45,474 research outputs found
Dust cleansing of star-forming gas: II. Did late accretion flows change the chemical composition of the solar atmosphere?
The possibility that the chemical composition of the solar atmosphere has
been affected by radiative dust cleansing of late and weak accretion flows by
the proto-sun itself, is explored. Estimates, using semi-analytical methods and
numerical simulations of the motion of dust grains in a collapsing non-magnetic
and non-rotating gas sphere with a central light source are made, to model
possible dust-cleansing effects. Our calculations indicate that the amounts of
cleansed material may well be consistent with the abundance differences
observed for the Sun when compared with solar-like stars and with the relations
found between these differences and condensation temperature of the element. It
seems quite possible that the proposed mechanism might have produced the
significant abundance effects observed for the Sun, provided that late and
relatively weak accretion did occur. The effects of cleansing may, however, be
affected by outflows from the Sun, the existence and dynamics of magnetic
fields and of the accretion disk, and the possible presence and location of the
Early Sun in a rich stellar cluster.Comment: 10 pages, accepted for publication in Astronomy & Astrophysics,
version with corrected typos and added information on magnetic field
On the intrinsic limitation of the Rosenbluth method at large momentum transfer
Correlations in the elastic electron proton scattering data show that the
Rosenbluth method is not reliable for the extraction of the electric proton
form factors at large momentum transfer, where the magnetic term dominates, due
to the size and the dependence of the radiative corrections.Comment: 10 pages, 4 figure
Nucleon electromagnetic structure: past, present, and future
We present the experimental status of electromagnetic hadron form factors.
New and surprising results, based on polarization measurements, have been
recently obtained for the electric proton and neutron form factors. In
particular, the electric and magnetic distributions inside the proton appear
not to be the same, in disagreement with results extracted from the unpolarized
cross section, using the Rosenbluth separation. The new findings have given
rise to a large number of papers and different speculations, as they question
directly the models of nucleon structure and the reaction mechanism itself
(based on -exchange), with a possible revision of the calculation of
radiative corrections, two-photon contribution etc. New data in time-like
region are also available, through annihilation reactions. A large interest in
this field arises, due also to the possibility of new measurements in polarized
electron nucleon elastic scattering at JLab, and also in the time-like region,
at Frascati and at the future FAIR international facility.Comment: 10 pages, 3 figures, to be included in a special issue of Nuovo
Cimento
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