3,868 research outputs found
Jet Deflection via Cross winds: Laboratory Astrophysical Studies
We present new data from High Energy Density (HED) laboratory experiments
designed to explore the interaction of a heavy hypersonic radiative jet with a
cross wind. The jets are generated with the MAGPIE pulsed power machine where
converging conical plasma flows are produced from a cylindrically symmetric
array of inclined wires. Radiative hypersonic jets emerge from the convergence
point. The cross wind is generated by ablation of a plastic foil via
soft-X-rays from the plasma convergence region. Our experiments show that the
jets are deflected by the action of the cross wind with the angle of deflection
dependent on the proximity of the foil. Shocks within the jet beam are apparent
in the data. Analysis of the data shows that the interaction of the jet and
cross wind is collisional and therefore in the hydro-dynamic regime. MHD plasma
code simulations of the experiments are able to recover the deflection
behaviour seen in the experiments. We consider the astrophysical relevance of
these experiments applying published models of jet deflection developed for AGN
and YSOs. Fitting the observed jet deflections to quadratic trajectories
predicted by these models allows us to recover a set of plasma parameters
consistent with the data. We also present results of 3-D numerical simulations
of jet deflection using a new astrophysical Adaptive Mesh Refinement code.
These simulations show highly structured shocks occurring within the beam
similar to what was observed in the experimentsComment: Submitted to ApJ. For a version with figures go to
http://web.pas.rochester.edu/~afrank/labastro/CW/Jet-Wind-Frank.pd
Hydrodynamical Models of Outflow Collimation in YSOs
We explore the physics of time-dependent hydrodynamic collimation of jets
from Young Stellar Objects (YSOs). Using parameters appropriate to YSOs we have
carried out high resolution hydrodynamic simulations modeling the interaction
of a central wind with an environment characterized by a moderate opening angle
toroidal density distribution. The results show that the the wind/environment
interaction produces strongly collimated supersonic jets. The jet is composed
of shocked wind gas. Using analytical models of wind blown bubble evolution we
show that the scenario studied here should be applicable to YSOs and can, in
principle, initiate collimation on the correct scales (R ~ 100 AU). The
simulations reveal a number of time-dependent non-linear features not
anticipated in previous analytical studies including: a prolate wind shock; a
chimney of cold swept-up ambient material dragged into the bubble cavity; a
plug of dense material between the jet and bow shocks. We find that the
collimation of the jet occurs through both de Laval nozzles and focusing of the
wind via the prolate wind shock. Using an analytical model for shock focusing
we demonstrate that a prolate wind shock can, by itself, produce highly
collimated supersonic jets.Comment: Accepted by ApJ, 31 pages with 12 figures (3 JPEG's) now included,
using aasms.sty, Also available in postscript via a gzipped tar file at
ftp://s1.msi.umn.edu/pub/afrank/SFIC1/SFIC.tar.g
Large Scale Flows from Orion-South
Multiple optical outflows are known to exist in the vicinity of the active
star formation region called Orion-South (Orion-S). We have mapped the velocity
of low ionization features in the brightest part of the Orion Nebula, including
Orion-S, and imaged the entire nebula with the Hubble Space Telescope. These
new data, combined with recent high resolution radio maps of outflows from the
Orion-S region, allow us to trace the origin of the optical outflows. It is
confirmed that HH 625 arises from the blueshifted lobe of the CO outflow from
136-359 in Orion-S while it is likely that HH 507 arises from the blueshifted
lobe of the SiO outflow from the nearby source 135-356. It is likely that
redshifted lobes are deflected within the photon dominated region behind the
optical nebula. This leads to a possible identification of a new large shock to
the southwest from Orion-S as being driven by the redshifted CO outflow arising
from 137-408. The distant object HH 400 is seen to have two even further
components and these all are probably linked to either HH 203, HH 204, or HH
528. Distant shocks on the west side of the nebula may be related to HH 269.
The sources of multiple bright blueshifted Herbig-Haro objects (HH 202, HH 203,
HH 204, HH 269, HH 528) remain unidentified, in spite of earlier claimed
identifications. Some of this lack of identification may arise from the fact
that deflection in radial velocity can also produce a change in direction in
the plane of the sky. The best way to resolve this open question is through
improved tangential velocities of low ionization features arising where the
outflows first break out into the ionized nebula.Comment: Astronomical Journal, in press. Some figures are shown at reduced
resolution. A full-resolution version is available at
http://ifront.org/wiki/Orion_South_Outflows_Pape
The Semiclassical Coulomb Interaction
The semiclassical Coulomb excitation interaction is at times expressed in the
Lorentz gauge in terms of the electromagnetic fields and a contribution from
the scalar electric potential. We point out that the potential term can make
spurious contributions to excitation cross sections, especially when the the
decay of excited states is taken into account. We show that, through an
appropriate gauge transformation, the excitation interaction can be expressed
in terms of the electromagnetic fields alone.Comment: 12 pages. Phys. Rev. C, Rapid Communication, in pres
Numerical Simulations of HH 555
We present 3D gasdynamic simulations of the Herbig Haro object HH 555. HH 555
is a bipolar jet emerging from the tip of an elephant trunk entering the
Pelican Nebula from the adjacent molecular cloud. Both beams of HH 555 are
curved away from the center of the H II region. This indicates that they are
being deflected by a side-wind probably coming from a star located inside the
nebula or by the expansion of the nebula itself. HH 555 is most likely an
irradiated jet emerging from a highly embedded protostar, which has not yet
been detected.
