235 research outputs found
Cassiopeia A and its Clumpy Presupernova Wind
The observed shock wave positions and expansion in Cas A can be interpreted
in a model of supernova interaction with a freely expanding stellar wind with a
mass loss rate of ~3e-5 Msun/yr for a wind velocity of 10 km/s. The wind was
probably still being lost at the time of the supernova, which may have been of
Type IIn or IIb. The wind may play a role in the formation of very fast knots
observed in Cas A. In this model, the quasi-stationary flocculi (QSFs)
represent clumps in the wind, with a density contrast of several 1000 compared
to the smooth wind. The outer, unshocked clumpy wind is photoionized by
radiation from the supernova, and is observed as a patchy HII region around Cas
A. This gas has a lower density than the QSFs and is heated by nonradiative
shocks driven by the blast wave. Denser clumps have recombined and are observed
as HI compact absorption features towards Cas A.Comment: 13 pages, ApJL, in pres
Dynamical Expansion of H II Regions from Ultracompact to Compact Sizes in Turbulent, Self-Gravitating Molecular Clouds
The nature of ultracompact H II regions (UCHRs) remains poorly determined. In
particular, they are about an order of magnitude more common than would be
expected if they formed around young massive stars and lasted for one dynamical
time, around 10^4 yr. We here perform three-dimensional numerical simulations
of the expansion of an H II region into self-gravitating, radiatively cooled
gas, both with and without supersonic turbulent flows. In the laminar case, we
find that H II region expansion in a collapsing core produces nearly spherical
shells, even if the ionizing source is off-center in the core. This agrees with
analytic models of blast waves in power-law media. In the turbulent case, we
find that the H II region does not disrupt the central collapsing region, but
rather sweeps up a shell of gas in which further collapse occurs. Although this
does not constitute triggering, as the swept-up gas would eventually have
collapsed anyway, it does expose the collapsing regions to ionizing radiation.
We suggest that these regions of secondary collapse, which will not all
themselves form massive stars, may form the bulk of observed UCHRs. As the
larger shell will take over 10^5 years to complete its evolution, this could
solve the timescale problem. Our suggestion is supported by the ubiquitous
observation of more diffuse emission surrounding UCHRs.Comment: accepted to ApJ, 40 pages, 13 b/w figures, changes from v1 include
analytic prediction of radio luminosity, better description of code testing,
and many minor changes also in response to refere
On Hydrodynamic Motions in Dead Zones
We investigate fluid motions near the midplane of vertically stratified
accretion disks with highly resistive midplanes. In such disks, the
magnetorotational instability drives turbulence in thin layers surrounding a
resistive, stable dead zone. The turbulent layers in turn drive motions in the
dead zone. We examine the properties of these motions using three-dimensional,
stratified, local, shearing-box, non-ideal, magnetohydrodynamical simulations.
Although the turbulence in the active zones provides a source of vorticity to
the midplane, no evidence for coherent vortices is found in our simulations. It
appears that this is because of strong vertical oscillations in the dead zone.
By analyzing time series of azimuthally-averaged flow quantities, we identify
an axisymmetric wave mode particular to models with dead zones. This mode is
reduced in amplitude, but not suppressed entirely, by changing the equation of
state from isothermal to ideal. These waves are too low-frequency to affect
sedimentation of dust to the midplane, but may have significance for the
gravitational stability of the resulting midplane dust layers.Comment: 36 pages, 19 figures. ApJ accepte
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