Pallas, Vesta, Ceres and Victoria; photoelectric photometry.

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Abstract
Photoelectric observations of several asteroids were made at the Goethe Link Observatory with the 36-inch reflector during the summer and fall of 1950. The possibility of short-period variation, caused by a combination of rotation, and either irregular shape or surface markings, was especially examined. Pallas, followed intensively for 6 hours on June 6, and less intensively on five other nights, showed no variation greater in amplitude than .03 mag. of period less than 8 hours. Although the observations are concurrent in time and color with those of 0. Gunther,1 they do not confirm the period .23720 day and amplitude .i8 mag. he found photographically. In the Link observations, an indication of some period of the order of 4.9 days and amplitude of .04 mag. was noticed, but not with certainty. Vesta, observed sufficiently to permit the determination of the complete light curve, shows maxima and minima of two different shapes. Thus, we confirm a previous result by Harris. One maximum is characterized by a flattened downward sloping portion, while the other is rounded and symmetrical. The minimum succeeding the flattened maximum is definitely broader than the other minimum, although the depths are equal. The flattened maximum is .013 mag. fainter than the symmetrical one. Our period for Vesta is, therefore, .445230 day, or twice the usual period. A new determination of the phase coefficient gives .026 mag. per degree of phase angle. Vesta's light variation may be explained by analysis of the light curve into two components, one a sine curve due to a rotating ellipsoid of relative semi-axes I ,I, 1.13, and the other a much smaller component, consisting of the deviations of the actual light curve from the sine component. These deviations may be caused by spots, one of the elongated ellipsoid's sides having more spots than the other. Three major large dark areas on Vesta are suggested by this analysis. A different interpretation, in terms of a slightly smaller ellipsoid of the same proportions, will explain the light curve in terms of three large mountain ranges on Vesta. Present observations do not permit distinguishing between the spot model and the mountain model, and the need for observations far from opposition is emphasized. For Ceres and Victoria, a negative result was obtained, although variation greater in amplitude than .03 mag. and any period between 5 minutes and several hours should have been easily recognized. In these two cases, however, our observations are not extensive enough for further conclusions. Financial assistance from the Office of Naval Research in this project is gratefully acknowledged. I.A. N. 280, 87, 1951. Goethe Link Observatory, Indiana University, Bloomington, Ind.

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