Papers by Turlough Downes
We use numerical simulations to clarify the mass-velocity and intensity-velocity relations for je... more We use numerical simulations to clarify the mass-velocity and intensity-velocity relations for jet-driven outflows. The simulated CO profiles reproduce the broken power laws observed of molecular outflows remarkably well, with no need for two distinct entrainment regimes. An analytic model is proposed to explain the exponent =~ - 1.5 of the underlying smooth mass-velocity relation.
Mon Notic Roy Astron Soc, Dec 20, 2006
Diffusive processes are known to restrict the stability of conventional numerical schemes which a... more Diffusive processes are known to restrict the stability of conventional numerical schemes which are not implicit in nature. Furthermore, recent work establishes that a large Hall term can impose an additional severe stability limit on standard explicit schemes. Following a previous paper which presented the one-dimensional case, we describe a fully three-dimensional method which relaxes the normal restrictions on explicit schemes for multifluid processes. This is achieved by applying the little known Super TimeStepping technique to the symmetric (ambipolar) component of the evolution operator for the magnetic field in the local plasma rest-frame, and the new Hall Diffusion Scheme to the skew-symmetric (Hall) component.
Numerical Simulations of H$_2$ Emission in Jets from Young Stellar Objects
Astrophysical letters & communications
In this paper we briefly review the basic theory of shock waves in relativistic hydrodynamics and... more In this paper we briefly review the basic theory of shock waves in relativistic hydrodynamics and magneto-hydrodynamics, emphasising some astrophysically interesting cases. We then present an overview of the theory of particle acceleration at such shocks describing the methods used to calculate the spectral indices of energetic particles. Recent results on acceleration at ultra-relativistic shocks are discussed.
The strong cosmic ray pressure gradient expected in shock precursors can drive bulk turbulent mot... more The strong cosmic ray pressure gradient expected in shock precursors can drive bulk turbulent motions through the mechanism responsible for the acoustic instability of the precursor region discussed in Drury and Falle [4]. The results of recent three-dimensional MHD simulations of this process will be presented and the implications for the shock precursor structure discussed.
Astronomy and Astrophysics
While it is generally thought that molecular outflows from young stellar objects (YSOs) are accel... more While it is generally thought that molecular outflows from young stellar objects (YSOs) are accelerated by underlying stellar winds or highly collimated jets, the actual mechanism of acceleration remains uncertain. The most favoured model, at least for low and intermediate mass stars, is that the molecules are accelerated at jet-driven bow shocks. Here we investigate, through high resolution numerical simulations, the efficiency of this mechanism in accelerating ambient molecular gas without causing dissociation. The efficiency of the mechanism is found to be surprisingly low suggesting that more momentum may be present in the underlying jet than previously thought. We also compare the momentum transferring efficiencies of pulsed versus steady jets. We find that pulsed jets, and the corresponding steady jet with the same average velocity, transfer virtually the same momentum to the ambient gas. The additional momentum ejected sideways from the jet beam in the case of the pulsed jet ...
A numerical simulation of the counter-jet in the HH46/47 system
... J., 24(1), 31-34, (1997) TP DOWNES © Irish Astronomical Journal • Provided by the NASA Astrop... more ... J., 24(1), 31-34, (1997) TP DOWNES © Irish Astronomical Journal • Provided by the NASA Astrophysics Data System ) A NUMERICAL SIMULATION OF THE COUNTER-JET IN THEHH46/47 SYSTEM TP DOWNES Dublin Institute for Advanced Studies, School of Cosmic ...
Astronomy and Astrophysics
We present the results of simulations of the development of the Kelvin-Helmholtz (KH) instability... more We present the results of simulations of the development of the Kelvin-Helmholtz (KH) instability in a cooled, slab symmetric system. The parameters were chosen to approximate the physical conditions typically found in jets from young stellar objects (YSOs). The effect of different methods of maintaining the initial equilibrium were examined for varying density. In addition, the effect of adjusting the width of the shear layer between the jet and ambient material was studied and found not to have significant long-term effects on the development of the instability. We find that, in general, cooling acts to: increase the level of mixing between jet and ambient material through the `breaking' of KH induced waves on the surface of the jet; increase the amount of momentum transferred from jet material to ambient material; increase the time taken for shocks to develop in the flow; reduce the strength of these shocks; reduce the rate of decollimation of momentum flux. The first and sec...
Exploring the Gamma Ray Universe, Sep 1, 2001
We present simulations of the complete evolution of a fireball model for gamma ray bursts using a... more We present simulations of the complete evolution of a fireball model for gamma ray bursts using a relativistic hydrodynamic code. Particle acceleration at both the forward and reverse shocks produces energetic electrons which then emit synchrotron radiation in the ambient magnetic field. The observed synchrotron spectrum is calculated.
In this paper we examine the effect that the Kelvin-Helmholtz instability can have on the propaga... more In this paper we examine the effect that the Kelvin-Helmholtz instability can have on the propagation of rotating YSO jets. There is strong evidence from observations to suggest that jets from young stars rotate. Rotation will change the expected growth rates of certain instabilities, which will have an effect on the predicted observational properties of these jets.
