The young massive SMC cluster NGC 330 seen by MUSE II. Multiplicity
  properties of the massive-star population
  Observations of massive stars in young open clusters (< ~8 Myr) have shown
that a majority of them are in binary systems, most of which will interact
during their life. Populations of massive stars older than ~20 Myr allow us to
probe the outcome of such interactions after many systems have experienced mass
and angular momentum transfer. Using multi-epoch integral-field spectroscopy,
we investigate the multiplicity properties of the massive-star population in
NGC 330 (~40 Myr) in the Small Magellanic Cloud to search for imprints of
stellar evolution on the multiplicity properties. From six epochs of VLT/MUSE
observations supported by adaptive optics we extract spectra and measure radial
velocities for stars brighter than F814W = 19. We identify single-lined
spectroscopic binaries through significant RV variability as well as
double-lined spectroscopic binaries, and quantify the observational biases for
binary detection. The observed spectroscopic binary fraction is 13.2+/-2.0 %.
Considering period and mass ratio ranges from log(P)=0.15-3.5, and q = 0.1-1.0,
and a representative set of orbital parameter distributions, we find a
bias-corrected close binary fraction of 34 +8 -7 %. This seems to decline for
the fainter stars, which indicates either that the close binary fraction drops
in the B-type domain, or that the period distribution becomes more heavily
weighted towards longer orbital periods. Both fractions vary strongly in
different regions of the color-magnitude diagram which probably reveals the
imprint of the binary history of different groups of stars. We provide the
first homogeneous RV study of a large sample of B-type stars at a low
metallicity. The overall bias-corrected close binary fraction of B stars in NGC
330 is lower than the one reported for younger Galactic and LMC clusters. More
data are needed to establish whether this result from an age or a metallicty
effect.
