Astrophys. J., 887, 12-12 (2019/December-2)
Carbon-deficient red giants.
Abstract (from CDS):
Carbon-deficient red giants (CDRGs) are a rare class of peculiar red giants, also called "weak G-band" or "weak CH" stars. Their atmospheric compositions show depleted carbon, a low 12C/13C isotopic ratio, and an overabundance of nitrogen, indicating that the material at the surface has undergone CN-cycle hydrogen burning. I present Stromgren uvby photometry of nearly all known CDRGs. Barium stars, having an enhanced carbon abundance, exhibit the "Bond-Neff effect"-a broad depression in their energy distributions at ∼4000 Å, recently confirmed to be due to the CH molecule. This gives Ba II stars unusually low Stromgren c1 photometric indices. I show that CDRGs, lacking CH absorption, exhibit an "anti-Bond-Neff effect"-higher c1 indices than normal red giants. Using precise parallaxes from Gaia DR2, I plot CDRGs in the color-magnitude diagram (CMD) and compare them with theoretical evolution tracks. Most CDRGs lie in a fairly tight clump in the CMD, indicating initial masses in the range ∼2-3.5M☉, if they have evolved as single stars. It is unclear whether they are stars that have just reached the base of the red-giant branch and the first dredge-up of CN-processed material, or are more highly evolved helium-burning stars in the red-giant clump. About 10% of CDRGs have higher masses of ∼4-4.5M☉, and exhibit unusually high rotational velocities. I show that CDRGs lie at systematically larger distances from the Galactic plane than normal giants, possibly indicating a role of binary mass transfer and mergers. CDRGs continue to present a major puzzle for our understanding of stellar evolution.
© 2019. The American Astronomical Society. All rights reserved.
Evolved stars - Chemically peculiar giant stars - Carbon-nitrogen cycle - Stromgren photometric system - Stellar abundances - Stellar parallax - Stellar rotation - Binary stars
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