The Occultation of Beta Scorpii by Jupiter and Io. II. Io

Photoelectric observations of the occultation of p Sco C (p2 Sco) by the Jovian satellite lo were made on 14 May 1971 from St. Thomas in the Virgin Islands. Accurate times of ingress and egress were determined, and the effects of Fresnel diffraction at the limb of lo were observed. In spite of the extreme sensitivity of such an observation to refraction, no effects due to the presence of an atmosphere were detected. This places an upper limit of P <7 X l0- (T/ i for any atmosphere on Io. In carrying out this analysis it was necessary to take account of the angular diameter of p Sco C which may be estimated from astrophysical arguments at about 2.0X l0- arc sec while the best observational fit corresponds to l.6X l0- arc sec. The detailed struc- ture of the trace strongly suggests the presence of a faint companion to p Sco C.