Carbon-Deficient Red Giants

TL;DR

This paper investigates carbon-deficient red giants, analyzing their photometric properties, evolutionary status, and possible origins, revealing their unique characteristics and raising questions about their formation and evolution.

Contribution

It provides the first comprehensive photometric and astrometric analysis of CDRGs, highlighting their evolutionary placement and potential binary interaction origins.

Findings

Most CDRGs have initial masses of 2-3.5 solar masses.

They exhibit higher c1 indices due to lack of CH absorption.

CDRGs are located farther from the Galactic plane than normal giants.

Abstract

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 A, 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 to 3.5 Msun, 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 to 4.5 Msun, 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.