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The Most Metal-Poor Stars in the Galaxy

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. Why the Fascination with Large Numbers of MP Stars ?.

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The Most Metal-Poor Stars in the Galaxy

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  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

  3. A Little Nomenclature(see Beers & Christlieb 2005) • [Fe/H] = log(NFe/NH)* - log(NFe/NH)o • Metal-Poor [Fe/H] < -1.0 100,000+ • Very Metal-Poor [Fe/H] < -2.0 20,000+ • Extremely Metal-Poor [Fe/H] < -3.0 400+ • Ultra Metal-Poor [Fe/H] < -4.0 4 • Hyper Metal-Poor [Fe/H] < -5.0 2 • Mega Metal-Poor [Fe/H] < -6.0 0

  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 HES HK Survey Note that while HK survey includes more stars … … HES finds more VMP stars

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

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

  8. Latest and Greatest from SEGUE SDSS/SEGUE N = 16002 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 < 10-15 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 > 15-20 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. 2004 / 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. HERESSurvey Spectra andResults to Date • HERES is based on “snapshot” high-resolution spectroscopy • Neutron-capture-enhanced stars indicated by presence of Eu 4129 • 8 new r-II starswith [r/Fe] ≥ +1.0 • 35 new r-I starswith [r/Fe] ~ + 0.5 • 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] > 1.0 0.5 < [Eu/Fe] < 1.0

  17. A NEW r-II Star with Measured Uranium – HE 1523-0901 (Frebel et al. 2007) V = 11.1 [Fe/H] = -2.95 [r/Fe] = +1.8

  18. Two New r-II Stars (Hayek et al 2007) • CS 29491-069, with [Fe/H] = -2.51, [r/Fe] = +1.1 • HE 1219-0312 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 31082-001 and CS 30336-132 • 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] < -2.0 and [C/Fe] > +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. CEMP-s CEMP-s CEMP-no CEMP-no Differences in [Fe/H] DistributionCEMP-s vs. CEMP-no (Aoki et al. 2006) Does s-process “shut down” at low [Fe/H], or is C from high-mass, rapidly rotating [Fe/H] < -6 stars ?

  22. HE 1305+0132 [Fe/H] = -2.5 [F/Fe] = +2.9 Measurement of Fin 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 !

  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 Cooler, Ultra Metal-Poor Warmer, Slightly Metal-Poor HE 2228-0137: [Fe/H] = -3.60, [C/Fe] = +1.87 HE 2218+0127: [Fe/H] = -1.04, [C/Fe] = +0.24 (J-K)o = 0.67 (J-K)o = 0.36

  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 22876-032; [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. Where is the Field Headed Next? • 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

  35. 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-rphotometric 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] < 0.0

  36. Kinematics at the NGP 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.

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