2011/03/31 by Daniela Carollo, D. Carollo, Timothy C. Beers +15
Engineering · Physics and Astronomy · #Astronomy and Astrophysical Research #Galactic halo #Halo #K-type main-sequence star #Milky Way #Sky #Space Technology and Applications #Stars #Stellar, planetary, and galactic studies #astro-ph.GA
paper · pdf · doi:10.1088/0004-637x/744/2/195
Accepted for publication in the Astrophysical Journal, 32 pages, 15 figures
arxiv created 2011/11/01 · openalex publication_date 2011/12/22 · arxiv updated 2015/05/27 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/05
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 ("carbonicity") 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]〉 ∼+1.0 at [Fe/H] =−1.5 to 〈[C/Fe]〉 ∼+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 | ∼2 kpc to 10% at | Z | ∼10 kpc. For stars with [Fe/H] <−2.0, the frequency grows from 8% at | Z | ∼2 kpc to 25% at | Z | ∼10 kpc. For stars with −2.0 < [Fe/H] <−1.5, the mean carbonicity is 〈[C/Fe]〉 ∼+1.0 for 0 kpc < | Z | < 10 kpc, with little dependence on | Z |; for [Fe/H] <−2.0, 〈[C/Fe]〉 ∼+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] ∼+0.5]); the outer halo has a greater portion of stars with large enhancements ([C/Fe] ∼+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.