751 research outputs found
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 3-mode, Variable Velocity Jet Model for HH 34
Variable ejection velocity jet models can qualitatively explain the
appearance of successive working surfaces in Herbig-Haro (HH) jets. This paper
presents an attempt to explore which features of the HH-34 jet can indeed be
reproduced by such a model. From previously published data on this object, we
find evidence for the existence of a 3-mode ejection velocity variability, and
then explore the implications of such a variability. From simple, analytic
considerations it is possible to show that the longer period modes produce a
modulation on the shorter period modes, resulting in the formation of
``trains'' of multiple knots. The knots observed close to the source of HH-34
could correspond to such a structure. Finally, a numerical simulation with the
ejection velocity variability deduced from the HH-34 data is computed. This
numerical simulation shows a quite remarkable resemblance with the observed
properties of the HH-34 jet.Comment: 28 pages LaTex, 10 postscript figure
Interaction of Infall and Winds in Young Stellar Objects
The interaction of a stellar or disk wind with a collapsing environment holds
promise for explaining a variety of outflow phenomena observed around young
stars. In this paper we present the first simulations of these interactions.
The focus here is on exploring how ram pressure balance between wind and
ambient gas and post-shock cooling affects the shape of the resulting outflows.
In our models we explore the role of ram pressure and cooling by holding the
wind speed constant and adjusting the ratio of the inflow mass flux to the wind
mass flux (Mdot_a/Mdot_w) Assuming non-spherical cloud collapse, we find that
relatively strong winds can carve out wide, conical outflow cavities and that
relatively weak winds can be strongly collimated into jet-like structures. If
the winds become weak enough, they can be cut off entirely by the infalling
environment. We identify discrepancies between results from standard snowplow
models and those presented here that have important implications for molecular
outflows. We also present mass vs. velocity curves for comparison with
observations.Comment: 35 pages, 11 figures (PNG and EPS
Radiation from matter entrainment in astrophysical jets: the AGN case
Jets are found in a variety of astrophysical sources, from young stellar
objects to active galactic nuclei. In all the cases the jet propagates with a
supersonic velocity through the external medium, which can be inhomogeneous,
and inhomogeneities could penetrate into the jet. The interaction of the jet
material with an obstacle produces a bow shock in the jet in which particles
can be accelerated up to relativistic energies and emit high-energy photons. In
this work, we explore the active galactic nuclei scenario, focusing on the
dynamical and radiative consequences of the interaction at different jet
heights. We find that the produced high-energy emission could be detectable by
the current gamma-ray telescopes. In general, the jet-clump interactions are a
possible mechanism to produce (steady or flaring) high-energy emission in many
astrophysical sources in which jets are present.Comment: 4 pages, 2 figures. Accepted for publication in the Proceedings of
the 275 IAU Symposium: "Jets at all Scales", held in Buenos Aires, September
13-17, 201
On the parameterization of turbulent fluxes over the tropical Eastern Pacific
We present estimates of turbulent fluxes of heat and momentum derived from low level (~30 m) aircraft measurements over the tropical Eastern Pacific and provide empirical relationships that are valid under high wind speed conditions (up to 25 ms<sup>−1</sup>). The estimates of total momentum flux and turbulent kinetic energy can be represented very accurately (<I>r</I><sup>2</sup>=0.99, when data are binned every 1 ms<sup>−1</sup>) by empirical fits with a linear and a cubic terms of the average horizontal wind speed. The latent heat flux shows a strong quadratic dependence on the horizontal wind speed and a linear relationship with the difference between the air specific humidity and the saturated specific humidity at the sea surface, explaining 96% of the variance. The estimated values were used to evaluate the performance of three currently used parameterizations of turbulence fluxes, varying in complexity and computational requirements. The comparisons with the two more complex parameterizations show good agreement between the observed and parameterized latent heat fluxes, with less agreement in the sensible heat fluxes, and one of them largely overestimating the momentum fluxes. A third, very simple parameterization shows a surprisingly good agreement of the sensible heat flux, while momentum fluxes are again overestimated and a poor agreement was observed for the latent heat flux (<I>r</I><sup>2</sup>=0.62). The performance of all three parameterizations deteriorates significantly in the high wind speed regime (above 10–15 ms<sup>−1</sup>). The dataset obtained over the tropical Eastern Pacific allows us to derive empirical functions for the turbulent fluxes that are applicable from 1 to 25 ms<sup>−1</sup>, which can be introduced in meteorological models under high wind conditions
Estimating the impact of natural and anthropogenic emissions on cloud chemistry: the influence of organic compounds
International audienceIn order to estimate the anthropogenic influence of gas and aerosol emissions from the Petroleum Industry in maritime zones with clouds of small vertical extent, a numerical 1D Eulerian cloud-chemical model with detailed microphysics (Alfonso and Raga, 2002) is used to simulate the influence of water soluble organic compounds (WSOC) and organic+inorganic gas emissions on cloud development. Following Mircea et al. (2002), we tested the sensitivity of the cloud and precipitation development in the classical inorganic case (CIC) and the inorganic+organic case (IOC) with respect to CCN compositions. The results indicate an increase in the droplet concentration for the IOC, and a delay in the development of precipitation. The pH spectral evolution was studied during both the development and precipitation stages. The influence of the diffusion of formic acid and its generation by oxidation of hydrated formaldehyde in the aqueous phase result in a reduction in the pH of precipitation in the range between 0.05 and 0.15 pH units (from 1 to 3%) for the high ambient SO2 concentration (20 ppb) and between 0.2?0.5 pH units (from 4 to 10%) for the low ambient SO2 concentration (1 ppb) case
The influence of organic compounds on the development of precipitation acidity in maritime clouds
International audienceIn order to estimate the anthropogenic influence of gas and aerosol emissions from the Petroleum Industry in maritime zones with clouds of small vertical extent, a numerical 1-D Eulerian cloud-chemical model with detailed microphysics (Alfonso and Raga, 2002) is used to simulate the influence of water soluble organic compounds (WSOC) and organic+inorganic gas emissions on cloud development. Following Mircea et al. (2002), we tested the sensitivity of the cloud and precipitation development in the classical inorganic case (CIC) and the inorganic+organic case (IOC) with respect to CCN compositions. The results indicate an increase in the droplet concentration for the IOC, and a delay in the development of precipitation. The pH spectral evolution was studied during both the development and precipitation stages. The influence of the diffusion of formic acid and its generation by oxidation of hydrated formaldehyde in the aqueous phase result in a reduction in the pH of precipitation in the range between 0.05 and 0.15 pH units (from 1 to 3%) for the high ambient SO2 concentration (20 ppb) and between 0.2-0.5 pH units (from 4 to 10%) for the low ambient SO2 concentration (1 ppb) case
Bipolar molecular outflows driven by hydromagnetic protostellar winds
We demonstrate that magnetically-collimated protostellar winds will sweep
ambient material into thin, radiative, momentum-conserving shells whose
features reproduce those commonly observed in bipolar molecular outflows. We
find the typical position-velocity and mass-velocity relations to occur in
outflows in a wide variety of ambient density distributions, regardless of the
time histories of their driving winds.Comment: 4 pages, 1 figure, submitted to ApJ
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