Early-type stars in the young open cluster NGC 2244 and in the Monoceros OB2 association I. The multiplicity of O-type stars

Aims. We present the results obtained from a long-term spectroscopic campaign to study the multiplicity of O-type stars in both the young open cluster NGC2244 and the Mon OB2 association. Methods. Our spectroscopic monitoring was performed over several years, allowing us to investigate different timescales. For each star, several spectral diagnostic tools were applied to search for line shifts and profile variations. We also measured the projected rotational velocity and revisited the spectral classification. Results. Several stars in our sample have been previously considered to be spectroscopic binaries, although only a few scattered observations were available. Our results now have identified a more complex situation for two new spectroscopic binaries (HD46 149 in NGC2244 and HD46 573 in MonOB2). The first object is a long-period double-lined spectroscopic binary, although the exact value of its period remains uncertain and the second object is classified as an SB1 system with a period of about 10.67 days but the time series of our observations do not enable us to derive a unique orbital solution for this system. We also find another star to be variable in radial velocity (HD46 150) and detect line profile variations in two rapid rotators (HD46 056 and HD46 485). Conclusions. This spectroscopic investigation places a firm lower limit (17%) on the binary fraction of O-stars in NGC2244 and reveals the lack of short-period O+OB systems in this cluster. In addition, a comparison of these new results with two other well-studied clusters (NGC6231 and IC1805) puts forward possible hints of a relation between stellar density and binarity, which could provide constraints on the theories of the formation and early evolution of hot stars.