The first complete orbital solution for the double-lined spectroscopic binary system γ 2 Velorum, obtained from measurements with the Sydney University Stellar Interferometer (SUSI), is presented. This system contains the closest example of a Wolf-Rayet star and the promise of full characterization of the basic properties of this exotic high-mass system has subjected it to intense study as an archetype for its class. In combination with the latest radial-velocity results, our orbital solution produces a distance of 336 +8 −7 pc, significantly more distant than the Hipparcos estimation. The ability to fully specify the orbital parameters has enabled us to significantly reduce uncertainties and our result is consistent with the Very Large Telescope Interferometer (VLTI) observational point, but not with their derived distance. Our new distance, which is an order of magnitude more precise than prior work, demands critical reassessment of all distance-dependent fundamental parameters of this important system. In particular, membership of the Vela OB2 association has been re-established, and the age and distance are also in good accord with the population of young stars reported by Pozzo et al. We determine the O-star primary component parameters to be M V (O) = −5.63 ± 0.10 mag, R(O) = 17 ± 2 R and M(O) = 28.5 ± 1.1 M . These values are consistent with calibrations found in the literature if a luminosity class of II-III is adopted. The parameters of the Wolf-Rayet (WR) component are M v (WR) = −4.33 ± 0.17 mag and M(WR) = 9.0 ± 0.6 M .

We present the first multiplicity-dedicated long baseline optical interferometric survey of the Scorpius-Centaurus-Lupus-Crux association. We used the Sydney University Stellar Interferometer to undertake a survey for new companions to 58 Sco-Cen Btype stars and have detected 24 companions at separations ranging from 7-130 mas, 14 of which are new detections. Furthermore, we use a Bayesian analysis and all available information in the literature to determine the multiplicity distribution of the 58 stars in our sample, showing that the companion frequency is F = 1.35 ± 0.25 and the mass ratio distribution is best described by q γ with γ = −0.46, agreeing with previous Sco-Cen high mass work and differing significantly from lower-mass stars in Tau-Aur. Based on our analysis, we estimate that among young B-type stars in moving groups, up to 23% are apparently single stars. This has strong implications for the understanding of high-mass star formation, which requires angular momentum dispersal through some mechanism such as formation of multiple systems.

Stellar angular diameters determined interferometrically are generally established by fitting the observed visibility data with a curve appropriate for a uniformly illuminated disc. The resulting uniform‐disc diameters must be corrected for the effects of limb darkening in order to determine the true angular diameters of the stars. An extensive grid of limb‐darkening corrections, based directly on the centre‐to‐limb intensity variations for Kurucz model stellar atmospheres, has been computed without the intermediate step of a parametrized representation of the centre‐to‐limb variation. The limitations of this method of correction are discussed.

The Sydney University Stellar Interferometer (SUSI) is a new long‐baseline optical interferometer located in northern New South Wales, Australia. SUSI has been developed to tackle a range of problems in stellar astrophysics, and its design is based on a successful prototype instrument. In its initial configuration, observations are made with a single baseline selected from an array of fixed north‐‐south baselines covering the range from 5 to 640 m. Small apertures, wavefront‐tilt correction and rapid signal sampling are employed to overcome the effects of atmospheric turbulence, and optical path equality is maintained by a dynamic optical delay line. The planned astrophysical programmes, the resulting design criteria, the instrument and its current status are described.

The binary star δ Sco (HD143275) underwent remarkable brightening in the visible in 2000, and continues to be irregularly variable. The system was observed with the Sydney University Stellar Interferometer (SUSI) in 1999, 2000, 2001, 2006 and 2007. The 1999 observations were consistent with predictions based on the previously published orbital elements. The subsequent observations can only be explained by assuming that an optically bright emission region with an angular size of ≳2 ± 1 mas formed around the primary in 2000. By 2006/2007 the size of this region grew to an estimated ≳4 mas. We have determined a consistent set of orbital elements by simultaneously fitting all the published interferometric and spectroscopic data as well as the SUSI data reported here. The resulting elements and the brightness ratio for the system measured prior to the outburst in 2000 have been used to estimate the masses of the components. We find MA= 15 ± 7 M⊙ and MB= 8.0 ± 3.6 M⊙. The dynamical parallax is estimated to be 7.03 ± 0.15 mas, which is in good agreement with the revised Hipparcos parallax.