Stellar X-ray sources

Abstract

The solar corona is the source both of energetic quanta and of the solar wind. Being maintained at a temperature of ≥ 1 MK by the energy of progressive waves emitted by the top layers of the hydrogen convective zone or the adjacent transition region, it converts a fraction of the heat flow, which in the case of the Sun is quite small, into the energy of radiation in the XUV range and of particles with one kiloelectronvolt per nucleon by an order of magnitude. The question which other stars possess similar coronae, is clearly of considerable interest for our understanding of the structure of the outer layers of such stars and of their evolution, in view also of the connected processes of the loss of mass and of angular momentum of rotation. It might furthermore be of some interest in connexion with the question of the density of ionizing radiation in the soft X-ray range in interstellar space. Though indirect conclusions can be reached from the known properties of stellar chromospheres and, in some cases, of stellar winds, the only observational approach to this problem known so far is to measure the emission of such stellar coronae in the soft X-ray range. This paper attempts to provide order of magnitude estimates of the fluxes of quanta to be expected, on the basis of the solar data for stars of similar spectral type and luminosity and of theoretical inferences concerning the coronae of stars of other spectral and luminosity classes. The limited transparency of interstellar space makes it necessary to discuss also the value of the interstellar density between clouds, which is at present rather uncertain and may be as low as 1 hydrogen atom per 10 cm 3 and the influence of the interstellar helium, which on the basis of new measurements seems to be less important than was thought until recently. Finally, the observability of stellar flares will be discussed briefly, because such events would contribute quanta of higher energy than that of the quanta ordinarily to be expected from stellar coronae.