A nova re-accretion model for J-type carbon stars ^⋆

Argelander-Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
e-mail: sutirtha@astro.uni-bonn.de

Received: 1 August 2013
Accepted: 3 October 2013

Abstract

The J-type carbon (J)-stars constitute 10–15% of the observed carbon stars in both our Galaxy and the Large Magellanic Cloud (LMC). They are characterized by strong ${\mathrm{13}}^{}\mathrm{C}$ absorption bands with low ¹²C/¹³C ratios along with other chemical signatures peculiar for typical carbon stars, e.g. a lack of s-process enhancement. Most of the J-stars are dimmer than the N-type carbon stars some of which, by hot-bottom burning, make ${\mathrm{13}}^{}\mathrm{C}$ only in a narrow range of masses. We investigate a binary-star formation channel for J-stars involving re-accretion of carbon-rich nova ejecta on main-sequence companions to low-mass carbon-oxygen white-dwarfs. The subsequent evolution of the companion stars in such systems is studied with a rapid binary evolutionary code to predict chemical signatures of nova pollution in systems which merge into giant single stars. A detailed population synthesis study is performed to estimate the number of these mergers and compare their properties with observed J-stars. Our results predict that such nova polluted mergers evolve with low luminosities as well as low ¹²C/¹³C ratios like the majority of observed J-stars (e.g. in the LMC) but cannot account for the observed fraction of J-stars in existing surveys of carbon stars.