The Dichotomy of the Galactic Halo and Abundance Patterns in the Most MetalPoor Stars

1
The Dichotomy of the Galactic Halo and Abundance
Patterns in the Most Metal-Poor Stars

• Timothy C. Beers
• Department of Physics Astronomy
• Michigan State University
• JINA Joint Institute for Nuclear Astrophysics

SDSS
2
Outline of Talk

• Part I
• Early results on the halo metallicity
  distribution function from SDSS/SEGUE
• The dichotomy of the Galactic halo, and the
  search for new probes of early nucleosynthesis
• Part II
• New results on light neutron-capture elements
• (Z 47)

3
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

4
New Efforts for Finding Very Metal-Poor Stars

• Stellar observations at medium-resolution have
  been obtained during the course of the Sloan
  Digital Sky Survey (SDSS)
• Calibration of spectrophotometry / telluric bands
• Directed studies (e.g., BHB stars, C-rich stars)
• Failed QSO targets
• New stellar observations being obtained during
  the course of SDSS extension program SEGUE
• See http//www.sdss.org/dr6/start/aboutsegue.html

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SEGUE The Sloan Extension for Galactic
Understanding and Exploration

• Use existing SDSS hardware and software to
  obtain
• 3500 square degrees of additional ugriz imaging
  at lower Galactic latitudes
• Stripes chosen to complement existing areal
  coverage includes several vertical stripes
  through Galactic plane
• Medium-resolution spectroscopy of 250,000
  optimally selected stars in the thick disk and
  halo of the Galaxy
• 200 spectroscopic plate pairs of 45 / 135 min
  exposures
• Objects selected to populate distances from 1 to
  100 kpc along each line of site
• Proper motions available (from SDSS) for stars
  within 5 kpc

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SEGUE uses stellar probes of increasing absolute
brightness to probe increasing distances in the
disk, thick disk and Milky Way halo.
K III
d lt 100 kpc
BHB/BS
d lt 50 kpc
Streams and outer halo stars
MSTO/F
d lt 15 kpc
G
thin, thick disk stars
d lt 6 kpc
Inner and outer halo stars
KV
d lt 1 kpc
r 1.5kpc
Other spectroscopic surveys will not probe as
deep, for instance, Blue Horizontal Branch Stars
(BHBs) from a survey with Vlt 12 are from a
volume within 1.5 kpc of the sun.
8 kpc
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Likely (?) Numbers of Detected MP Stars from SEGUE

• Actual numbers will depend on the shape of the
  halo Metallicity Distribution Function
• Fe/H lt -2.0 20,000 (VMP)
• Fe/H lt -3.0 2,000 (EMP)
• Fe/H lt -4.0 200 ? (UMP)
• Fe/H lt -5.0 20 ? (HMP)
• Fe/H lt -6.0 2 ? (MMP)

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The Low-Metallicity Tail of the Metallicity
Distribution Function of SDSS-I Stars
N 4225 S/N gt 10/1
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The Low-Metallicity Tail of the Metallicity
Distribution Function of SEGUE Stars
N 2414 S/N gt 10/1
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Nature of the Galactic Halo(s) Conclusions First

• 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
  history
• Outer Halo Dominant at R gt 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

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See Carollo et al. 2007 (astro-ph/0706.3005)

• The Dichotomy of the Galactic Halo of the Milky
  Way
• Daniela Carollo, Timothy C. Beers, Young Sun Lee,
  Masashi Chiba, John E. Norris , Ronald Wilhelm,
  Thirupathi Sivarani, Brian Marsteller, Jeffrey A.
  Munn, Coryn A. L. Bailer-Jones, Paola Re
  Fiorentin, Donald G. York

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Galactic Velocity Components

• Proper motions obtained from the re-calibrated
  USNO-B Catalog, typical accuracy 3-4 mas/yr (Munn
  et al. 2004)
• Used in combination with the measured radial
  velocities and estimated distances from the SSPP
  to derive the full space motion components (U, V,
  W) relative to the local standard of rest

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Fe/H vs. V Component
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MDF for Retrograde Stars
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Fe/H vs. Eccentricity / The History
ELS 1962
Fe/H -1.5
Fe/H 0
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The orbital parameters were evaluated adopting a
Galactic potential of the Stackel form (Chiba
Beers 2000 for details). For the first time we
can clearly distinguish the presence of the thick
disk (and MWTD) population as a separate entity
from the local halo(s) stars.
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Flattened Inside / Spherical Outside Inversion
from Kinematics to Density Prediction

• By making simplifying assumptions about nature of
  galactic potential, e.g., that the Jeans theorem
  applies
• One can invert motions to recover the underlying
  density field armchair cartography
• May Binney (1986)
• Sommer-Larsen Zhen (1990)
• Chiba Beers (2000)
• Note progression from flattened to spherical with
  decreasing metallicity

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Towards a Virtual Galaxy Courtesy J. Tumlinson
Work is beginning now to couple stochastic
chemical evolution to dark matter dynamics within
N-body simulations (Gadget2), to calculate
realistic observables in the full 6D
position/velocity and 20D chemical spaces of
modern surveys then full hydro.
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Whats Next ?

