Origin of the early-type R stars: a binary-merger solution to a century-old problem?

Abstract

The early-R stars are carbon-rich K-type giants. They are enhanced in C12, C13 and N14, have approximately solar oxygen, magnesium isotopes, s-process and iron abundances, have the luminosity of core-helium burning stars, are not rapid rotators, are members of the Galactic thick disk and, most peculiarly of all, are all single stars. Conventional single-star stellar evolutionary models cannot explain such stars, but mergers in binary systems have been proposed to explain their origin. We have synthesized binary star populations to calculate the number of merged stars with helium cores which could be early-R stars. We find many possible evolutionary channels. The most common of which is the merger of a helium white dwarf with a hydrogen-burning red giant branch star during a common envelope phase followed by a helium flash in a rotating core which mixes carbon to the surface. All the channels together give ten times more early-R stars than we require to match recent Hipparcos observations - we discuss which channels are likely to be the true early-R stars and which are not. For the first time we have constructed a viable model of the early-R stars with which we can test some of our ideas regarding common envelope evolution in giants, stellar mergers, rotation, the helium flash and the origin of the early-R stars.

17 Pages, accepted for publication in A&A

References in corpus (5)

• AGB stars in the Magellanic Clouds. II. The rate of star formation across the LMC
• The blue stragglers formed via mass transfer in old open clusters
• CH stars at High Galactic Latitudes
• On the angular momentum evolution of merged white dwarfs
• The s-process in stellar population synthesis: a new approach to understanding AGB stars

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• Characterisation of Galactic carbon stars and related stars from Gaia-EDR3
• J-type carbon stars: A dominant source of 14N-rich presolar SiC grains of type AB
• Carbon stars from LAMOST DR2 data
• Properties of carbon stars in the Solar neighbourhood based on Gaia DR2 astrometry
• Discovery of carbon-rich Miras in the Galactic bulge
• A close-in giant planet escapes engulfment by its star
• Core radii and common-envelope evolution
• The carbon star mystery: forty years later
• Lithium in coalesced non-compact stars
• A Small Step on the Long Road to Understanding the R-Stars: CNO Cycling in Candidate R-Star Progenitors
• A Main Sequence CH-star in the globular cluster M55 (NGC 6809)