S. M. Owens

The fundamental parameters of the roAp star α Circini

We have used the Sydney University Stellar Interferometer to measure the angular diameter of α Cir. This is the first detailed interferometric study of a rapidly oscillating A (roAp) star, α Cir being the brightest member of its class. We used the new and more accurate Hipparcos parallax to determine the radius to be 1.967 ± 0.066 R⊙ . We have constrained the bolometric flux from calibrated spectra to determine an effective temperature of 7420 ± 170 K . This is the first direct determination of the temperature of an roAp star. Our temperature is at the low end of previous estimates, which span over 1000 K and were based on either photometric indices or spectroscopic methods. In addition, we have analysed two high-quality spectra of α Cir, obtained at different rotational phases and we find evidence for the presence of spots. In both spectra we find nearly solar abundances of C, O, Si, Ca and Fe, high abundance of Cr and Mn, while Co, Y, Nd and Eu are overabundant by about 1 dex. The results reported here provide important observational constraints for future studies of the atmospheric structure and pulsation of α Cir.

The radius and mass of the subgiant star β Hyi from interferometry and asteroseismology

We have used the Sydney University Stellar Interferometer to measure the angular diameter of β Hydri. This star is a nearby G2 subgiant the mean density of which was recently measured with high precision using asteroseismology. We determine the radius and effective temperature of the star to be 1.814 ± 0.017 R⊙ (0.9 per cent) and 5872 ± 44 K (0.7 per cent) respectively. By combining the radius with the mean density, as estimated from asteroseismology, we make a direct estimate of the stellar mass. We find a value of 1.07 ± 0.03 M⊙ (2.8 per cent), which agrees with published estimates based on fitting in the Hertzsprung–Russell diagram, but has much higher precision. These results place valuable constraints on theoretical models of β Hyi and its oscillation frequencies.

The radius and other fundamental parameters of the F9 V star β Virginis

We have used the Sydney University Stellar Interferometer (SUSI) to measure the angular diameter of the F9 V star β Virginis. After correcting for limb darkening and combining with the revised Hipparcos parallax, we derive a radius of 1.703 ± 0.022 R ⊙ (1.3 per cent). We have also calculated the bolometric flux from published meas urements which, combined with the angular diameter, implies an effective temperature of 6059 ± 49 K (0.8 per cent). We also derived the luminosity of β Vir to be L = 3.51 ± 0.08 L ⊙ (2.1 per cent). Solar-like oscillations were measured in this star by Carrier et al. (2005) an d using their value for the large frequency separation yields the mean stellar density with an uncertainty of about 2 per cent. Our constraints on the fundamental parameters of β Vir will be important to test theoretical models of this star and its oscillations.

The Sydney University Stellar Interferometer: A Major Upgrade to Spectral Coverage and Performance

A new beam-combination and detection system has been installed in the Sydney University Stellar Interferometer working at the red end of the visual spectrum (λλ 500–950 nm) to complement the existing blue-sensitive system (λλ 430–520 nm) and to provide an increase in sensitivity. Dichroic beam-splitters have been introduced to allow simultaneous observations with both spectral systems, albeit with some restriction on the spectral range of the longer wavelength system (λλ 550–760 nm). The blue system has been upgraded to allow remote selection of wavelength and spectral bandpass, and to enable simultaneous operation with the red system with the latter providing fringe-envelope tracking. The new system and upgrades are described and examples of commissioning tests presented. As an illustration of the improvement in performance the measurement of the angular diameter of the southern F supergiant δ CMa is described and compared with previous determinations.

Progress in commissioning the Sydney University Stellar Interferometer (SUSI)

The Sydney University Stellar Interferometer (SUSI) is a long baseline optical amplitude interferometer. In its initial configuration it is a two aperture, single ro instrument with wavefront-tilt corrections and dynamic optical path length compensation. It has been designed to measure the angular dimensions of stars of essentially all spectral types as well as the angular separations of binary stars. SUSI is located alongside the Australia Telescope at the Paul Wild Observatory, near Narrabri in northern New South Wales, Australia. It has a North-South array of input stations giving baselines covering the range from 5 m to 640 m. The baselines are being progressively commissioned, starting with the shorter ones, in parallel with an observing program aimed at fine-tuning the performance of the instrument. Progress and results from the commissioning program and the current status of the instrument are described.

The Emergent Flux and Effective Temperature of δ Canis Majoris

New angular diameter determinations for the bright southern F8 supergiant δ CMa enable the bolometric emergent flux and effective temperature of the star to be determined with improved accuracy. The spectral flux distribution and bolometric flux have been determined from published photometry and spectrophotometry and combined with the angular diameter to derive the bolometric emergent flux ℱ = (6.50 ± 0.24) × 107 Wm−2 and the effective temperature T eff = 5818 ± 53 K. The new value for the effective temperature is compared with previous interferometric and infrared flux method determinations. The accuracy of the effective temperature is now limited by the uncertainty in the bolometric flux rather than by the uncertainty in the angular diameter.

SUSI: an update on instrumental developments and science

The Sydney University Stellar Interferometer is a long baseline optical interferometer located in northern New South Wales, Australia. It has a North-South array of eleven fixed input siderostat stations giving a range of baselines from 5 to 640 m. Currently ten baselines from 5 to 160 m are fully operational and beam-combination and detection systems for the spectral ranges 430-520nm and 550-950nm are available. Dichroic beam-splitters have been introduced to allow simultaneous observations with both spectral systems. The original blue beam-combination system has been upgraded to improve sensitivity and to allow rapid wavelength switching. A software scheduler has been developed to automate much of the observational procedure including the acquisition of a star, fringe search and acquisition, recording of fringe scans, and the taking of photometric scans. A data pipeline for processing the observational data has been further developed to include seeing corrections and this has improved the calibration of the observational data. Preliminary results of scientific observations with both blue and red systems, including observations of single stars, binary stars and Cepheid variables are described.

The SUSI instrument: new science and technology

The Sydney University Stellar Interferometer (SUSI) has now been operating with a significanly more sensitive beam combiner system (the Red Table) for several years. A number of novel results from the realms of single, binary and pulsating star astrophysics are presented. Simultaneous dual beam-combiner operation with the Red Table and an upgraded Blue Table has been demonstrated, enabling a high spectral dispersion oberving mode for stellar studies. Plans for a major upgrade are briefly described. One component of this will be remote operation of the array for routine observational data taking, with first steps on this path reported here with the inception of the Sydney Remote Operations Center. The backbone of future plans with SUSI center around the installation of the southern of the twin PAVO instruments. PAVO is also discussed in more detail in Ireland et al, this proceedings.

An Overview of the Susi Control System

The SUSI control system is a distributed real-time system currently consisting of 17 processors. A custom real-time operating system and network protocols ensure synchronous operation of servo loops across multiple processors.

Cepheid Observations with the Sydney University Stellar Interferometer: ℓCarinae and βDoradus

Observations of the southern Cepheids lCar and β Dor to yield the mean angular diameters and angular pulsation amplitudes have been made with the Sydney University Stellar Interferometer (SUSI)at a wavelength of 700 nm. The results of a preliminary analysis are compared with those obtained with the VLTI at 2.2 μm and excellent agreement between the results from the two instruments is found for lCar but there are significant differences for βDor.