1 The Most Metal-Poor Stars in the Galaxy Timothy C. Beers Department of Physics & Astronomy Michigan State University & JINA: Joint Institute for Nuclear Astrophysics SDSS

2 Why the Fascination with Large Numbers of MP Stars ? Extremely MP stars have recorded the heavy element abundances produced in the first generations of stars The shape of the low-metallicity tail of the Metallicity Distribution Function (MDF) will (eventually) show structure that reveals the characteristic abundances of major epochs of star formation in early Galaxy Change in the nature of the MDF as a function of distance may reveal the assembly history of the MW Determination of the frequency of various elemental abundance signatures, e.g., enhancement of [C/Fe], [alpha/Fe], etc. Identification of relatively rare objects amongst MP stars, e.g., r-process / s-process enhanced stars

A Little Nomenclature (see Beers & Christlieb 2005) [Fe/H] = log(N Fe /N H ) * - log(N Fe /N H ) o Metal-Poor [Fe/H] < ,000+ Very Metal-Poor [Fe/H] < ,000+ Extremely Metal-Poor [Fe/H] < Ultra Metal-Poor [Fe/H] < Hyper Metal-Poor [Fe/H] < Mega Metal-Poor [Fe/H] <

4 Past Efforts Have Now Identified Significant Numbers of Very ([Fe/H] < -2.0) and Extremely ([Fe/H] < -3.0) Metal-Poor Stars HK Survey of Beers and colleagues Hamburg/ESO Survey of Christlieb and colleagues Stellar observations obtained during the course of: –the original Sloan Digital Sky Survey (SDSS) –and its extension (SDSS-II), which includes –SEGUE: Sloan Extension for Galactic Understanding and Exploration

5 MDFs from Medium-Resolution Efforts To Date HK Survey HES Note that while HK survey includes more stars … … HES finds more VMP stars

6 Adding HK + HES Together N = 9669 < -2 = 2756 < -3 = 380 Note potential “bumps” in combined MDF at -2.2 and -3.0

7 The Low-Metallicity Tail of the MDFs for SDSS-I and SEGUE Stars SDSS-I N = 4225 S/N > 10/1 SEGUE N = 2414 S/N > 10/1 Represents about 1/3 rd of VMP stars to be found in SDSS/SEGUE

Latest and Greatest from SEGUE 8 SDSS/SEGUE N = S/N > 10/1

9 Nature of the Galactic Halo(s) (see Carollo et al. 2007, Nature 450,1022) The structural components of the stellar populations in the Galaxy have been known for (at least) several decades: –Bulge / Thin Disk / Thick Disk (MWTD) / Halo New results from SDSS have now revised this list: –Halo  Halos –Inner Halo: Dominant at R < kpc Highly eccentric (slightly prograde) orbits Metallicity peak at [Fe/H] = -1.6 Likely associated with major/major collision of massive components early in galactic history –Outer Halo: Dominant at R > kpc Uniform distribution of eccentricity (including highly retrograde) orbits Metallicity peak around [Fe/H] = -2.2 Likely associated with accretion from dwarf-like galaxies over an extended period, up to present

10 [Fe/H] vs. V Component

11 Implications One can now target outer-halo stars in order to elucidate their chemical histories ([α/Fe], [C/Fe]), and possibly their accretion histories One can now preferentially SELECT outer-halo stars based on proper motion cuts in the local volume (SDSS-III/SEGUE-2) One can now take advantage of the lower [Fe/H], in general, of outer-halo stars to find the most metal- poor stars (all three stars with [Fe/H] < -4.5 have properties consistent with outer halo membership) One can soon constrain models for formation / evolution of the Galaxy that take all of the chemical and kinematic information into account (e.g., Tumlinson 2006)

12 What is Required Now ? Need to collect statistical samples, based on medium- and high-resolution follow-up of both [Fe/H] < -2.0 and < -3.0 stars in order to address: –Definitive trends and scatters of Li, CNO, alpha- element, iron-peak, and heavy elements –Correlations of the elemental properties with kinematics and location –Connections, or not, with presently observable dwarf galaxies, in particular the low-luminosity dwarf spheroidal galaxies identified by SDSS-II imaging

13 Progress is Being Made! HERES (Christlieb et al / Barklem et al. 2005) New r-II stars with high-resolution abundance studies (Hayek et al. 2007) First Stars n-capture element study (Francois et al. 2007) Subaru studies of CEMP stars (Aoki et al. 2006, 2007) F observations of CEMP stars with Gemini/Phoenix (Schuler et al. 2007) CNO survey of CEMP stars from SOAR/OSIRIS (Beers et al. 2007) Li observations (Bonifacio / Gonzalez-Hernandez / Aoki et al. 2007) CASH Survey follow-up from HET (Frebel / Beers et al., in progress ) UMP Survey from Subaru / Keck / Magellan (Norris/Aoki et al., in progress)

14 The Power Of Large N: 274 Stars from HERES

15 HERES Survey Spectra and Results to Date HERES is based on “snapshot” high- resolution spectroscopy Neutron-capture-enhanced stars indicated by presence of Eu new r-II stars with [r/Fe] ≥ new r-I stars with [r/Fe] ~ The apparent frequency of r-II stars is ~ 5 % of giants with [Fe/H] < -2.5

16 Distribution of [Fe/H] for R-process Enhanced Stars from HERES (Barklem et al. 2005) [Eu/Fe] > < [Eu/Fe] < 1.0

17 A NEW r-II Star with Measured Uranium – HE (Frebel et al. 2007) V = 11.1 [Fe/H] = [r/Fe] = +1.8

18 Two New r-II Stars (Hayek et al 2007) CS , with [Fe/H] = -2.51, [r/Fe] = +1.1 HE with [Fe/H] = -2.96, [r/Fe] = +1.5 Both appear to exhibit the “actinide boost” phenomenon (Th/Eu, U/Eu unusually high) –Seen also in CS and CS –Of 12 r-II stars with published analysis, about 30% exhibit actinide boosts –Why ?

