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The Frequency of Carbon-Enhanced Stars in HERES and SDSS

The Frequency of Carbon-Enhanced Stars in HERES and SDSS. Timothy C. Beers Department of Physics & Astronomy Michigan State University & JINA: Joint Institute for Nuclear Astrophysics. Collaborators on This Project. Sara Lucatello (Univ. of Padova)

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The Frequency of Carbon-Enhanced Stars in HERES and SDSS

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  1. The Frequency of Carbon-Enhanced Stars in HERES and SDSS Timothy C. Beers Department of Physics & Astronomy Michigan State University & JINA: Joint Institute for Nuclear Astrophysics

  2. Collaborators on This Project • Sara Lucatello (Univ. of Padova) • Brian Marsteller, Thirupathi Sivarani (MSU / JINA) • Paul Barklem, Norbert Christlieb (Uppsala Univ.) • Thomas Masseron (Univ. of Montpellier) • Silvia Rossi (Univ. of Sao Paulo) • Gillian Knapp (Princeton Univ.)

  3. From Discovery to Understanding • “Carbon Stars” have been recognized as a class of astronomical object for more than a century • Only in the past 10 years have samples of carbon-enhanced stars reached numbers that suggest we might soon be better able to understand the multiple mechanisms responsible for its origin and evolution • Development of theory has also provided crucial additional information and testable predictions • Expansion of available samples of carbon-enhanced stars now, and in the next few years, will fill in the detailed information required to identify • Their origin, and the varieties of their origin • The astrophysical significance of the carbon-enhancement phenomenon at low metallicity

  4. The Original Detection of the Phenomenon • In the HK objective prism survey of Beers, Preston, & Shectman (1985,1992) it was noticed that many of the low [Fe/H] stars exhibited G-band strengths that were far higher than expected • Why should this be so? • What does it mean ?

  5. Carbon Production at [Fe/H] < -5.0 May be From Pop. III The two stars with the lowest [Fe/H] known: HE 0107-5240 (-5.2) HE 1327-2326 (-5.4) have [C/Fe] = +4.0 40% of stars known with [Fe/H] < -3.5 have [C/Fe] > +1.0 HE 0107-5240

  6. SEGUE Sample Spectra – Low Metallicity Stars

  7. Example Main-Sequence Turnoff Stars of Low Metallicity Teff = 6426 / log g = 4.20 / [Fe/H] = -1.70 Teff = 6450 / log g = 3.88 / [Fe/H] = -2.20 Teff = 6457 / log g = 3.63 / [Fe/H] = -2.69 Teff = 6475 / log g = 4.13 / [Fe/H] = -3.08 Teff = 6416 / log g = 4.20 / [Fe/H] = -3.59

  8. SEGUE Sample Spectra – Carbon Enhanced Metal-Poor Stars

  9. The Pertinent Observational Questions • What are the general trends and scatter of carbon abundances for stars over a wide range of metallicities, from [Fe/H]= -5.4 to 0.0, but especially below [Fe/H] = -2.0 ? • What is the frequency and degree of carbon enhancement exhibited by stars within specific intervals of metallicity and evolutionary states ? • What are the elemental (and isotopic) abundance patterns associated with carbon-enhanced stars? • Provide the keys to understanding of multiple origins • Provide insight into required new observations and theories

  10. Presently Available Observational Samples • The HK Survey of Beers and colleagues • Beers, Preston, & Shectman (1985, 1992) • Beers (1999) • Beers et al. (in preparation) • The Hamburg/ESO Survey of Christlieb and colleagues • Christlieb et al. (2001) • Christlieb et al. (in preparation / several collaborations) • The Sloan Digital Sky Survey • SDSS-I (e.g., Margon et al. 2002; Downes et al. 2004) • SDSS-II (SEGUE) • Sloan Extension For Galactic Understanding and Exploration

  11. The Zoo of Carbon-Enhanced Stars • Beers & Christlieb (ARA&A, 2005) define the following categories; surely subject to revision

  12. The Behavior of Carbon in HERES • The Hamburg/ESO R-Process Enhanced Star survey (HERES) has obtained high-resolution (R ~ 20,000), moderate S/N (30/-1 to 50/1) spectra of ~ 380 candidate [Fe/H] < -2.0 stars from the HES (Christlieb et al. 2004; Barklem et al. 2005) • Distribution of [C/Fe] among these stars provides present best insight into the nature of carbon-enhancement yet obtained • Advantages • Star selection criteria well understood, relative to HK survey • Unbiased (within limits) with respect to carbon-enhancement (controllable) • Limitations • Still too small in number of stars • Limited primarily to cooler stars (by choice) • Color selection, and influence of carbon, not well controlled

  13. New Results for the HERES Sample (Lucatello et al. 2006)Includes 75 Stars not Analyzed by Barklem et al. (2005) Note, while upper envelope of [C/H] is approximately constant, the upper envelope of [C/Fe] increases with declining [Fe/H].

