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The Subaru Seminar is
usually held in Room 104 of the Hilo Base Facility, adjacent
to the main lobby. Everyone is welcome to attend. If you are
interested in giving a seminar, please contact Subaru seminar organizers
(Tadayuki Kodama, Kumiko S. Usuda, Naoyuki Tamura, Tomonori Usuda)
by email : sseminar_at_subaru.naoj.org (please change"_at_" to @).
June 28, Tuesday, at 11:00 am
" The formation of the Milky Way stellar halo as traced by chemicalabundances of metal-poor stars "
Miho Ishigaki
(NAOJ, Mitaka)
Chemical abundances in atmospheres of metal-poor stars are well-preserved
quantities from the early Universe, which is one of the only observational
signatures on the ancient stellar and supernovae nucleosynthesis in the
Milky Way Galaxy. Together with phase-space coordinates (position, distance
and 3-d velocity components), chemical abundances in old stars have provided
tremendous implications on the chemodynamical evolution of the early stage
of our Galaxy.
In this talk, I will summarize how chemical compositions of nearby stars
have been used to make constraints on the formation of each structural
component of our Galaxy, namely, the thin disk, thick disk and stellar halo.
Then, I will talk about the state of our ongoing Subaru HDS study on
homogenous chemical abundance analyses of nearby halo stars. Our sample
includes ~90 metal-poor (-4<[Fe/H]<-0.5) dwarf and giant stars located
within a few kilo-persec from the Sun, that show a wide range in orbital
characteristics (e.g. large eccentricity or highly retro-grade motion). In
particular, the sample stars whose orbit reaches the outer part of the
Galactic halo are quite suitable for examining the merging history of our
Galaxy, since remnants of tidally disrupted stellar systems (e.g. dwarf
galaxy), may largely persist in the outer halo. Chemical abundances of
key-elements, including alpha, iron-peak and neutron-capture elements have
been obtained with a LTE abundance analysis code. We show that the estimated
alpha-elements-to-iron abundance ratios ([alpha/Fe]), one of the important
diagnostic for star formation history in a stellar system, show a mild
decreasing gradient with metallicity. This result is broadly consistent with
a hypothesis that the halo stars are partly originated from a system with a
lower star formation rate where delayed enrichments of Fe from Type Ia SNe
are responsible for the decrease in the [alpha/Fe]. Implications on the
building blocks of the Milky Way stellar halo will be discussed based on the
estimated abundances of various key-elements will be presented.