Journal
ASTROPHYSICAL JOURNAL LETTERS
Volume 829, Issue 2, Pages -Publisher
IOP PUBLISHING LTD
DOI: 10.3847/2041-8205/829/2/L24
Keywords
Galaxy: halo; stars: abundances; stars: individual (G64-12, G64-37); stars: Population II
Categories
Funding
- National Science Foundation [PHY-1430152]
- CAPES
- FAPESP [2012/24392-2]
- CNPq (Bolsa de Produtividade)
- Division Of Physics
- Direct For Mathematical & Physical Scien [1430152] Funding Source: National Science Foundation
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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 ratio spectra (S/N similar to 700/1) with resolving power R similar to 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 carboni cities, 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) versus [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 (T-eff similar to 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 similar to 2-3, and explosion energies by factors of similar to 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.
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