Early-type stars in the young open cluster NGC 2244 and in the Monoceros OB2 association

Abstract

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 NGC 2244 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 (HD 46 149 in NGC 2244 and HD 46 573 in Mon OB2). 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 (HD 46 150) and detect line profile variations in two rapid rotators (HD 46 056 and HD 46 485). Conclusions. This spectroscopic investigation places a firm lower limit (17%) on the binary fraction of O-stars in NGC 2244 and reveals the lack of short-period O+OB systems in this cluster. In addition, a comparison of these new results with two other wellstudied clusters (NGC 6231 and IC 1805) 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.

A spectroscopic investigation of early-type stars in the young open cluster Westerlund 2

“…With only five binaries or binary candidates (including WR 20a and the three eclipsing OB systems, two of which host B-type primaries) out of the fifteen stars investigated, our results are more reminiscent of the situation encountered in IC 1805 (De Becker et al 2006;Hillwig et al 2006) and NGC 2244 (Mahy et al 2009). If we restrict ourselves to O-type stars, the fraction of confirmed or candidate binaries drops to about 15% (2 out of 13).…”

“…If we restrict ourselves to O-type stars, the fraction of confirmed or candidate binaries drops to about 15% (2 out of 13). Given the higher stellar density in the core of NGC 6231 than in both NGC 2244 and IC 1805, Mahy et al (2009) tentatively suggested that the number of short-period early-type binary systems might be correlated with the cluster density. However, with its rather large cluster density and low fraction of short-period massive binaries, Westerlund 2 apparently does not follow such a correlation.…”

Surface abundances of ON stars

“…The nature of the tentative variability observed in the giant sample is also unclear: binarity or wind or photospheric variability? Of the five dwarfs or subgiants, HD 48279 is constant in radial velocity (Mahy et al 2009), HD 12323 is a single-lined spectroscopic binary (SB1; Bolton & Rogers 1978), HD 14633 an SB1 with a low-mass component and high eccentricity (Boyajian et al 2005), HD 102415 is reported as "likely variable" by Walborn et al (2011), and HD 201345 has an unclear status. Figure 11 provides more information on the line-profile and radial-velocity variations of ten of the ON stars studied in the present paper.…”

A quantitative study of O stars in NGC 2244 and the Monoceros OB2 association

“…In addition to shaping the surface of the star and modifying the surface temperature (poles being hotter than the equator), it triggers internal mixing processes A&A 538, A39 (2012) Notes. Spectral types are from Sota et al (2011) and Mahy et al (2009). Magnitudes are from Maíz-Apellániz et al (2004).…”

“…As a result, this region contains a homogeneous sample of O stars with different masses and luminosities, but most likely the same age. Binaries are also present (Mahy et al 2009). As such, this sample is well suited to investigating the effects of rotation on massive stars surface abundances.…”