• 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 (SEGUE-II)
• 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
  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)

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Ongoing/Planned SDSS/SEGUE Follow-Up Spectroscopy

• Hobby-Eberly Telescope / HRS
• R 15,000 observations of up to 1000 stars with
  Fe/H lt -2.0, and g lt 16.0
• Subaru Telescope / HDS
• R 22,000 observations of up to 200 stars with
  Fe/H lt -2.5 and 16.0 lt g lt 17.0
• Keck II Telescope / ESI
• R 7000 observations of stars with
• Fe/H lt -3.0 and 17.0 lt g lt 18.5

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Part II New Results for Light Neutron-Capture
Elements

• It has been known for some time that the class of
  highly r-process-enhanced stars (r-II) exhibit
  excellent agreement with a pure r-process pattern
  for 56 Z lt 83
• These same stars exhibit very large star-to-star
  scatter in e.g., Eu/Fe, possibly indicating a
  largely unmixed early Galaxy
• Lighter neutron-capture elements, with Z 47,
  e.g.,Sr, Y, Zr, Mo, Rh, Pd, Ag exhibit deviations
  from a scaled (to heavier elements) solar
  r-process pattern, suggesting a different origin
  than heavier elements

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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
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The LEPP of Travaglio et al. (2004)

• At the lowest Fe/H, Ba and Eu are known to be
  dominated by contributions from classical
  r-process, rather than s-process.
• But what of the lighter n-capture elements ?
• Travaglio et al (2004), based on a literature
  sample of Sr, Y, Zr/Ba or Eu, argued that
  dominant n-capture process for these elements
  differs from that of the heavier n-capture
  species.
• Proposed a new primary process, the Lighter
  Element Primary Process, or LEPP, probably
  associated with massive stars.
• Cescutti et al. (2005), and Qian Wasserburg
  (2007) have come to similar conclusions, although
  discussions on the likely source still underway

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A New Sample of Measured Neutron-capture Elements
for VMP and EMP Stars

• Francois et al. (2007)
• Report accurate, homogeneous n-capture abundance
  measurements for 32 VMP stars (Fe/H lt -2.0),
  including 22 EMP stars (Fe/H lt -3.0), based on
  sample from First Stars project of Cayrel et al.
  (2004)
• Determinations, or upper limits for 16 n-capture
  elements
• Sr, Y, Zr, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Dy,
  Ho, Er, Tm, and Yb for all stars
• Identification of trends and scatter for most
  species

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Examples of r-process Scaled Patterns (to Ba)
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Trends of Sr, Y, Zr vs. Fe/H

• Rather similar behavior shown for X/Fe for all
  three elements
• Note only normal stars shown, e.g., no CEMP
  stars
• CS 31082-001 shown
• Note the importance of upper limits at the lowest
  Fe/H, making sense of trends below Fe/H
  -3.0 much more clear
• All three elements are roughly in solar
  proportion above -3.0, clearly lower below -3.0

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Patterns of Sr,Y,Zr/Ba vs. Ba/H

• For exploration of LEPP, Ba/H more natural
  reference than Fe/H
• Note general trend toward rising X/Ba for
  Ba/H lt -2
• Note possible change of behavior at Ba/H
  -5

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Average Sr,Y,Zr/Ba Residuals vs. Ba/H

• Residuals relative to Arlandini et al. (1999)
  Solar System r-process pattern

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Summary and Conclusions

• SDSS/SEGUE is in the process of obtaining up to
  20,000 stars with Fe/H lt -2.0 unclear how many
  will be lt -4.0
• Clarification of an inner/outer structure of the
  halo opens the window toward directed studies of
  likely outer-halo stars, with hopes of greatly
  enlarging the number of known UMP, HMP, and MMP
  stars
• An example shown of the power of the lowest
  metallicity stars to inform on the nature of
  nuclear physics processes operating in the early
  Galaxy
• More data (especially high-resolution follow-up
  data) is needed, and hopefully, will be obtained
  in the near future.