19 Examples of r-process Patterns for “normal” VMP Stars (Francois et al. 2007)

20 Carbon-Enhanced Metal-Poor Stars Stars with [Fe/H] +1.0 –At least 20% of ALL stars with [Fe/H] < -2.0 are CEMP –At least 40% of ALL stars with [Fe/H] < -3.5 are CEMP –100% of three stars with [Fe/H] < -4.0 are CEMP Different patterns of n-capture elements –r-process rich (AGB mass-transfer) –r/s-process rich (??) –No n-capture (??) –r-process rich (SN progenitors ?) CEMP stars at MSTO particularly valuable –Probably preserve totality of mass from intermediate mass (2-7 Mo companion) –[C/Fe] ratio reflects that of companion

21 Differences in [Fe/H] Distribution CEMP-s vs. CEMP-no (Aoki et al. 2006) CEMP-s CEMP-no CEMP-s CEMP-no Does s-process “shut down” at low [Fe/H], or is C from high-mass, rapidly rotating [Fe/H] < -6 stars ?

22 Measurement of F in CEMP Stars (Schuler et al. 2007) First detection of F in a CEMP star (using Gemini/Phoenix Can help distinguish between origins of elements in CEMP stars –AGB –Primordial massive stars –Both ! HE [Fe/H] = -2.5[F/Fe] = +2.9

23 SDSS Spectra of CEMP TO Stars

24 Close-Up View

25 Observations with Subaru/HDS (Aoki et al. 2007) R = 60,000 S/N ~ 30/1-70/1 per pixel Typically at least two exposures to check for rapid radial velocity changes

26 Membership in the Outer Halo Kinematic properties suggest dominance by outer halo

27 CEMP-IMF-CMB (Tumlinson 2007) As expected for a CMB-dependent IMF, the fraction of CEMP stars is high at low metallicity, and declines with increasing [Fe/H] as the typical formation redshift of EMPs declines.

28 Medium-Resolution Spectra of Sample CEMP Stars HE : [Fe/H] = -3.60, [C/Fe] = HE : [Fe/H] = -1.04, [C/Fe] = (J-K)o = 0.67 (J-K)o = 0.36 Cooler, Ultra Metal-Poor Warmer, Slightly Metal-Poor

29 Measurements of Nitrogen Strongest line of N is a NH molecule at 3360 A, which requires large telescopes with blue- sensitive spectrographs (feasible with SOAR 4.1m and Goodman spectrograph)

30 Measurements of Oxygen in Carbon-Enhanced Stars Estimates of the C and N abundances can often be obtained from observations in the optical region using moderate-sized telescopes. O measurements in the optical usually requires 8m- 10m class telescopes. By observing in the near- infrared with SOAR/OSIRIS, the abundance of O can be inferred due to its influence on the strong CO molecular bands that are visible in this portion of the spectrum.

31 New Li Measurements for EMP Stars ([Fe/H] < -3.0) Bonifacio et al. (2007)

32 Lithium in CS ; [Fe/H] = -3.6

33 Where is the Field Headed Next? Near Term (next 1-3 years) –Completion of CNO survey for 100 CEMP stars –Completion of CASH (Chemical Abundances of Stellar Halo) with HET (N = 1000 stars with [Fe/H] < -2.0) –Additional measurements of F in CEMP stars (Gemini/VLT) –Additional measurements of Li in VMP/EMP stars (VLT/Subaru) –Completion of medium-res spectra for 500,000 stars with SEGUE-II

34 Far Term (next 5-10 years) –Completion of medium-res survey for 2,500,000 stars with LAMOST –Completion of high-res near-IR survey of 100,000 stars with APOGEE (SDSS-III) –Completion of high-res optical survey of 1,000,000 stars with WFMOS (Subaru) –Coupling of abundance measurements with proper motions and parallaxes from GAIA Where is the Field Headed Next?

A Metallicity Map of the Milky Way Based on the spectroscopic determinations of atmospheric parameters from the SSPP, one can calibrate a u-g vs. g-r photometric estimator of [Fe/H] For main-sequence F and G stars Accuracy on the order of 0.25 dex –Set by photometric errors of a few percent –Covers region -2.0 < [Fe/H] <

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Kinematics at the NGP 38 By choosing directions close to the NGP, the proper motions (obtained from a re-calibration of the USNO-B catalog) sample only the U and V velocity components. This enables determination of the rotational properties for Galactic components as a function of distance and metallicity This map shows results for some 60,000 stars.