The massive star binary fraction in young open clusters - III. IC 2944 and the Cen OB2 association

Abstract

Using an extended set of multi‐epoch high‐resolution high signal‐to‐noise ratio optical spectra, we readdress the multiplicity properties of the O‐type stars in IC 2944 and in the Cen OB2 association. We present new evidence of binarity for five objects and we confirm the multiple nature of another two. We derive the first orbital solutions for HD 100099, HD 101436 and HD 101190 and we provide additional support for HD 101205 being a quadruple system. The minimal spectroscopic binary fraction in our sample is fmin= 0.57. Using numerical simulations, we show that the detection rate of our observational campaign is close to 90 per cent, leaving thus little room for undetected spectroscopic binary systems. The statistical properties of the O‐star population in IC 2944 are similar, within the uncertainties, to the results obtained in the earlier papers in this series despite the fact that sample size effects limit the significance of the comparison. Using newly derived spectroscopic parallaxes, we reassess the distance to IC 2944 and obtained 2.3 ± 0.3 kpc, in agreement with previous studies. We also confirm that, as far as the O stars are concerned, the IC 2944 cluster is most likely a single entity.

Radial Velocities of Galactic O-Type Stars. I. Short-Term Constant Velocity Stars

“…This may be an indication of either an unresolved, low-amplitude shortperiod spectroscopic binary, or a long-period spectroscopic binary. The latter conclusion is supported by new observations from Sana et al (2011), who argue that the system consists of two hot components in a 37 day period.…”

“…Stickland & Lloyd (2001) added two more measurements from IUE spectra, 11.7 and −7.2 km s −1 , that are comparable to our results. On the other hand, Sana et al (2011) obtained 10 high dispersion spectra between 2005 and 2008 that appear to indicate a long-period variation of three to six months. Furthermore, they found evidence of a companion spectrum at the velocity extremum that suggests a B2-3 V classification.…”

Self-Regulated Shocks in Massive Star Binary Systems

“…As this is relatively close to periastron, we cannot appeal to the larger separation that will occur at apastron to improve the match against our model results. Adopting the recently derived orbital solution for HD101190 with an eccentricity of ∼0.3 (Sana et al 2011) does not help the agreement between our model and its log(L X /L bol ) data point either. A more detailed hydrodynamical model of a windphotosphere collision is warranted to investigate the systematic discrepancy in kT values and log(L X /L bol ).…”

“…However, the agreement between the model and observations is reasonably good for log(L X /L bol ), with the exception of the two systems with orbital periods of roughly 6 days: HD93205 (Townsley et al 2011;Nazé et al 2011) and HD101190 (Chlebowski et al 1989;Sana et al 2011;Gagne et al 2013). We remind the reader that orbital periods have been converted to binary separations under the basic assumption of circular orbits, which is accurate for the majority of the systems in the sample.…”

Massive open star clusters using the VVV survey

“…For DBS 132, Obj 42 has a RV that is much higher than the dispersion interval, so it is probably a field star. One should note that this analysis is based on a single epoch observation and follow-up observations of other epochs would be needed to eliminate bias due to binarity, especially since the binary fraction among OB and WR stars is high ( f bin > 0.5 for OB stars; Sana et al 2011 and references therein; and f bin ∼ 0.4 for WR stars; Schnurr et al 2008 and references therein). The average RVs of each cluster are listed in Table 3.…”