The concentration of X's in the cores of low-mass stars in advanced evolutionary stages

Abstract

It has been suggested in a preceding paper that X's—massive (≈4mp) elementary components of dark matter—have cross-sections for collisions with nucleons ≳2·10−37 cm2 and that in these collisions an appreciable fraction of the available energy can be emitted in the form of easily escaping «dark radiation». X's would steadily accrete onto stars and concentrate in their cores; an energy sinkLx≈Lo could result from their presence in the core of a main-sequence star of mass≈1Mo. As the star evolves, the concentration of X's shrinks faster than the stellar core; at some time during the giant stage the central density of X's would grow higher than that of ordinary matter; after that time the concentration could not provide a stellar-energy sink any longer However, this undesirable clogging of the central energy sink is avoided if the X's are (non-Majorana) fermions, as we assume. On the other hand, the carriers of dark radiation should be the massless quanta of a pseudoscalar field strongly coupled to the Dirac field of the X's. The hypothesis of the X's would lead to a picture of the structure and evolution of lowmass stars quite different from the conventional picture; in particular, the central temperature of a red giant would be of some thousands of kelvins only; H-burning would provide the energy source of DA white dwarfs; the DA stage could be followed by the planetary or the non-DA stage. Different ways of testing the hypothesis of the X's are discussed in the last section of the paper.