Photometry of asteroids.

Abstract

The magnitudes in the present Ephemerides of Minor Planets are not on any known photometric system, and have a probable error of 0.3 mag. In the Yerkes-McDonald Asteroid Survey, under the direction of Kuiper (Groeneveld and Kuiper 1954), the measures on the plates are made with the Ross photometer and calibrated with Mount Wilson photographic magnitudes in Selected Areas. Corrections are being applied for differential extinction, for field effects on the plate, for trailing of the asteroid image and, as found in unpublished work by Baum, a systematic correction to the Selected Areas. A simple method was developed, for both photographic and photoelectric work, for reducing apparent to absolute magnitude, namely g = pg - 5 log rp - aF, where pg is the observed magnitude, r and p are respectively the heliocentric and geocentric distance in astronomical units, a is the phase angle and F is the phase factor 6pg/~a. Photographically, with the Yerkes 24- inch reflector, an average phase factor of 0.023 mag/degree, between phase angles 100 and 200, was found from 22 asteroids. Light-curves of asteroid 20 were obtained (Gehrels 1956) photoelectrically with the ~~-inch on 6 nights, at phase angles ranging from near 00 to 200; a sharp increase of the phase factor was found at phase angles smaller than 70 The magnitude systems of all regular asteroid programs were compared with that of the YerkesMcDonald Survey. Systematic differences as large as 3 mag. were found; the difference between Survey magnitudes and those in the Ephemerides is +1.7 + o.i(pg - 15.0). Data from the regular asteroid programs and from the Survey are now being combined to mean absolute magnitudes on the International Photographic System, with a probable error of some o.o8 mag. Gebrels, T. 1956, Ap. J. 123, 331. Groeneveld, Ingrid and Kuiper, Gerald P. 1954, Ap. J. 120, 200. Verkes Observatory, Williams Bay, Wis.