Inferred Versus Actual Characteristics of Molecular Outflows - Some Pitfalls
Multifluid Magnetohydrodynamics: An Explicit Approach
We present a model of a pulsar-driven supernova remnant, by using a hydrodynamics code, which sim... more We present a model of a pulsar-driven supernova remnant, by using a hydrodynamics code, which simulates the evolution of a pulsar wind nebula when the pulsar is moving at a high velocity through its expanding supernova remnant. The simulation shows four different stages of the pulsar wind nebula: the supersonic expansion stage, the reverse shock interaction stage, the subsonic expansion stage and ultimately the bow shock stage. Due to the high velocity of the pulsar, the position of the pulsar is located at the head of the pulsar wind bubble, after the passage of the reverse shock. The resulting morphology of the pulsar wind bubble is therefore similar to the morphology of a bow shock pulsar wind nebula. We show how to distinguish these two different stages, and apply this method to the SNR G327.1-1.1, for which we argue that there is no bow shock around its pulsar wind nebula.
Numerical investigations of astrophysical plasma flows often rely on the ideal magnetohydrodynami... more Numerical investigations of astrophysical plasma flows often rely on the ideal magnetohydrodynamic (MHD) approximation. In the case of weakly ionized plasmas the most questionable assumption of this is that the gas may be adequately approximated as a perfectly conducting single fluid. In direct consequence, the field lines are frozen into the bulk flow and may exert unrealistic forces on the gas in situations where magnetic diffusion should be important. A more appropriate treatment under these conditions is to discard the flux-freezing approximation by allowing charged species (i.e. electrons, ions, charge carrying dust grains) to have relative motions and compete in their interactions with the neutral gas component and the magnetic field. Taking such a multifluid approach admits ambipolar and Hall diffusion effects which can have a significant influence on the dynamics of the plasma. Conventional explicit numerical schemes have been shown to have a vanishing stable time step limit...
Dyson. 2002
We use numerical simulations to clarify the mass-velocity and intensity-velocity relations for je... more We use numerical simulations to clarify the mass-velocity and intensity-velocity relations for jet-driven outows. The simulated CO pro les reproduce the broken power laws observed of molecular outows remarkably well, with no need for two distinct entrainment regimes. An analytic model is proposed to explain the exponent ' 1:5 of the underlying smooth mass-velocity relation.
Monthly Notices of the Royal Astronomical Society, 2014
Observations of non-thermal emission from several supernova remnants suggest that magnetic fields... more Observations of non-thermal emission from several supernova remnants suggest that magnetic fields close to the blastwave are much stronger than would be naively expected from simple shock compression of the field permeating the interstellar medium (ISM). We investigate in some detail a simple model based on turbulence generation by cosmic-ray pressure gradients. Previously this model was investigated using 2D MHD simulations.
Proceedings of The International Astronomical Union, 1997
We present 2D slab symmetric simulations of the development of the symmetric modes of the Kelvin-... more We present 2D slab symmetric simulations of the development of the symmetric modes of the Kelvin-Helmholtz (KH) instability in non-adiabatic jets. The simulations are run for parameters which are thought to be close to those of jets from young stellar objects (YSOs). Two different radiative cooling functions are investigated: one which is calculated for a gas of cosmic abundances cooling
We present results of a suite of simulations of turbulent decay in molecular clouds. These simula... more We present results of a suite of simulations of turbulent decay in molecular clouds. These simulations include magnetic fields and, in a simplified manner, non-ideal effects such as ambipolar diffusion and the Hall effect. It has been speculated that the Hall effect may have some importance in molecular clouds in spite of its rather low level in comparison with ambipolar diffusion. We use our simulation results to compare turbulent decay with and without non-ideal effects and we also explore the effect of the individual non-ideal effects incorporated.
Initial results from simulations of 4-fluid MHD turbulence in molecular clouds are presented. The... more Initial results from simulations of 4-fluid MHD turbulence in molecular clouds are presented. The species included in the simulations are ions, electrons, negatively charged dust grains and neutrals. The results indicate that, on length scales of a few tenths of a parsec, multifluid effects have a significant impact on the properties of the turbulence. In particular, the power spectra of the velocity and magnetic fields are significantly softened, while the PDF of the densities of the charged and neutral fluids are appreciably different. Indeed, the magnetic field strength displays much less spatial structure on all lengthscales up to 1 pc than in the ideal MHD case. The assumptions of ideal MHD therefore appear to be inadequate for simulating turbulence in molecular clouds at these length scales.
Multifluid Simulations of the Kelvin-Helmholtz Instability in a Weakly Ionised Plasma
Astrophysics and Space Science Proceedings, 2009
ABSTRACT Multifluid simulations of the Kelvin-Helmholtz instability were carried out in order to ... more ABSTRACT Multifluid simulations of the Kelvin-Helmholtz instability were carried out in order to observe the effect of including Hall and ambipolar diffusion. We found that the initial growth of the instability is similar in each case, but the following non-linear regimes differ greatly. In the presence of Hall diffusion, the plasma experiences a magnetic dynamo, and the strength of the magnetic field is seen to grow steadily even after the instability reaches a maximum. In the presence of ambipolar diffusion, the magnetic field quickly decreases, most likely as a result of magnetic reconnection.
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Papers by Turlough Downes