Multiplicity of red supergiants in NGC 330

When virial equilibrium is assumed, the dynamical mass of the cluster is log(M_dyn_/M_{sun}_)=5.20+/-0.17, in good agreement with previous upper limits. We detect significant radial velocity variability in our multi-epoch observations and distinguish between variations caused by atmospheric activity and those caused by binarity. The binary fraction of NGC 330 RSGs is estimated by comparisons with simulated observations of systems with a range of input binary fractions.

In this way, we account for observational biases and estimate the intrinsic binary fraction for RSGs in NGC 330 as f_RSG_=0.3+/-0.1 for orbital periods in the range 2.3<logP[days]<4.3, with q>0.1. Using the distribution of the luminosities of the RSG population, we estimate the age of NGC 330 to be 45+/-5Myr and estimate a red straggler fraction of 50%. We estimate the binary fraction of RSGs in NGC 330 and conclude that it appears to be lower than that of main-sequence massive stars, which is expected because interactions between an RSG and a companion are assumed to effectively strip the RSG envelope.