Evidence for Halo Kinematics among Cool Carbon-Rich Dwarfs

TL;DR

This study analyzes the kinematics of approximately 600 cool carbon-rich dwarf stars, revealing that a significant portion exhibit halo-like motions, suggesting an ancient, metal-poor origin and potential dominance in the Galaxy's carbon-enhanced star population.

Contribution

It provides the first kinematic evidence that a large fraction of cool carbon-rich dwarfs are members of the Galactic halo, indicating their ancient and metal-poor nature.

Findings

30-60% of stars show halo-like kinematics

Many stars lie below an old, metal-poor disk isochrone

Carbon-rich dwarfs likely dominate the Galaxy's metal-poor, carbon-enhanced stars

Abstract

This paper reports preliminary yet compelling kinematical inferences for N ~ 600 carbon-rich dwarf stars that demonstrate around 30% to 60% are members of the Galactic halo. The study uses a spectroscopically and non-kinematically selected sample of stars from the SDSS, and cross-correlates these data with three proper motion catalogs based on Gaia DR1 astrometry to generate estimates of their 3-D space velocities. The fraction of stars with halo-like kinematics is roughly 30% for distances based on a limited number of parallax measurements, with the remainder dominated by the thick disk, but close to 60% of the sample lie below an old, metal-poor disk isochrone in reduced proper motion. An ancient population is consistent with an extrinsic origin for C/O >1 in cool dwarfs, where a fixed mass of carbon pollution more readily surmounts lower oxygen abundances, and with a lack of detectable ultraviolet-blue flux from younger white dwarf companions. For an initial stellar mass function that favors low-mass stars as in the Galactic disk, the dC stars are likely to be the dominant source of carbon-enhanced, metal-poor stars in the Galaxy.