ANGULAR DIAMETER AND LIMB DARKENING OF ARCTURUS

The diameter of Arcturus has been measured with the MkIII Optical Inte rferometer at 5 wavelengths between 450 and 500 nm. The wavelength dep endence of the uniform disk diameter agrees quantitatively with predic tions from limb-darkening coefficients for a grid of model atmospheres tabulated by Manduca et al. (1977) and Manduca (1979). The true limb- darkened diameter of Arcturus is 21.0 +/- 0.2 mas. By combination of t his value with the bolometric flux of (5.04 +/- 0.20) x 10(-8) Wm(-2) from the literature, the effective temperature is determined to be 430 3 +/- 47 K. It is also shown that the visibility function (i.e., visib ility as a function of baseline length) cannot be fitted with uniform disk models close to and beyond the first null. The measured shape of the visibility function agrees well, however, with the Manduca et al. limb-darkened disk models.