In our simulations we vary the incident photon flux, which in one of our
models is equal to the flux coming from a star 1 pc away emitting 5x10^48
ionizing (i. e., with energies above the H Lyman limit) photons per second. An
external, plane-parallel flow (a ``side-wind'') is coming from the same
direction as the photoionizing flux. We have made four simulations, decreasing
the photon flux by a factor of 10 in each simulation. We discuss the properties
of the flow and we compute Halpha emission maps (integrated along lines of
sight).
We show that the level of the incident photon flux has an important influence
on the shape and visibility of the jet. If the flux is very high, it causes a
strong evaporation of the neutral clump, producing a photoevaporated wind
traveling in the direction opposite to the incident flow. The interaction of
the two flows creates a double shock ``working surface'' around the clump
protecting it and the jet from the external flow. The jet only starts to curve
when it penetrates through the working surface.Comment: 14 pages, 4 figures, accepted by Ap
A study of local approximation for polarization potentials
We discuss the derivation of an equivalent \textit{l}-independent
polarization potential for use in the optical Schr\"{o}dinger equation that
describes the elastic scattering of heavy ions. Three diffferent methods are
used for this purpose. Application of our theory to the low energy scattering
of the halo nucleus Li from a C target is made. It is found that
the notion of \textit{l}-independent polarization potential has some validity
but can not be a good substitute for the \textit{l}-dependent local equivalent
Feshbach polarization potential.Comment: 8 pages, 4 figure
Trapped Protostellar Winds and their Breakout
Observations show that high-velocity jets stem from deeply embedded young
stars, which may still be experiencing infall from their parent cloud cores.
Yet theory predicts that, early in this buildup, any outgoing wind is trapped
by incoming material of low angular momentum. As collapse continues and brings
in more rapidly rotating gas, the wind can eventually break out. Here we model
this transition by following the motion of the shocked shell created by impact
of the wind and a rotating, collapsing envelope. We first demonstrate, both
analytically and numerically, that our previous, quasi-static solutions are
dynamically unstable. Our present, fully time-dependent calculations include
cases both where the wind is driven back by infall to the stellar surface, and
where it erupts as a true outflow. For the latter, we find that the time of
breakout is sim 50,000 yr for wind speeds of 200 km/s. The reason for the delay
is that the shocked material, including the swept-up infall, must be able to
climb out of the star's gravitational potential well.
We explore the critical wind speed necessary for breakout as a function of
the mass transport rates in the wind and infall, as well as the cloud rotation
rate Omega0 and time since the start of infall. Breakout does occur for
realistic parameter choices. The actual breakout times would change if we
relaxed the assumption of perfect mixing between the wind and infall material.
Our expanding shells do not exhibit the collimation of observed jets, but
continue to expand laterally. To halt this expansion, the density in the
envelope must fall off less steeply than in our model.Comment: 44 pages, 10 figures, accepted to Ap
Stellar parameters for stars of the CoRoT exoplanet field
Aims:To support the computation and evolutionary interpretation of periods
associated with the rotational modulation, oscillations, and variability of
stars located in the CoRoT fields, we are conducting a spectroscopic survey for
stars located in the fields already observed by the satellite. These
observations allow us to compute physical and chemical parameters for our
stellar sample. Method: Using spectroscopic observations obtained with UVES/VLT
and Hydra/Blanco, and based on standard analysis techniques, we computed
physical and chemical parameters (, , ,
, , , and ) for a large
sample of CoRoT targets. Results: We provide physical and chemical parameters
for a sample comprised of 138 CoRoT targets. Our analysis shows the stars in
our sample are located in different evolutionary stages, ranging from the main
sequence to the red giant branch, and range in spectral type from F to K. The
physical and chemical properties for the stellar sample are in agreement with
typical values reported for FGK stars. However, we report three stars
presenting abnormal lithium behavior in the CoRoT fields. These parameters
allow us to properly characterize the intrinsic properties of the stars in
these fields. Our results reveal important differences in the distributions of
metallicity, , and evolutionary status for stars belonging to
different CoRoT fields, in agreement with results obtained independently from
ground-based photometric surveys. Conclusions: Our spectroscopic catalog, by
providing much-needed spectroscopic information for a large sample of CoRoT
targets, will be of key importance for the successful accomplishment of several
different programs related to the CoRoT mission, thus it will help further
boost the scientific return associated with this space mission.Comment: 43 pages, 17 figures, accepted for publication in A&
Theory of Multiphonon Excitation in Heavy-Ion Collisions
We study the effects of channel coupling in the excitation dynamics of giant
resonances in relativistic heavy ions collisions. For this purpose, we use a
semiclassical approximation to the Coupled-Channels problem and separate the
Coulomb and the nuclear parts of the coupling into their main multipole
components. In order to assess the importance of multi-step processes, we
neglect the resonance widths and solve the set of coupled equations exactly.
Finite widths are then considered. In this case, we handle the coupling of the
ground state with the dominant Giant Dipole Resonance exactly and study the
excitation of the remaining resonances within the Coupled-Channels Born
Approximation. A comparison with recent experimental data is made.Comment: 29 pages, 7 Postscript figures available upon reques
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