The MiMeS survey of magnetism in massive stars: CNO surface abundances of Galactic O stars

Abstract

Context. The evolution of massive stars is still partly unconstrained. Mass, metallicity, mass loss, and rotation are the main drivers of stellar evolution. Binarity and the magnetic field may also significantly affect the fate of massive stars. Aims. Our goal is to investigate the evolution of single O stars in the Galaxy. Methods. For that, we used a sample of 74 objects comprising all luminosity classes and spectral types from O4 to O9.7. We relied on optical spectroscopy obtained in the context of the MiMeS survey of massive stars. We performed spectral modelling with the code CMFGEN. We determined the surface properties of the sample stars, with special emphasis on abundances of carbon, nitrogen, and oxygen. Results. Most of our sample stars have initial masses in the range of 20 to 50 M . We show that nitrogen is more enriched and carbon and oxygen are more depleted in supergiants than in dwarfs, with giants showing intermediate degrees of mixing. CNO abundances are observed in the range of values predicted by nucleosynthesis through the CNO cycle. More massive stars, within a given luminosity class, appear to be more chemically enriched than lower mass stars. We compare our results with predictions of three types of evolutionary models and show that for two sets of models, 80% of our sample can be explained by stellar evolution including rotation. The effect of magnetism on surface abundances is unconstrained. Conclusions. Our study indicates that in the 20−50 M mass range, the surface chemical abundances of most single O stars in the Galaxy are fairly well accounted for by stellar evolution of rotating stars.

Surface abundances of OC supergiants

“…Figure 1 shows the log g -T eff diagram with evolutionary tracks from Ekström et al (2012). We added the morphologically normal late-O supergiants of Martins et al (2015a) for comparison. All stars are located at, or just beyond, the terminal age main sequence.…”

“…Synthetic spectra calculated from Sota et al (2011Sota et al ( , 2014. (2) Star analyzed by Martins et al (2015a).…”

“…In this follow-up study, we focus on OC stars. Martins et al (2015a) analyzed a large sample of O stars including one OC object (HD 152249). The latter turned out to show little nitrogen enrichment, unlike similar objects without the "C" qualifier.…”

Massive stars, successes and challenges

“…This fact has for consequence that it is difficult from observed stars to isolate the specific correlation between the surface enrichments and rotation. When sufficient care has been taken for selecting the stars presenting properties allowing to isolate the effect of rotation (isolated stars of about the same initial mass, age and metallicity) then a good agreement is found between rotating stellar models and the observations (Maeder et al 2009, see also Przybilla et al 2010, Martins et al 2015.…”

Bayesian least squares deconvolution

“…Meanwhile, the spectral line itself presents correlations that will make it appear above the noise after adding many spectral lines. This idea was later improved with the introduction of the least squares deconvolution technique (LSD; Donati et al 1997), which has facilitated the detection of polarimetric signals in a wide variety of stars and the generation of a set of interesting surveys for the detection of magnetic fields in different stellar types (e.g., Wade et al 2012;Marsden et al 2014;Martins et al 2015).…”