Nebula

Abstract: Spectia of sixty-seven H Ir regions from 3 to 24 kpc from the nucleus of M31 have been obtained with the DTM image-tube spectrograph at a dispersion of 135 A mm '. Radial velocities, principally from Ha, have been determined with an accuracy of 10 km sec ' for most regions. Rotational velocities have been calculated under the assumption of circular motions only. For the region interior to 3 kpc where no emission regions have been identified, a narrow [N 13] x6583 emission line is observed. Velocities from this line indicate a rapid rotation in the nucleus, rising to a maximum circular velocity of V = 225 km at R = 400 pc, and falling to a deep minimum near R = 2 kpc. From the rotation curve for R 3 kpc, agree with the 21-cm observations, although the maximum rotational velocity, V = 270 t 10 km sec ', is slightly higher than that obtained from 21-cm observations.

Abstract: A steady state, spherically symmetric, magnetohydrodynamic model of the Crab nebula is constructed. A highly relativistic positronic pulsar wind is terminated by a strong MHD shock that decelerates the flow and increases its pressure to match boundary conditions imposed by the recently discovered supernova remnant that surrounds the nebula. If the magnetic luminosity of the pulsar wind upstream of the shock is about 0.3 percent of its particle luminosity, the pressure and velocity boundary conditions imposed by the remnant place the shock where it is inferred to be: near the outer boundary of an underluminous region observed to surround the pulsar. It is necessary to include the weak magnetization of the wind to satisfy the boundary conditions and to calculate the regular synchrotron radiation self-consistently.

Abstract: A compilation of data on 78 elements in the nine groups of chondrites shows each to be isochemical with the exception of a few volatiles. With the exception of the most volatile elements, the groups have solar abundances to within a factor of two. The solar abundances and the chemical and physical properties of phases in the leastaltered chondrites indicate formation by grain agglomeration in the preplanetary nebula. Planets formed by the gradual growth of bodies in the solar nebula. Because there is no evidence for the formation of non-chondritic bodies in the nebula, the simplest model calls for the bulk compositions of the terrestrial planets to be chondritic. Mercury is enriched in metal, perhaps either because of high loss of silicates due to enhanced radial drag in the innermost part of the nebula, or because of enhanced accretion of metallic cores from disrupted asteroids. Chondritic compositions should be considered as boundary conditions for planetary models.