Massive binaries in the Cepheus OB2/3 region Constraining the formation mechanism of massive stars

Context. Two different formation scenarios for stars of masses larger than 10 M-circle dot exist. Although simulations within both scenarios are capable of producing stars up to the highest observed masses, the relevance of the two formation scenarios for massive star-formation is not yet clear. Aims. We aim to detect companions to massive stars to constrain the binary parameters of the multiple systems. These findings will help to constrain the formation of massive stars. Methods. We performed z'-band observations of massive and intermediate-mass stars in the Cep OB2/3 associations with the Lucky imaging camera AstraLux on the 2.2 m telescope of the Calar Alto observatory. The analysis aimed at detecting binary systems with separations out to 2 ''.2 (similar to 1700AU), the inner limit depending on the contrast. The maximum contrast of 7 mag in z' (corresponding to a mass contrast of 17: 1 versus an O9V primary) requires a minimum separation of 0 ''.7 (similar to 550AU). Results. We found 28 new companions for our sample of 148 intermediate-mass and massive stars in Cep OB2/3. The companion star fraction of the massive stars is 0.7; about 50% of the systems are triples. The mass function of the companions to the massive stars is strongly top-heavy. We found that the sample parameters closely agreed with those found in the Orion Trapezium cluster. Conclusions. The multiplicity of massive stars seems to be significantly higher than that of intermediate-mass stars, independently of the environment. The comparison of our findings with the theories of massive star-formation favor the formation of massive stars by the fragmentation of proto-stellar cores combined with competitive accretion.