The Role of Binaries in the Carbon Star Phenomenon

Abstract

Abstract. This presentation reviews the role that binaries play in the production of Barium, CH, and S stars. New radial velocity observations confirming the binary nature of subgiant CH stars are also discussed. Evidence is presented that the early R-type carbon stars exhibit no binaries. It is suggested that they were once all binaries, but having small separations, a coalescing companion has caused them to mix near the helium core flash. IntroductionIn a review of the roles of the various carbon stars, Scalo (1976) pointed out that the classical carbon stars lie above the luminosity of the onset of helium shell flashing on the asymptotic giant branch (AGB), as shown by Crabtree et al. (1976). But he noted that most Barium stars are too faint to be identified with double-shell models. Along with CH stars, their population II equivalents, they did not fit into the AGB scenario for production of carbon stars, even though they exhibit many of the same abundance characteristics. It seemed that they must be associated with the helium core flash, but standard stellar models did not predict mixing of the core flash material. The Barium, CH, and S stars 2

A binary origin for low-luminosity carbon stars

“…The standard picture of repeated third dredgeup in AGB stars produces intrinsic carbon stars when the products of helium burning are mixed to the surface. An alternative possibility is that mass transfer in a binary system is responsible for non-TPAGB carbon stars, the extrinsic carbon stars (Van Eck et al 1998;Mowlavi 1998;McClure 2000). Here we use a new synthetic TPAGB code (Izzard et al 2004) coupled with a synthetic binary stellar evolution code (Hurley, Tout & Pols 2002) to determine the effect of mass transfer in binary stars on the carbon star luminosity function.…”