51 research outputs found
The dynamics of internal working surfaces in MHD jets
The dynamical effects of magnetic fields in models of radiative, Herbig-Haro
(HH) jets have been studied in a number of papers. For example, magnetized,
radiative jets from variable sources have been studied with axisymmetric and 3D
numerical simulations. In this paper, we present an analytic model describing
the effect of a toroidal magnetic field on the internal working surfaces that
result from a variability in the ejection velocity. We find that for parameters
appropriate for HH jets the forces associated with the magnetic field dominate
over the gas pressure force within the working surfaces. Depending on the ram
pressure radial cross section of the jet, the magnetic field can produce a
strong axial pinch, or, alternatively, a broadening of the internal working
surfaces. We check the validity of the analytic model with axisymmetric
numerical simulations of variable, magnetized jets.Comment: 14 pages, 4 figures. ApJ in pres
3-D Kinematics of the HH 110 jet
We present new results on the kinematics of the jet HH 110. New proper motion
measurements have been calculated from [SII] CCD images obtained with a time
baseline of nearly fifteen years. HH 110 proper motions show a strong asymmetry
with respect to the outflow axis, with a general trend of pointing towards the
west of the axis direction. Spatial velocities have been obtained by combining
the proper motions and radial velocities from Fabry-Perot data. Velocities
decrease by a factor ~3 over a distance of ~10 cm, much shorter than the
distances expected for the braking caused by the jet/environment interaction.
Our results show evidence of an anomalously strong interaction between the
outflow and the surrounding environment, and are compatible with the scenario
in which HH 110 emerges from a deflection in a jet/cloud collision.Comment: (1)Universitat de Barcelona; (2)UNAM; (3)UPC; (4)University of
Hawaii; (5)Southern Astrophysical Research Telescope. 9 pages; 7 Figures
Accepted by A&
Position-Velocity Diagrams for the Maser Emission coming from a Keplerian Ring
We have studied the maser emission from a thin, planar, gaseous ring in
Keplerian rotation around a central mass observed edge-on. The absorption
coefficient within the ring is assumed to follow a power law dependence with
the distance from the central mass as, k=k0r^{-q}. We have calculated
position-velocity diagrams for the most intense maser features, for different
values of the exponent q. We have found that, depending on the value of q,
these diagrams can be qualitatively different. The most intense maser emission
at a given velocity can either come mainly from regions close to the inner or
outer edges of the amplifying ring or from the line perpendicular to the line
of sight and passing through the central mass (as is commonly assumed).
Particularly, when q>1 the position-velocity diagram is qualitatively similar
to the one observed for the water maser emission in the nucleus of the galaxy
NGC 4258. In the context of this simple model, we conclude that in this object
the absorption coefficient depends on the radius of the amplifying ring as a
decreasing function, in order to have significant emission coming from the
inner edge of the ring.Comment: 13 pages, 7 figures, to appear in the 2007 July 20 issue of The
Astrophysical Journa
Photoevaporation of Minihalos during Reionization
We present the first gas dynamical simulations of the photoevaporation of
cosmological minihalos overtaken by the ionization fronts which swept through
the IGM during reionization in a LCDM universe, including the effects of
radiative transfer. We demonstrate the phenomenon of I-front trapping inside
minihalos, in which the weak, R-type fronts which traveled supersonically
across the IGM decelerated when they encountered the dense, neutral gas inside
minihalos, becoming D-type I-fronts, preceded by shock waves. For a minihalo
with virial temperature T_vir < 10,000 K, the I-front gradually burned its way
through the minihalo which trapped it, removing all of its baryonic gas by
causing a supersonic, evaporative wind to blow backwards into the IGM, away
from the exposed layers of minihalo gas just behind the advancing I-front.
Such hitherto neglected feedback effects were widespread during reionization.
N-body simulations and analytical estimates of halo formation suggest that
sub-kpc minihalos such as these, with T_vir < 10,000 K, were so common as to
dominate the absorption of ionizing photons. This means that previous estimates
of the number of ionizing photons per H atom required to complete reionization
which neglected this effect may be too low. Regardless of their effect on the
progress of reionization, however, the minihalos were so abundant that random
lines of sight thru the high-z universe should encounter many of them, which
suggests that it may be possible to observe the processes described here in the
absorption spectra of distant sources.Comment: 4 pages, 3 figures, to appear in, The Emergence of Cosmic Structure,
the 13th Annual October Astrophysics Conference in Maryland, S. Holt & C.
Reynolds, eds (AIP
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