Observations of the southern Milky Way at 21 centimeters.

Abstract

Whipple 1950, 1951), if we assume that the meteors originated in the outer layers of the parent comet's nucleus, under relatively weak pressure. This would imply that comet Giacobini-Zinner became a producer of meteors only in recent times, possibly after the great Jupiter perturbation of 1897, immediately preceding the comet's discovery. An analysis of other showers shows that the Taurid meteors display a low degree of fragmentability. This is consistent with the above explanation, since their parent comet, Halley's, is large and old, so that present-day Taurids probably came from layers which originally were deep inside the nuclear ices. A survey of beginning heights, decelerations and fragmentation indices for some of the major showers, a-Capricornids, Taurids, t-Aquarids, Geminids, ô-Aquarids, Lyrids, Perseids, Orionids, based exclusively on Super-Schmidt material, shows that each shower has its own distinct peculiarities, revealed by systematic deviations from the mean, as given by nonshower, sporadic meteors. These deviations are in both directions, plus and minus, with no special preference ; in particular they do not indicate a systematically greater height of appearance for shower meteors. In view of the many factors which may combine to cause each single deviation, any attempt at a detailed explanation seems to be premature at this stage.

Bending instabilities at the origin of persistent warps: A new constraint on dark matter halos

“…Since the first observation of the Milky Way warp at the end of the fifties (Kerr & al., 1956;Burke, 1957), half a century has passed without a clear explanation of this feature (see Binney, 1992, for a review). The warp problematic comes essentially from its high frequency in spirals.…”