33. Commission de la Structure et de la Dynamique du Systeme Galactique

Abstract

COMMISSION 33 of galactic novae is discussed in ch. 2 of C. Payne-Gaposchkin's monograph The Galactic Novae (North Holland Publ. Co., Amsterdam, 1957). For a description of radio observations dealing with the halo we refer to the report on radio astronomy. So far, there has been practically no common ground between the optical and radio observations of the halo. Luyten and Carpenter [7] reported on a systematic survey of faint blue stars which has been extended by Luyten and Seyfert [8]. Colour indices and proper motions were measured for stars found on Tucson plates in 45 Selected Areas. They appear to be objects of intermediate luminosity, not fainter than + 5 absolute magnitude; surprisingly few white dwarfs have turned up in the survey. Another extensive survey of blue stars in high galactic latitudes is being carried out at Tonantzintla (Iriarte and Chavira) [9]. Surveys of variable stars have been inaugurated by Shapley [10] and by the Kapteyn Laboratory at Groningen in collaboration with the Mount Wilson and Palomar observatories. For details concerning the latter project we refer to the report on the Stockholm conference on Coordination of Galactic Research (see below). The aim of the Groningen-Palomar programme is the determination of the shape of the equi-density surfaces of the population of variable stars in the halo. This programme is being supplemented by an objective-prism survey of late-type variable stars by Blanco at Cleveland, based on plates taken at the Nassau station of the Case Observatory and reaching 19-5 photographic magnitude. SPIRAL STRUCTURE Radio observations Since the previous report of this commission, various important 21-cm surveys of the distribution of neutral interstellar hydrogen have been brought to a provisional completion. The southern sky has been covered by the Australian observers, and the northern observers have extended their surveys to the inner parts of the Galaxy. Of these latter, one published by Schmidt [n] covers longitudes 340 0 to 50° and shows that the Orion and Sagittarius arms can be traced throughout this inner region; both arms appear to be trailing and seem to be connected by a hydrogen bridge near the Sun. Schmidt suggests that this bridge, between / = 34o° and 20 0 , explains the large difference between the small inclination of the spiral arms as found from the radio observations and the larger value derived from the positions of O-associations. The hydrogen in the inner parts of the Galaxy is found to be confined to a very thin layer of only 220 pc thickness between half-density planes. The average density between 6 and 8 kpc distance from the centre is 0-85 H atoms per cc, and, between 4 and 6 kpc, o-6 H atoms per cc. Results of the Australian survey have been published by Kerr, Hindman and Carpenter [12] and by Kerr and Hindman [13]. The multi-armed structure of the Galaxy is confirmed as well as the trailing tendency of the arms, and there are suggestions that the major arms can be followed through several revolutions. The question of possible sys...