Southern massive stars at high angular resolution. Physical separations and mass ratios

Context. A key property of massive stars is their high degree of multiplicity, which can impact their evolution and end-of-life products. The Southern Massive Stars at High Angular Resolution survey (smash+) use interferometric and high-angular resolution techniques to detect companions at intermediate separations, from about 1 milli-arsec to 8$\arcsec$, a domain that so far has remained largely unexplored. Aims. In this paper, we convert the angular separations and magnitude contrasts into physical units, i.e. projected physical separations and mass ratios. We also derive the sensitivity of the survey for various physical and orbital parameters and we investigate the orbital separation and mass distributions. Methods. We develop a spectral type/luminosity class - H-band luminosity - mass calibration based on existing grids of physical parameters of O stars and we use these to obtain the photometric distance to each system. We also derive the individual masses of the primaries and of each detected companion. Results. The probability of detecting companions is very uniform within the sensitivity limits of the smash+ survey, which can be considered near-complete for binaries with 1 $<$ log(a/AU ) $<$ 4 and q = M2/M1 $>$ 0.2. The projected separations follow a uniform distribution in log-separation. The mass ratios follow a power-law distribution $f_q \propto q^{\kappa}$ with $\kappa$ values that decrease towards larger separation. For resolved companions within 100 AU, we find $\kappa_{<100} = -0.6^{+0.9}_{-0.7}$ which is both compatible with the power-law distributions derived for spectroscopic binaries ($\kappa \sim 0$) and that proposed in an earlier study by Moe \& Di Stefano ($\kappa = -1.4 \pm 0.4$). Beyond $\sim$1000 AU, we observe a clear lack of (near)equal-mass companions, with an upper mass-ratio limit declining towards larger separations.