Surface abundances of OC supergiants

Abstract

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

5 pages, 6 figures. Accepted as 'Research Note' in Astronomy and Astrophysics

References in corpus (17)

• Grids of stellar models with rotation - I. Models from 0.8 to 120 Msun at solar metallicity (Z = 0.014)
• Physical Properties of Wolf-Rayet Stars
• Rotating Massive Main-Sequence Stars I: Grids of Evolutionary Models and Isochrones
• Southern Massive Stars at High Angular Resolution: Observational Campaign and Companion Detection
• The Galactic O-Star Spectroscopic Survey (GOSSS). II. Bright Southern Stars
• The Galactic O-Star Spectroscopic Survey. I. Classification System and Bright Northern Stars in the Blue-Violet at R~2500
• The IACOB project: I. Rotational velocities in Northern Galactic O and early B-type stars revisited. The impact of other sources of line-broadening
• The VLT-FLAMES Tarantula Survey XII. Rotational velocities of the single O-type stars
• Massive star models with magnetic braking
• The massive star binary fraction in young open clusters I. NGC 6231 revisited
• Early Results from the Galactic O-Star Spectroscopic Survey: C III Emission Lines in Of Spectra
• The MiMeS Survey of Magnetism in Massive Stars: CNO surface abundances of Galactic O stars
• The weak magnetic field of the O9.7 supergiant zeta Orionis A
• The magnetic field of zeta Orionis A
• On the formation of CIII 4647-50-51 and CIII 5696 in O star atmospheres
• Further Results from the Galactic O-Star Spectroscopic Survey: Rapidly Rotating Late ON Giants
• 2dF-AAOmega spectroscopy of massive stars in the Magellanic Clouds: The north-eastern region of the Large Magellanic Cloud

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