433 research outputs found
Protostellar Jets: The Best Laboratories for Investigating Astrophysical Jets
Highly collimated supersonic jets are observed to emerge from a wide variety
of astrophysical objects, ranging from Active Nuclei of Galaxies (AGN's) to
Young Stellar Objects (YSOs) within our own Galaxy. Despite their different
physical scales (in size, velocity, and amount of energy transported), they
have strong morphological similarities. Thanks to the proximity and relatively
small timescales, which permit direct observations of evolutionary changes, YSO
jets are, perhaps, the best laboratories for cosmic jet investigation. In this
lecture, the formation, structure, and evolution of the YSO jets are reviewed
with the help of observational information, MHD and purely hydrodynamical
modeling, and numerical simulations. Possible applications of the models to AGN
jets are also addressed.Comment: 19 pages, PostScript (9 figures upon request). Invited review for
proceedings of the International Conference on Plasma Physics (Foz do
Iguassu, November 1994) eds. P. Sakanaka et al
Particle Acceleration by Magnetic Reconnection
Observational data require a rich variety of mechanisms to accelerate fast
particles in astrophysical environments operating under different conditions.
The mechanisms discussed in the literature include varying magnetic fields in
compact sources, stochastic processes in turbulent environments, and
acceleration behind shocks. An alternative, much less explored mechanism
involves particle acceleration within magnetic reconnection sites. In this
chapter we discuss this mechanism and show that particles can be efficiently
accelerated by reconnection through a first order Fermi process within large
scale current sheets (specially when in the presence of local turbulence which
speeds up the reconnection and make the acceleration region thicker) and also
through a second order Fermi process in pure MHD turbulent environments.Comment: 24 pages, 8 figures. arXiv admin note: text overlap with
arXiv:1103.2984, arXiv:1202.5256, arXiv:1008.1981 by other author
Evolution of the ISM of Starburst galaxies: the SN heating efficiency
The interstellar medium heated by SN explosions may acquire an expansion
velocity larger than the escape velocity and leave the galaxy through a
supersonic wind. SN ejecta are transported out of the galaxies by such winds
which thus affect the chemical evolution of the galaxies. The effectiveness of
the processes mentioned above depends on the heating efficiency (HE) of the
SNe, that is a matter of debate. We have constructed a simple semi-analytic
model, considering the essential ingredients of a SB environment which is able
to qualitatively trace the thermalisation history of the ISM in a SB region and
determine the HE evolution. We find that, as long as the mass-loss rate of the
clouds remains larger than the rate at which the SNRs interact one with each
other, the SN heating efficiency remains very small, as radiative cooling of
the gas dominates. We conclude that the HE value has a time-dependent trend
that is sensitive to the initial conditions of the system.Comment: 17 pages, 18 figures, A&A accepte
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