomy & Astrophysics, Academia Sinica)

Abstract
========
We present an investigation of the chemical abundance patterns of
halo substructures using high-resolution spectroscopic measurements. In
particular, we study the abundances of the alpha-like element titanium (Ti)
and the s-process elements yttrium (Y) and lanthanum (La) for M giant
candidates of (a) the Sagittarius (Sgr) dwarf spheroidal (dSph) + tidal tail
system, (b) the Galactic Anticenter Stellar Structure (GASS), also known as
the Monoceros Ring, and (c) the Triangulum-Andromeda (TriAnd) Star Cloud. As
expected, the majority of the Sgr stars show peculiar abundance patterns
compared to those of nominal Milky Way stars, but as a group the stars form
a coherent picture of chemical enrichment of the Sgr dSph from [Fe/H] = -1.4
to solar abundance. The overall [Ti/Fe], [Y/Fe], [La/Fe] and [La/Y] patterns
with [Fe/H] of the Sgr stream plus Sgr core do, for the most part, resemble
those seen in the Large Magellanic Cloud (LMC) and other current Milky Way
satellites, only shifted by Delta[Fe/H] ~ +0.4 from the LMC and by ~+1 dex
from Milky Way dSph satellites; these relative shifts reflect the faster
and/or more efficient chemical evolution of Sgr compared to the other
satellites, and show that Sgr has had an enrichment history more like the
LMC than the dSph satellites. We also apply "chemical fingerprinting" to the
GASS/Monoceros Ring and TriAnd Star Cloud, to explore the origins of the two
systems and the hypothesized connections between them. GASS has been debated
either to originate from the Galactic accretion of a satellite creating a
tidal stream, or as a part of the disk, dynamically induced through warping
or resonances, etc. Our exploration shows that GASS is indeed made of stars
from a dSph, and that it is distinct in chemistry from outer disk stars. And
whereas the TriAnd Star Cloud has been assumed to come from the tidal
disruption of the same accreted Milky Way satellite as the GASS/Monoceros
Ring, our comparison of the abundance patterns in Monoceros and TriAnd M
giants suggests that the TriAnd Star Cloud is likely an independent halo
substructure unrelated to Monoceros.