  14. Additional Results from Lucatello et al. On the left, two different definitions of CEMP stars. One from Beers & Christlieb (2005), the other from Aoki et al. (2006). On the right, there appears to be a dilution of the outer envelope.

  15. Frequencies of CEMP Stars Based on the Lucatello et al. Analysis • When adopting the definition of CEMP from Beers & Christlieb ([C/Fe] > +1.0) • f (CEMP) < -2.0 = 20 ± 2 % • When adopting the definition of CEMP from Aoki et al. (2006) • f (CEMP) < -2.0 = 23 ± 2 % • Note that neither definition adequately takes into account “evolutionary dilution”, which would drive fraction up further still • A better sample would be comprised of stars that are sufficiently unevolved to not suffer from evolutionary effects

  16. Recent CEMP Observations from Subaru (Aoki et al. 2006)

  17. CEMP-s CEMP-s CEMP-no CEMP-no Differences in [Fe/H] DistributionCEMP-s vs. CEMP-no CEMP-no stars occur at uniformly LOW [Fe/H]

  18. Additional Evidence for Evolutionary Dilution A sample of 64 CEMP stars from the literature, including the Aoki et al. objects. Note the similar behavior as seen in the Lucatello et al analysis of HERES stars. A decline of up to 0.5 dex in C, N, and C+N with increasing luminosity

  19. SDSS -- The Telescope and Data ARC 2.5m SDSS Telescope (3 deg FOV)

  20. SEGUE: The Sloan Extension for Galactic Understanding and Exploration • Fully funded ($15 Million: Sloan Foundation / NSF / Partners (JINA) for operation through July 2008 • Use existing SDSS hardware and software to obtain: • 3500 square degrees of additional ugriz imaging at lower latitudes • 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

  21. SEGUE uses stellar probes of increasing absolute brightness to probe increasing distances in the disk, thick disk and Milky Way halo. K III d < 100 kpc BHB/BS d < 50 kpc Streams and outer halo stars MSTO/F d < 15 kpc G thin, thick disk stars d < 6 kpc Inner and outer halo stars KV 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

  22. SEGUE observing plan and status as of June 2006 SDSS Imaging scan Declination = -20 degrees Planned SEGUE grid pointings (200) Planned SEGUE scan (3500 sq deg) Planned targeted SEGUE pointings(60) Sgr stream planned scan Completed SEGUE imaging Completed SEGUE plate pointing

  23. 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) • Tests indicate we expect to find ~ 5000 CEMP stars among the SEGUE sample of MP stars

  24. New CEMP Stars Selected from SDSS-I PHO and RED Sample (5700 K < Teff < 6700 K ) • MP FTO stars targeted during normal SDSS operation as spectrophotometric flux calibrators (PHO/RED: 16 per plug-plate / reasonably bright, g < 16.5) • [Fe/H] from spectroscopic pipeline (5 estimators) [C/Fe] estimated using [C/Fe] = f (GP,KP) and spectral synthesis • Through DR-4: N (PHO; [Fe/H] < -1.0) = 5321

  25. New Results from SDSS-I Only Includes Suspected CEMP Stars From Marsteller et al. (in preparation)

  26. New Results from SDSS-I Note large number of upper limits at high Teff

  27. Production of Carbon at Low Metallicity • Fujimoto et al. (2000) • Enhanced convective mixing of interior carbon (at low and intermediate mass) post He flash in stars with [Fe/H] < -4.0 • Meynet et al. (2005), Hirschi et al. (2006) argue for extremely efficient C (and N and O) production in massive low metallicity stars • Rapid rotation drives efficient mixing to outer layers, CNO opacity, as well as molecules from them, drives large mass loss • e.g., an 85 Mo low [Fe/H] star loses 75% of its mass prior to explosion !

  28. Summary of Results • Distribution of [C/Fe] vs [Fe/H] in HERES strongly suggests multiple origins of C-enhancement • AGB transfer (80%) • Primordial (20%) • Distribution of [C/Fe] vs (J-K) in HERES strongly suggests dilution of C for cooler stars • Best (high-res) estimate (HERES sample) • Freq C-enhancement for [Fe/H] < -2.0: f ([C/Fe] > +1.0) > 20 % • Possible explanation at low [Fe/H] associated with rapidly rotating high-mass stars with [Fe/H] < -6.0 (MMP) • Exploration of CEMP fractions at main-sequence turnoff in SDSS-I/SEGUE will provide best available sample for exploration of phenomenon, for many thousands of stars

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