Carbon-enhanced Metal-poor Stars in the Inner and Outer Halo Components of the Milky Way

Abstract

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 lang[C/Fe]rang ~+1.0 at [Fe/H] =–1.5 to lang[C/Fe]rang ~+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 lang[C/Fe]rang ~+1.0 for 0 kpc < |Z| < 10 kpc, with little dependence on |Z|; for [Fe/H] <–2.0, lang[C/Fe]rang ~+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.