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

Mahy L., Naze Y., Rauw G., Gosset E., De Becker M., Sana H., Eenens P.

(ULg, ESO, U. de Guanajuato)

Aims. We present the results obtained from a long-term spectroscopic campaign devoted to the multiplicity of O-type stars in the
young open cluster NGC2244 and in the Mon OB2 association.
Methods. Our spectroscopic monitoring was performed over several years, allowing us to probe different time-scales. For each star,
several spectral diagnostic tools are applied, in order to search for line shifts and profile variations. We also measure the projected
rotational velocity and revisit the spectral classification.
Results. In our sample, several stars were previously considered as spectroscopic binaries, though only a few scattered observations
were available. Our results now reveal a more complex situation. Our study identifies two new spectroscopic binaries (HD46149 in
NGC2244 and HD46573 in MonOB2). The first object is a long-period double-lined spectroscopic binary, though 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 classify another star as variable in
radial velocity (HD46150) and we detect line profile variations in two rapid rotators (HD46056 and HD46485).
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 about the formation and early evolution of hot stars.

Reference: A&A in press
Status: Manuscript has been accepted

Weblink: http://arxiv.org/abs/0905.1592


Email: mahy@astro.ulg.ac.be