HELIUM PHOTODISINTEGRATION AND NUCLEOSYNTHESIS: IMPLICATIONS FOR TOPOLOGICAL DEFECTS, HIGH ENERGY COSMIC RAYS, AND MASSIVE BLACK HOLES

We consider the production of $^3$He and $^2$H by $^4$He photodisintegration initiated by non-thermal energy releases during early cosmic epochs. We find that this process cannot be the predominant source of primordial $^2$H since it would result in anomalously high $^3$He/D ratios in conflict with standard chemical evolution assumptions. We apply this fact to constrain topological defect models of highest energy cosmic ray (HECR) production. Such models have been proposed as possible sources of ultrahigh energy particles and gamma-rays with energies above $10^{20}$eV. The constraints on these models derived from $^4$He-photodisintegration are compared to corresponding limits from spectral distortions of the cosmic microwave background radiation (CMBR) and from the observed diffuse gamma-ray background. It is shown that for reasonable primary particle injection spectra superconducting cosmic strings, unlike ordinary strings or annihilating monopoles, cannot produce the HECR flux at the present epoch without violating at least the $^4$He-photodisintegration bound. The constraint from the diffuse gamma-ray background rules out the dominant production of HECR by the decay of Grand Unification particles in models with cosmological evolution assuming standard fragmentation functions. Constraints on massive black hole induced photodisintegration are also discussed.Comment: 20 latex pages, 1 figure added via figures comman

Circumstellar gas in beta Pic

International audienceFrom the very early observations completed in 1984, it was clear that some gas was present in the circumstellar environment. In particular the CaII lines were showing very strong absorption signatures never observed in the local ISM toward similarly nearby stars. From the comparison of the two first observations of these lines it became also clear that these lines were time variable. This lead to numerous observational campaigns showing that stable gas as well as infalling gas was present in the system. The concept of falling and evaporating bodies (FEBs) was born: unexpectedly, exocomets were observed before exoplanets ! The lifetime of the continuously produced gas being very short, it lead to a needed additional concept of a stable braking gas at rest relative to the system and able to stop all other species through collisions. The nature of this braking gas is still debated and its location unclear. The distance of the gas is shown to be very extended, some of it being at less than 1 AU, mainly from absorption studies and some at more than 100 AU from the star, mainly from the observation of emissions signatures. The nature, origin and composition of the gas seem to be due to either the evaporation of FEBs for the closer gas or to the evaporation of distant orbiting and evaporating bodies (OEBs) of all sizes including grains, for the more remote gas