3 research outputs found
HELIUM PHOTODISINTEGRATION AND NUCLEOSYNTHESIS: IMPLICATIONS FOR TOPOLOGICAL DEFECTS, HIGH ENERGY COSMIC RAYS, AND MASSIVE BLACK HOLES
We consider the production of He and H by He photodisintegration
initiated by non-thermal energy releases during early cosmic epochs. We find
that this process cannot be the predominant source of primordial H since it
would result in anomalously high 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 eV. The constraints on these models derived from
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 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