The Galactic Halo - Anglo

The AEGIS Survey
(and more …)
Timothy C. Beers
National Optical Astronomy Observatory
The AAOmega Evolution of Galactic
Structure (AEGIS) program aims:
Constrain the chemodynamical
evolution of the Milky Way through the
study of the halo and the outer thick
Determine the metal content,
kinematics, and chemical abundance
ratios for 70,000 stars over 4,900 deg2
using the AAOmega multi-object
spectrograph on the 3.9m Australian
Astronomical Telescope
Has been underway at the AAT for the past
~2.5 years, completed its intended
acquisition of stars, targeted from
Determination of stellar atmospheric
parameters (Teff, logg, [Fe/H]), as well as
[C/Fe] and [α/Fe], and accurate radial
velocities, making use of a modified version
of the SEGUE Stellar Parameter Pipeline
Targeted Fields
Target fields circa 1 year ago, about 60,000 stars /
now have ~ 70,000
AEGIS – Typical Spectra
AEGIS – Typical Spectra
AEGIS – Comparison with SkyMapper
Estimate of [Fe/H]
AEGIS – Distribution of Radial Velocities
AEGIS – Distribution of log g vs. Teff
AEGIS – Distribution of [Fe/H]
AEGIS – Distribution of V_GSR vs. [Fe/H]
AEGIS – Distribution of [C/Fe] vs. [Fe/H]
AEGIS -- The Path Forward
Complete visual inspection of AEGIS spectra, kicking out
problematic parameter estimates, optimize metallicities
Proper motions from UCAC4, SPM 4, PPMXL have been
gathered (about 80-90% of stars)
Combine PMs, RVs, and distance estimates in order to obtain
full space motions
Examination of full space motions, in combination with
chemistry ([Fe/H], [C/Fe], [/Fe]) in order to deduce
properties of the various Galactic components
High-resolution follow-up of most interesting EMP and
CEMP stars
New CEMP + VMP Star Survey Summary
 Placco, Beers, Kennedy et al. have been using “bad weather” time on the
Gemini N and S telescopes to search for NEW examples of CEMP and VMP
stars chosen from the HK and HES candidates
 Numerous examples of new CEMP stars found by targeting on the G-band
strength of scanned HES stars
 By taking advantage of the apparently strong correlation between large
C over-abundances and declining [Fe/H], rather than on the weakness
of the CaII K line for metal weakness, and obtaining C information later
from medium-res spectroscopic follow-up
 Numerous examples of new VMP stars found by targeting on previously
unobserved HK and HES candidates
 CEMP survey recently completed (~ 900 spectra / ~300 new CEMP stars)
 VMP survey just getting underway
 High-resolution work (AAT, Magellan, VLT/X-Shooter) – Just Starting
[C/Fe] vs. [Fe/H] (Medium-Res Results)
CEMP Stars -- The Path Forward
Expansion of numbers of identified CEMP stars, in particular
with [Fe/H] < —2.5, which include both CEMP-s and CEMPno stars, both from HK/HES and the ~ 5-10 million mediumres spectra coming from LAMOST
High-resolution follow-up spectroscopy of a core sample of
100-200 CEMP stars, in order to assign classifications based
on heavy elements, and to determine CNO, alpha elements,
and other light element abundances, in particular Be and Li
Radial velocity monitoring of CEMP stars, in order to
determine binary nature, as well as characterize
correlations between chemical patterns and nature of the
detected binary -- already begun (Hansen et al., 2013, and
in preparation)
Aoki et al. (Science, in press)
A Chemical Signature of First-Generation
Very-Massive Stars
W. Aoki, N. Tominaga, T. C. Beers, S. Honda, Y. S. Lee
Abstract: Numerical simulations of structure formation in the early Universe
predict the formation of some fraction of stars with masses several hundred
times the solar mass. No clear evidence of supernovae from such supermassive stars has, however, yet been found in the chemical compositions of
Milky Way stars. Here we report on an analysis of a very metal-poor star, SDSS
J001820.5−093939.2, which possesses elemental-abundance ratios that differ
significantly from any previously known star. This star exhibits low
[α-element/Fe] ratios and large contrasts between the abundances of odd and
even element pairs, such as Sc/Ti and Co/Ni. Such features have been
predicted by model calculations of the nucleosynthesis associated with a pairinstability supernova of a 130-260 solar-mass star, or a core-collapse
supernova of an even more massive star. The result suggests that the mass
distribution of first-generation stars might extend to 100 solar masses or larger.
Abundance Patterns Like No Other
SDSS J0018-0939 is a cool
(Teff ~ 4600) main-sequence
star with [Fe/H] = -2.5, NOT carbonenhanced, and with elementalabundance ratios unlike any
previously studied very lowmetallicity star.
Abundance ratios between adjacent
odd- and even-element pairs are
very low: [Na/Mg] = -0.56,
[Sc/Ti] < -0.99, [Co/Ni] = -0.77.
In addition, the n-capture elements
are quite low compared to other VMP
stars: [Sr/Fe] < -1.8, [Ba/Fe] < -1.3.
Frequency (~ 1/500) similar to
frequency of high-mass progenitors
predicted by Karlsson et al. (2008)
Alternative Supernova Models
 Core-collapse SN, M = 25 Mo
 Type Ia SN + core-collapse SN,
M = 25 Mo
 PISN, M = 130 Mo (black)
 Super-massive star,
M = 1000 Mo (purple)
VMS Progenitors -- The Path Forward
Establishment of the frequency of such objects as SDSS
J0018-0939, based on high-resolution spectroscopic surveys
of the many thousands of stars known with [Fe/H] < -2.5
Refinement of model-based SN abundance patterns for PISN
and very-massive stars
Full numerical GCE models, taking into account the effects
of local mixing, in order to match frequencies of CEMP stars
and C-normal stars, as well as Li-depletion phenomenon for
very metal-poor stars

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