Asteroid photometry at small phase angles

Kurzfassung

One of us (JP) carried out photoelectric photometry for 12 asteroids at Lowell Observatory in 1984-85. We determined the synodic rotation period for (83) Beatrix (10.11 hr), (121) Hermione (5.6 hr),(162) Laurentia (11.87 hr), (508) Princetonia (52.8 hr), (510) Mabella (19.40 hr?), and (636) Erika (14.62 hr).The previously published periods for (276) Adelheid, (344) Desiderata, and (984) Gretia are confirmed. We also present B-V colour indices and provide new information for the taxonomic classification of three of the objects. The observations are submitted for publication in Astronomy and Astrophysics. (83) Beatrix was also observed at 1988, 1990 and 1992 apparitions by other observers and we were thus able to calculate its pole position. The purpose of the observations was to study the behavior of asteroid lightcurves at as wide a range of phase angles as possible. Some previous observations, mainly of high-albedo asteroids indicate the presence of an opposition spike at very low phase angles (e.g. Harris and Young, 1986, Bull.Amer.Astron.Soc.,18, 797). Recent theoretical work by Muinonen (1990, PhD. Dissertation,University of Helsinki, Helsinki, Finland, and references therein) has suggested that wave-optical effects may become important at very small solar phase angles. The phase curves of (83) Beatrix, (276) Adelheid, and (344) Desiderata reach subdegree phase angles. No opposition spike is seen in the phase curves of these asteroids. This seems to certify that opposition spike is mainly a property of high-albedo asteroids and planetary satellites.