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SDSS

Timothy C. BeersNational Optical Astronomy Observatory

The AEGIS Survey (and more …)

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 disk

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 SkyMapper

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 (a-SSPP)

AEGIS

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]

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

AEGIS -- The Path Forward

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)

Expansion of numbers of identified CEMP stars, in particular with [Fe/H] < —2.5, which include both CEMP-s and CEMP-no stars, both from HK/HES and the ~ 5-10 million medium-res 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)

CEMP Stars -- The Path Forward

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 super-massive 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 pair-instability 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 OtherSDSS J0018-0939 is a cool (Teff ~ 4600) main-sequencestar with [Fe/H] = -2.5, NOT carbon-enhanced, and with elemental-abundance ratios unlike any previously studied very low-metallicity 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)

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

VMS Progenitors -- The Path Forward