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Mining the Milky Way Galaxy with SDSS. Timothy Beers Dept. of Physics & Astronomy and JINA: Joint Institute for Nuclear Astrophysics Michigan State University. The Sloan Digital Sky Survey. The most ambitious astronomy project ever undertaken

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Mining the milky way galaxy with sdss

Mining the Milky Way Galaxy with SDSS

Timothy Beers

Dept. of Physics & Astronomy and

JINA: Joint Institute for Nuclear Astrophysics

Michigan State University

The sloan digital sky survey
The Sloan Digital Sky Survey

  • The most ambitious astronomy project ever undertaken

    • Obtain accurately calibrated imaging of 10,000 square degrees of (northern) sky, in five filters (ugriz)

    • Obtain moderate-resolution spectroscopy for

      • 1,000,000 galaxies

      • 100,000 quasars

  • Has been fully operational since ~ Jan 1999

  • Scheduled to complete (or nearly so) its primary imaging mission in July 2005

  • In many ways, a precursor of similar surveys to follow

The telescope and detectors
The Telescope and Detectors

ARC 2.5m SDSS Telescope (3 deg FOV)

The sdss imaging camera
The SDSS Imaging Camera

The actual camera and a block

diagram of the CCD layout

The sdss scrolling sky
The SDSS Scrolling Sky

The sdss spectrograph plug plate
The SDSS Spectrograph Plug Plate

Identification of targets on the sky A prepped and drilled plate

The sdss spectrograph s
The SDSS Spectrograph(s)

Two spectrographs, with spectral

coverage from 3800 – 9000 A

Segue the s loan e xtension for g alactic u nderstanding and e xploration
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

SEGUE observing plan and status as of February 2005

SDSS Imaging scan

Declination = -20 degrees

Planned SEGUE grid pointings (140)

Planned SEGUE scan (3500 sq deg)

Planned targeted SEGUE pointings(60)

Sgr stream planned scan

Completed SEGUE imaging

Completed SEGUE plate pointing

The limits of the sdss pipeline
The Limits of the SDSS Pipeline

b = 20 o b = 7 o b = 2 o

In central field, stellar density is 120,000 stars per square

degree (r < 20) -- the present limits of current pipelines


(l,b) = (50, -15) Star Density = 50,000 per sq. deg with g<23

SDSS PHOTO pipeline detection and photometry meets

SEGUE requirements at this stellar density.

SEGUE uses stellar probes of increasing

absolute brightness to probe

increasing distances in the disk, thick

disk and Milky Way halo.


d < 100 kpc


d < 50 kpc

Streams and outer halo stars


d < 15 kpc


thin, thick

disk stars

d < 6 kpc

Inner and outer halo stars


d < 1 kpc

r = 1.5kpc

Other spectroscopic surveys will not probe as deep,

for instance, Blue Horizontal Branch Stars (BHBs) from a

survey with V< 12 are from a volume within 1.5 kpc of the sun.

8 kpc

Sdss dr3 distribution of fe h and velocity vs fe h
SDSS DR3 -- Distribution of [Fe/H] and Velocity vs. [Fe/H]

  • Even though SDSS does not specifically target the most metal-poor stars, it finds plenty of them !

  • There are N ~ 3000 stars with [Fe/H] < -2.0 (with well-measured parameters) and cooler than the halo main-sequence turnoff included in DR-3, and N ~ 100 such stars with [Fe/H] < -3.0

  • This is more than the SUM OF ALL PREVIOUS [Fe/H] < -2.0 stars found over the past half-century by other survey efforts

Likely numbers of detected mp stars from segue
Likely Numbers of Detected MP Stars from SEGUE

  • Actual numbers will depend on the shape of the halo Metallicity Distribution Function

    • [Fe/H] < -2.0 ~ 20,000 (VMP)

    • [Fe/H] < -3.0 ~ 2,000 (EMP)

    • [Fe/H] < -4.0 ~ 200 ? (UMP)

    • [Fe/H] < -5.0 ~ 20 ? (HMP)

    • [Fe/H] < -6.0 ~ 2 ? (MMP)

Why the fascination with large numbers of mp stars
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 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

  • Identification of relatively rare objects amongst MP stars, e.g., r-process / s-process enhanced stars that can be studied at higher resolution to understand detailed predictions of nucleosynthesis models

Segue target selection jina fied


SEGUE Target Selection– “JINA-fied”

Examples of recent progress
Examples of Recent Progress

  • Discovery of Hyper Metal-Poor star HE 0107-5240

    • [Fe/H] = -5.3 (Christlieb et al. 2002)

  • Discovery of Hyper Metal-Poor star HE 1317-2326

    • [Fe/H] = -5.6 (Frebel et al. 2005)

  • Hamburg/ESO R-Process-Enhanced Star Survey (HERES) observations of [Fe/H] < -2.0 giants

    • Barklem et al. (2005)

    • “Snapshot” spectroscopy (R ~ 20,000, S/N ~ 30/1) of ~ 400 VMP giants with VLT/UVES

    • Discovery of 10 new r-II stars ; 30 new r-I stars; numerous s-process-enhanced stars, numerous carbon-enhanced stars

    • Discovery of new “U Star”: CS 29497-004

The importance of r process enhanced metal poor stars
The Importance of r-process Enhanced Metal-Poor Stars

  • CS 22892-052: [Fe/H] = 3.1;

    [r/Fe] = +1.7

  • All r-I and r-II stars have patterns for 56 < Z < 76 that match the solar r-process component extremely well (Sneden et al. 2003)

  • Most have measurable lines of Th, and other stable r-process elements, upon which cosmo-chronometric age limits can be placed

  • Some have measurable lines of U, providing tighter constraints on age estimates

Z, The Proton Number 

A star with measurable uranium
A Star with Measurable Uranium

CS 31082-001 ([Fe/H] = -2.9); The First Meaningful Measurement

of Uranium Outside the Solar System (Cayrel et al. Nature 2001)

Heres survey other elements
HERES Survey: Other Elements !

CS 31082-001: [Fe/H] = -2.9 HERES Blue Spectrum

The power of large n 274 stars from heres
The Power Of Large N: 274 Stars from HERES

A new r process enhanced star with uranium detected cs 29497 004
A New R-Process Enhanced Star with Uranium Detected: CS 29497-004 !

Heres like follow up of vmp giants with b 17
HERES-Like Follow-Up of VMP Giants with B < 17