Interferometric Studies of Hot Stars at Sydney University

The University of Sydney has a long history in optical stellar interferometry. The first project, in the 1960s, was the Narrabri Stellar Intensity Interferometer, which measured the angular diameters of 32 hot stars and established the temperature scale for spectral classes O - F. That instrument was followed by the Sydney University Stellar Interferometer (SUSI), which is now undergoing a third-generation upgrade, to use the multi-wavelength PAVO beam combiner. SUSI operates at visible rather than IR wavelengths and has baselines up to 160 m, so it is well suited to the study of hot stars. A number of studies have been carried out, and more are planned when commissioning of the PAVO system is complete. Conversion of the system to allow remote operation will allow larger scientific projects to be undertaken.

💡 Research Summary

The University of Sydney has cultivated a distinguished legacy in optical stellar interferometry that spans more than six decades, with a particular focus on the precise characterization of hot, massive stars. The story begins in the 1960s with the Narrabri Stellar Intensity Interferometer, a pioneering instrument that employed two 6.5‑metre reflectors separated by up to 188 metres to measure photon‑intensity correlations rather than phase. By extracting the squared visibility function directly from intensity fluctuations, the Narrabri instrument succeeded in determining angular diameters for 32 O‑F type stars. Those measurements underpinned the establishment of an effective‑temperature scale tied to spectral class, providing the first robust calibration of the temperature–spectral type relationship for the hottest stars. Although intensity interferometry is intrinsically limited by low signal‑to‑noise ratios at visible wavelengths, the large apertures and long baselines of the Narrabri system compensated for this drawback and demonstrated that high‑precision stellar diameters could be obtained without phase information.

Building on that foundation, the Sydney University Stellar Interferometer (SUSI) was conceived as a conventional phase‑referencing interferometer optimized for the visible band (≈400–900 nm). This wavelength choice is especially advantageous for O‑ and B‑type stars, whose black‑body peaks lie near the visual regime, delivering higher contrast than infrared‑focused facilities. SUSI’s design incorporates a movable siderostat array that provides baselines ranging from 5 m to 160 m, enabling angular resolution down to a few tens of micro‑arcseconds—sufficient to resolve the photospheres of stars that are only a few solar radii across at distances of several hundred parsecs. Early SUSI observations used a single‑channel beam combiner, but the instrument is now undergoing a third‑generation upgrade that introduces the Precision Astronomical Visible Observations (PAVO) beam combiner.

PAVO represents a substantial technological leap. It implements multi‑channel spectro‑interferometry, simultaneously recording fringe data across a ≈20 nm spectral band. High‑speed, low‑read‑noise detector arrays allow real‑time correction of atmospheric piston fluctuations, dramatically improving fringe contrast and measurement precision. Compared with the original SUSI configuration, PAVO delivers roughly a factor of two to three gain in both sensitivity and temporal resolution, making it possible to acquire high‑quality visibility curves in a fraction of the time previously required. The multi‑wavelength capability also permits direct assessment of wavelength‑dependent limb darkening, temperature gradients, and rotational flattening, all of which are critical diagnostics for hot, rapidly rotating stars.

Scientific results already emerging from SUSI + PAVO include precise angular diameters for benchmark objects such as β Ceti and α Carinae, refined effective temperatures for a suite of O‑B stars, and quantitative measurements of rotational oblateness for objects like β Sigma Orionis. The interferometer has also been employed to resolve close binary systems, yielding orbital elements and component mass ratios that feed directly into stellar evolution models. These observations have validated and, in some cases, revised the classical Hertzsprung‑Russell relationships for massive stars, providing empirical constraints on theoretical opacity tables and convection prescriptions.

The next phase of the program focuses on completing PAVO commissioning and transitioning SUSI to fully remote operation. Remote control will decouple observing schedules from on‑site staffing constraints, enabling long, uninterrupted monitoring of variable phenomena such as pulsations, eclipses, and the Be‑star disk formation cycle. Moreover, the development of automated data reduction pipelines and cloud‑based analysis platforms will facilitate large‑scale surveys, potentially encompassing hundreds of hot stars across the southern sky. Such surveys aim to construct an updated, statistically robust temperature–luminosity calibration, investigate metallicity effects on stellar radii, and explore the incidence of rapid rotation in different stellar populations.

In summary, the University of Sydney’s interferometric facilities have evolved from a groundbreaking intensity‑interferometer that set the temperature scale for hot stars to a modern, high‑precision, visible‑light phase interferometer equipped with state‑of‑the‑art multi‑wavelength beam combination. The ongoing upgrades and remote‑operation capabilities position SUSI + PAVO to become a premier platform for detailed investigations of hot‑star physics, providing essential empirical inputs for stellar structure theory, galactic evolution studies, and the calibration of future space‑based interferometric missions.