A study of faint minor planets (m 20) was undertaken to extend the McDonald Asteroid Survey beyond m = 16.0. Plates were taken with the 48-inch Scirniidt camera of the Palomar Observatory; they cover a field of about 120 X 180 on the sky, centred on the vernal equinox. More than 2000 asteroids were discovered, of which 1800 yielded orbits which were sufficiently accurate for statistical treatment. Extension of the log number versus mean opposition magnitude relation to m0 = 20.6 showed the increase to be linear, of slope 0.39, except for the interval 13.0 m0 < 17.5, where the values of log N(m0) are lower than those given by the linear relation. Contrary to a result from the McDonald Survey, no clear-cut differences were found in the distribution of absolute magnitudes g in different parts of the asteroid ring beyond g = 12.0. Moreover, the discontinuity in the log N(g) distribution appears in the present material at the same value of g for thiee different distance groups, again in contradiction to the McDonald Survey. The space density of asteroids at 0.28 A.U. from the plane of the ecliptic is found to be half that in the plane, at 0.8 A.U. it is very small. The Kirkwood gaps in the distribution function of semi-major axes are similar for bright and faint asteroids, and no difference in the distribution functions of eccentricities and inclinations was noticed. The phase function of the faint asteroids is shown to be different from that for the bright ones, the linear part being steeper; the opposition effect, however, is the same for bright and faint objects. Many known groupings are represented among the fainter minor planets, e. g. Trojans, members of the Hilda and Hungaria groups and of the Hirayama families (except the Phocaea family). New families were also found, similar to the Hirayama fanlilies. Most of the loose groups introduced by Brouwer could not be recognized in this survey, and those which were found again had the character of small families or were part of larger ones. Apparently Brouwer's criterion for membership of these groups, the approximate constancy of the sum of the longitudes of proper node and proper perihelion, is irrelevant. The material was also searched for jet streams, as introduced by Mfve'n. One may have been found, within a family for which j> 40; other families could not be used because of the declination of the material. Of the objects with orbits 42 per cent are members of recognized families and groups. There are indications that some families occur in pairs.
Astronomy and Astrophysics Supplement Series
- Pub Date:
- October 1970