G64-12 and G64-37 are Carbon-Enhanced Metal-Poor Stars

TL;DR

This study provides high-resolution spectral analysis of two well-known stars, confirming their classification as carbon-enhanced metal-poor stars and refining models of their progenitors, impacting our understanding of early Galactic evolution.

Contribution

First reliable carbon abundance measurements for G64-12 and G64-37, classifying them as CEMP-no stars and refining progenitor models with high-quality data.

Findings

Both stars are confirmed as CEMP-no stars with [C/Fe] > +1.0.

High S/N spectra are crucial for accurate carbon abundance in warm, metal-poor stars.

Revised progenitor masses and explosion energies are lower than previous estimates.

Abstract

We present new high-resolution chemical-abundance analyses for the well-known high proper-motion subdwarfs G64-12 and G64-37, based on very high signal-to-noise spectra (S/N ~ 700/1) with resolving power R ~ 95,000. These high-quality data enable the first reliable determination of the carbon abundances for these two stars; we classify them as carbon-enhanced metal-poor (CEMP) stars based on their carbonicities, which both exceed [C/Fe] = +1.0. They are sub-classified as CEMP- no Group-II stars, based on their location in the Yoon-Beers diagram of absolute carbon abundance, A(C) vs. [Fe/H], as well as on the conventional diagnostic [Ba/Fe]. The relatively low absolute carbon abundances of CEMP-no stars, in combination with the high effective temperatures of these two stars (Teff ~ 6500 K) weakens their CH molecular features to the point that accurate carbon abundances can only be estimated from spectra with very high S/N. A comparison of the observed abundance patterns with the predicted yields from massive, metal-free supernova models reduces the inferred progenitor masses by factors of ~ 2-3, and explosion energies by factors of ~ 10-15, compared to those derived using previously claimed carbon abundance estimates. There are certainly many more warm CEMP-no stars near the halo main-sequence turnoff that have been overlooked in past studies, directly impacting the derived frequencies of CEMP-no stars as a function of metallicity, a probe that provides important constraints on Galactic chemical evolution models, the initial mass function in the early Universe, and first-star nucleosynthesis.