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Nuclear Reactions and Element Synthesis in the Surface of Stars.

The Astrophysical Journal Supplement Series1955Vol. 2, pp. 167–167

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William A. Fowler, G. R. Burbidge, E. M. Burbidge

Abstract

The astronomical evidence suggests that the anomalous overabundances, particularly of some heavy elements, in peculiar A stars with magnetic fields (Apm stars) are unlikely to be due to synthesis in the stellar interior. It is suggested in this paper that regions develop in which sufficient magnetic energy is available to accelerate particles on the stellar surface so that nuclear reactions can take place there. A a model it is supposed that the particles are accelerated in regions of changing magnetic field by the Swann betatron mechanism, modified to take account of the plasma and collision properties of the gas. The conditions necessary for element synthesis are plausible. They are, depending on the efficiency of the acceleration and nuclear processes, that the spot regions (assumed to have radii of 0 cm) have densities of 1 10- g/cc and local magnetic fields H = 5 X 10 -10 gauss and that average particle energies of 1 Mev are attained. One to 10 per cent of the spot is involved (or a much smaller fraction for the very large spots), and a fraction 0.01 .5 of the magnetic energy is converted into kinetic energy. The anomalous abundances occur in a surface layer reaching down to p = 10- g/cc. It is assumed that the time scales over which the mechanism is effective are sec. Radiative effects may limit the time scale to the order of 1-10 sec, but favorable choices for the other parameters involved will compensate for this. Depending on the details of acceleration, energy loss, and energy-sharing mechanisms, we discuss two specific models. a) An energy spectrum of the accelerated particles given byf(E) L-3, with about 1 per cent of the particles accelerated to Mev. The still higher energy particles in the spectrum are to be identified with the observed high-energy cosmic radiation, probably after some additional acceleration in interstellar fields. Direct synthesis then takes place by (a, n) . . . (a, 3 nucleons) and by (d, p) and (d, n) reactions. This model can be criticized on the grounds of energy loss to electrons by the accelerated nuclei at p = 10- (h7 g/cc, and it can be ruled out observationally if the D/H ratio in the Apm stars 0.01, since theoretically it builds up to 0.01. b) A Maxwell-Boltxmann energy distribution in a "hot spot" within the magnetic spot, with `kT I Mev (0.3 <kT < 1.5), and all the particles involved. Synthesis then takes place by neutron production through (p, n) processes principally on the light nuclei, followed by (n, ) capture processes on the metals. In the primary (p, n) process each stable target nucleus, X, serves catalytically to change protons into neutrons. The unstable residual isobar, Y, produced in the reaction X + p Y + n decays back to X by positron-neutrino emission or electron capture, i.e., Y H X + + + p or Y + e H X + p. In this case the theoretical D/H ratio might be as low as 10- . These results are applied to the observed anomalies in a2 CVn and HD 133029. By making plausible estimates about the spot activity and the amount of mixing, the overabundances of Sr, Y, Zr, La, the rare earths, and Pb can be reasonably explained if the activity has gone on for 5 X 108 years. Other anomalies, i.e., of 0, Si, and Ca may also be explicable. Stellar surface synthesis of the cosmical D, Li, Be, and B is discussed, since these are unstable at the temperatures of stellar interiors. It is suggested that they are produced by spallation reactions on C, N, and 0, either high up in the stellar atmospheres (p < 10- ) or less probably in "hot spots" similar to case b, but where kT 5 Mev. The postulated amount of magnetic activity in Apm stars is insufficient to produce reasonable abundances. If much smaller-scale activity on a larger number of stars (red dwarfs) is considered, then in 10 years a D/H ratio of 10- -8 and the cosmical abundances of Li, Be, and B can be produced, if a fairly high efficiency of production and ejection in these stars is assumed. The absolute upper limit on D/H is as high as 10-2 if equilibrium between stellar surfaces and interstellar material is reached. The natural radioactive elements may be produced by collisions between metal nuclei and the stable, heavy nuclei, such as lead and bismuth.

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Abstract

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