Research Note

Surface abundances of OC supergiants^⋆

¹ LUPM, Université de Montpellier, CNRS, Place Eugène Bataillon, 34095 Montpellier, France
e-mail: fabrice.martins@umontpellier.fr
² Aix-Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
³ Departamento de Física y Astronomía, Universidad de La Serena, Cisternas 1200 N, La Serena, Chile
⁴ Department of Physics & Astronomy, University College London, Gower St, London WC1E 6BT, UK

Received: 10 February 2016
Accepted: 26 February 2016

Abstract

Context. Some O and B stars show unusually strong or weak lines of carbon and/or nitrogen. These objects are classified as OBN or OBC stars. It has recently been shown that nitrogen enrichment and carbon depletion are the most likely explanations for the existence of the ON class.

Aims. We investigate OC stars (all being supergiants) to check that surface abundances are responsible for the observed anomalous line strengths.

Methods. We perform a spectroscopic analysis of three OC supergiants using atmosphere models. A fourth star was previously studied by us. Our sample thus comprises all OC stars known to date in the Galaxy. We determine the stellar parameters and He, C, N, and O surface abundances.

Results. We show that all stars have effective temperatures and surface gravities fully consistent with morphologically normal O supergiants. However, OC stars show little, if any, nitrogen enrichment and carbon surface abundances consistent with the initial composition. OC supergiants are thus barely chemically evolved, unlike morphologically normal O supergiants.