CARBON-ENHANCED METAL-POOR STARS IN THE INNER AND OUTER HALO COMPONENTS OF THE MILKY WAY

Journal Article · Tue Jan 10 04:00:00 EST 2012 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/744/2/195· OSTI ID:22004249

Carollo, Daniela; Norris, John E; Freeman, Ken C ^[1]; Beers, Timothy C; Lee, Young Sun; Kennedy, Catherine R ^[2]; Bovy, Jo ^[3]; Sivarani, Thirupathi ^[4]; Aoki, Wako ^[5]

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1. Research School of Astronomy and Astrophysics, Australian National University, Cotter Road, Weston, ACT 2611 (Australia)
2. Department of Physics and Astronomy and JINA (Joint Institute for Nuclear Astrophysics), Michigan State University, E. Lansing, MI 48824 (United States)
3. Center for Cosmology and Particle Physics Department of Physics, New York University, 4 Washington Place, New York, NY 10003 (United States)
4. Indian Institute of Astrophysics, II Block, Koramangala, Bangalore 560034 (India)
5. National Astronomical Observatory, Mitaka, Tokyo 181-8588 (Japan)

Carbon-enhanced metal-poor (CEMP) stars in the halo components of the Milky Way are explored, based on accurate determinations of the carbon-to-iron ([C/Fe]) abundance ratios and kinematic quantities for over 30,000 calibration stars from the Sloan Digital Sky Survey. Using our present criterion that low-metallicity stars exhibiting [C/Fe] ratios ({sup c}arbonicity{sup )} in excess of [C/Fe] =+0.7 are considered CEMP stars, the global frequency of CEMP stars in the halo system for [Fe/H] <-1.5 is 8%, for [Fe/H] <-2.0 it is 12%, and for [Fe/H] <-2.5 it is 20%. We also confirm a significant increase in the level of carbon enrichment with declining metallicity, growing from ([C/Fe]) {approx}+1.0 at [Fe/H] =-1.5 to ([C/Fe]) {approx}+1.7 at [Fe/H] =-2.7. The nature of the carbonicity distribution function (CarDF) changes dramatically with increasing distance above the Galactic plane, |Z|. For |Z| <5 kpc, relatively few CEMP stars are identified. For distances |Z| >5 kpc, the CarDF exhibits a strong tail toward high values, up to [C/Fe] > +3.0. We also find a clear increase in the CEMP frequency with |Z|. For stars with -2.0 < [Fe/H] <-1.5, the frequency grows from 5% at |Z| {approx}2 kpc to 10% at |Z| {approx}10 kpc. For stars with [Fe/H] <-2.0, the frequency grows from 8% at |Z| {approx}2 kpc to 25% at |Z| {approx}10 kpc. For stars with -2.0 < [Fe/H] <-1.5, the mean carbonicity is ([C/Fe]) {approx}+1.0 for 0 kpc < |Z| < 10 kpc, with little dependence on |Z|; for [Fe/H] <-2.0, ([C/Fe]) {approx}+1.5, again roughly independent of |Z|. Based on a statistical separation of the halo components in velocity space, we find evidence for a significant contrast in the frequency of CEMP stars between the inner- and outer-halo components-the outer halo possesses roughly twice the fraction of CEMP stars as the inner halo. The carbonicity distribution also differs between the inner-halo and outer-halo components-the inner halo has a greater portion of stars with modest carbon enhancement ([C/Fe] {approx}+0.5]); the outer halo has a greater portion of stars with large enhancements ([C/Fe] {approx}+2.0), although considerable overlap still exists. We interpret these results as due to the possible presence of additional astrophysical sources of carbon production associated with outer-halo stars, beyond the asymptotic giant-branch source that may dominate for inner-halo stars, with implications for the progenitors of these populations.

• OSTI ID: 22004249
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 2 Vol. 744; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
ABUNDANCE
ASTROPHYSICS
CALIBRATION
CARBON
DATA ANALYSIS
DISTRIBUTION FUNCTIONS
GALACTIC EVOLUTION
IRON
MILKY WAY
STARS