The Formation of Arms in Barred Spirals.

Abstract

view Abstract Citations References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Formation of Arms in Barred Spirals. Danby, J. M. A. Abstract Many barred spirals have condensations or blobs at the ends of the bars. A collinear configuration consisting of a bar and two blobs is unstable to the extent that it might be expected to become unrecognizable after one rotation of the bar. Yet actual configurations remain essentially unchanged over several revolutions, for the blobs are usually the seats of well-formed arms. In a reference system rotating with the bar, the equipotential surfaces of the bar will have saddle or librational points beyond the ends of its longest axis; these are positions of unstable equilibrium, but retrograde periodic orbits around them are possible. It is suggested that a blob initially consists of a shifting population of bodies, and appears as a condensation simply because its members are moving relatively slowly. Most members return to the vicinity of the bar. Those that cscape form a thin, well-defined spiral arm, regardless of the direction with which they escape from the point of libration, and, within reason, regardless of the speed with which they leave. The zero-velocity curves at this point seem to have a strong "focusing" effect on the orbits. The mean rotation of a blob is direct. If a blob becomes large enough to have a local gravitational influence, the region from which matter can escape from the system lies on the side away from the bar. The orbits of escaping particles continue to form thin spiral arms. Publication: The Astronomical Journal Pub Date: 1965 DOI: 10.1086/109533 Bibcode: 1965AJ.....70Q.320D full text sources ADS |