S U S I Stellar Angular Diameters Determined with SUSI J. Davis, M.J. Ireland, A.P. Jacob, J.R. North, S.M. Owens, J.G. Robertson, W.J. Tango, P.G. Tuthill Sydney University Stellar Interferometer Abstract SUSI’s newly commissioned beam- combination system, working at the red end of the visual spectrum, has provided a significant increase in sensitivity. The increased sensitivity has resulted in an improvement in calibration and accuracies of <±1% in angular diameters have been achieved. A program of accurate angular diameter determinations of stars of later spectral type than those measured with the Narrabri Stellar Intensity Interferometer is now underway. Why Measure Angular Diameters? The measurement of the angular diameters of stars leads to the direct determination of fundamental stellar properties  Emergent fluxes, effective temperatures, radii and luminosities The measurements also provide tests of model atmosphere predictions either directly, or in combination with other data such as spectrophotometry, flux measurements and parallaxes. Measuring the Angular Diameter of a Star The visibility of interference fringes formed by starlight decreases as the separation of the apertures (the baseline) of an interferometer increases. The rate of decrease depends on the angle subtended by the star (its angular diameter) as shown below. Visibility 2 (V 2 ) as a function of baseline is measured and this enables the angular diameter to be determined. For a circular disk that appears uniformly bright V 2 = |2J 1 (x)/x| 2 where x =  b , b is the projected baseline,  is the angular diameter of the disk and is the wavelength. The Angular Diameter of  CMa (HR 2693) As an example of our success in improving the accuracy of angular diameter measurements we list results for  CMa (F8 Ia). It was the faintest star and latest spectral type measured with the Narrabri Stellar Intensity Interferometer (Hanbury Brown, Davis & Allen, MNRAS, 167,121,1974). We have subsequently measured it with SUSI’s original blue beam-combination system (Davis et al, MNRAS, 303, 783, 1999) and more recently with SUSI’s new red beam combination system, each time significantly improving the accuracy as shown in the table. Limb Darkening Stars do not appear as uniformly illuminated disks but are limb darkened. Generally the function for a uniform disk is fitted to the measured values of V 2 and a correction, obtained from model stellar atmospheres, is applied to obtain the true limb- darkened angular diameter (see for example Davis, Tango & Booth. MNRAS, 318, 387, 2000 and Tango & Davis, MNRAS, 333, 642,2002) Observations in Progress and Conclusions We have shown that angular diameters with an accuracy of < ±1% can be measured with SUSI. The limiting magnitude is currently being established through observations of fainter stars. Currently R limit ~ +5.0 and accuracies of ~ ±1% in angular diameter at this limit are achievable.  Cen (HR 5288) The observations of  Cen (K0 IIIb), calibrated with interleaved observations of  Cen and  &  Lup, are shown in the diagram. The uncertainties are smaller than the symbols representing the data. NSII: 443 nm SUSI: 700 nm  UD = 3.29 ± 0.46 mas  UD = ± mas SUSI: 700 nm  UD = ± mas;  LD = ± mas (±0.6%) The calibrators for the SUSI observations were  CMa and  CMa. Combination of the angular diameter with the flux received from the star (6.1±0.3)  0 -9 Wm -2 gives: F = (1.38±0.07)  Wm -2 and T eff = 6100±80K Calibration Measurements of V 2 for a programme star are calibrated by interleaving the observations with observations of calibrators - unresolved stars or stars of known angular diameter. Combination of the angular diameter with the Hipparcos parallax (53.52±0.79mas) gives R/R o = 10.96±0.17 (±1.6%) Spectrophotometry is required for accurate surface flux, effective temperature and luminosity determinations. The Hipparcos parallax (1.82±0.56mas) results in low accuracy values for R/R o =215±65 and L/L o =(5.6±3.5)  10 4