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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 – PowerPoint PPT presentation

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Title: 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

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
  • 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

A Little Nomenclature(see Beers Christlieb
  • Fe/H log(NFe/NH) - log(NFe/NH)o
  • Metal-Poor Fe/H lt -1.0 100,000
  • Very Metal-Poor Fe/H lt -2.0 20,000
  • Extremely Metal-Poor Fe/H lt -3.0 400
  • Ultra Metal-Poor Fe/H lt -4.0 4
  • Hyper Metal-Poor Fe/H lt -5.0 2
  • Mega Metal-Poor Fe/H lt -6.0 0

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

MDFs from Medium-Resolution Efforts To Date
HK Survey
Note that while HK survey includes more stars
finds more VMP stars
Adding HK HES Together
Note potential bumps in combined MDF at -2.2
and -3.0
The Low-Metallicity Tail of the MDFs for SDSS-I
and SEGUE Stars
SDSS-I N 4225 S/N gt 10/1
Represents about 1/3rd of VMP stars to be found
Latest and Greatest from SEGUE
SDSS/SEGUE N 16002 S/N gt 10/1
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 lt 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
  • Outer Halo Dominant at R gt 15-20 kpc
  • Uniform distribution of
    (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

Fe/H vs. V Component
  • One can now target outer-halo stars in order to
    elucidate their chemical histories (a/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 lt
    -4.5 have properties consistent with outer halo
  • 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)

What is Required Now ?
  • Need to collect statistical samples, based on
    medium- and high-resolution follow-up of both
    Fe/H lt -2.0 and lt -3.0 stars in order to
  • 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

Progress is Being Made!
  • HERES (Christlieb et al. 2004 / Barklem et al.
  • 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,
  • 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)

The Power Of Large N 274 Stars from HERES
HERES Survey Spectra andResults to Date
  • HERES is based on snapshot high-resolution
  • Neutron-capture-enhanced stars indicated by
    presence of Eu 4129
  • 8 new r-II stars with r/Fe 1.0
  • 35 new r-I stars with r/Fe 0.5
  • The apparent frequency of r-II
  • stars is 5 of giants with Fe/H lt -2.5

Distribution of Fe/H for R-process Enhanced
Stars from HERES (Barklem et al. 2005)
Eu/Fe gt 1.0
0.5 lt Eu/Fe lt 1.0
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
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 ?

Examples of r-process Patterns for normal VMP
Stars (Francois et al. 2007)
Carbon-Enhanced Metal-Poor Stars
  • Stars with Fe/H lt -2.0 and C/Fe gt 1.0
  • At least 20 of ALL stars with Fe/H lt -2.0 are
  • At least 40 of ALL stars with Fe/H lt -3.5 are
  • 100 of three stars with Fe/H lt -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

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 lt -6
stars ?
Measurement of F in CEMP Stars (Schuler et al.
  • First detection of F in a CEMP star (using
  • Can help distinguish between origins of elements
    in CEMP stars
  • AGB
  • Primordial massive stars
  • Both !

SDSS Spectra of CEMP TO Stars
Close-Up View
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

Membership in the Outer Halo
Kinematic properties suggest dominance by outer
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.
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 22180127 Fe/H -1.04, C/Fe 0.24
(J-K)o 0.67

(J-K)o 0.36
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)

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

New Li Measurements for EMP Stars (Fe/H lt -3.0)
Bonifacio et al. (2007)
Lithium in CS 22876-032 Fe/H -3.6
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 lt -2.0)
  • Additional measurements of F in CEMP stars
  • Additional measurements of Li in VMP/EMP stars
  • Completion of medium-res spectra for 500,000
    stars with SEGUE-II

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

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 lt Fe/H lt 0.0

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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.