Turlough P. Downes

An explicit scheme for multifluid magnetohydrodynamics

When modeling astrophysical fluid flows, it is often appropriate to discard the canonical magnetohydrodynamic approximation thereby freeing the magnetic field to diffuse with respect to the bulk velocity field. As a consequence, however, the induction equation can become problematic to solve via standard explicit techniques. In particular, the Hall diffusion term admits fast-moving whistler waves which can impose a vanishing timestep limit. Within an explicit differencing framework, a multifluid scheme for weakly ionised plasmas is presented which relies upon a new approach to integrating the induction equation efficiently. The first component of this approach is arelatively unknown method of accelerating the integration of parabolic systems by enforcing stability over large compound timesteps rather than over each of the constituent substeps. This method, Super Time Stepping, proves to be very effective in applying a part of the Hall term up to a known critical value. The excess of the Hall term above this critical value is then included via a new scheme for pure Hall diffusion.

A three-dimensional numerical method for modelling weakly ionized plasmas

Astrophysical fluids under the influence of magnetic fields are often subjected to single- or twofluid approximations. In the case of weakly ionized plasmas, however, this can be inappropriate due to distinct responses from the multiple constituent species to both collisional and noncollisional forces. As a result, in dense molecular clouds and protostellar accretion discs, for instance, the conductivity of the plasma may be highly anisotropic leading to phenomena such as Hall and ambipolar diffusion strongly influencing the dynamics. 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.

Turbulent magnetic field amplification driven by cosmic ray pressure gradients

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 present a simple model which is capable of achieving sufficient magnetic field amplification to explain the observations. We propose that the cosmic-ray pressure gradient acting on the inhomogeneous ISM upstream of the supernova blastwave induces strong turbulence upstream of the supernova blastwave. The turbulence is generatedthroughthedifferentialaccelerationoftheupstreamISMwhichoccursasaresult of density inhomogeneities in the ISM. This turbulence then amplifies the pre-existing magnetic field. Numerical simulations are presented which demonstrate that amplification factors of 20 or more are easily achievable by this mechanism when reasonable parameters for the ISM and supernova blastwave are assumed. The length scale over which this amplification occurs is that of the diffusion length of the highest energy non-thermal particles.

Jet-driven molecular outflows from class 0 sources : younger and stronger than they seem?

Context. The momentum, age and momentum injection rate (thrust) of molecular outflows are key parameters in theories of star formation. Systematic biases in these quantities as inferred from CO line observations are introduced through simplified calculations. These biases were quantified for radially expanding flows. However, recent studies suggest that the youngest outflows may be better described by jet-driven bowshocks, where additional biases are expected. Aims. We investigate quantitatively the biases in momentum, age, and thrust estimates in the case of young jet-driven molecular outflows, and propose more accurate methods of determining these quantities. Methods. We use long-duration (1500 yr) high resolution numerical simulations in concert with the standard observational methods of inferring the relevant quantities to quantify the systematic biases in these calculations introduced, in particular, by dissociation, erroneous inclusion of transverse momentum, and hidden material at cloud velocity. Jet/ambient density contrasts of 0.1–1 are considered, leading to bow speeds of 60–135 km s −1 . Results. When mass-weighted velocities are used, lifetimes are overestimated by typically an order of magnitude. The molecular thrust is then underestimated by similar amounts. Using the maximum velocity in CO profiles gives better results, if empirical corrections for inclination are applied. We propose a new method of calculating the lifetime of an outflow which dramatically improves estimates of age and molecular thrust independent of inclination. Our results are applicable to younger flows which have not broken out of their parent cloud. Conclusions. Published correlations between the molecular flow thrust and the source bolometric luminosity obtained with the maximum CO velocity method should remain valid. However, dissociation at the bow head may cause the observable thrust to underestimate the total flow thrust by a factor of up to 2–4, depending on the bow propagation speed and the magnetic field strength. Detailed evaluation of this effect would greatly help to better constrain the efficiency of the ejection mechanism in protostars.

The mass-velocity and intensity-velocity relations in jet-driven molecular outflows

We use numerical simulations to examine the mass-velocity and intensity-velocity relations in the CO J = 2-1 and H 2 S(1)1-0 lines for jet-driven molecular outflows. Contrary to previous expectations, we find that the mass-velocity relation for the swept-up gas is a single power-law, with a shallow slope ≃-1.5 and no break to a steeper slope at high velocities. An analytic bowshock model with no post-shock mixing is shown to reproduce this behaviour very well. We show that molecular dissociation and the temperature dependence of the line emissivity are both critical in defining the shape of the line profiles at velocities above ∼20 kms - 1 . In particular, the simulated CO J = 2-1 intensity-velocity relation does show a break in slope, even though the underlying mass distribution does not. These predicted CO profiles are found to compare remarkably well with observations of molecular outflows, both in terms of the slopes at low and high velocities and in terms of the range of break velocities at which the change in slope occurs. Shallower slopes are predicted at high velocity in higher excitation lines, such as H 2 S(1)1-0. This work indicates that, in jet-driven outflows, the CO J = 2-1 intensity profile reflects the slope of the underlying mass-velocity distribution only at velocities ≤20km s - 1 , and that higher temperature tracers are required to probe the mass distribution at higher speed.

The Evolution and Simulation of the Outburst from XZ Tauri - A Possible EXor?

Received date ;accepted date Abstract. We report on multi-epoch HST/WFPC2 images of the XZ Tauri binary, and its outflow, covering the period from 1995 to 2001. Data from 1995 to 1998 have already been published in the literature. Additional images, from 1999, 2000 and 2001 are presented here. These reveal not only further dynamical and morphological evolution of the XZ Tauri outflow but also that the suspected outflow source, XZ Tauri North has flar ed in EXor-type fashion. In particular our proper motion studies suggests that the recently discovered bubble-like shock, driven by the the XZ Tauri outflow, is slowing down (its tangent ial velocity decreasing from 146 km s 1 to 117 km s 1 ). We also present simulations of the outflow itself, with pla usible ambient and outflow parameters, that appear to reproduce not only the dynamical evolution of the flow, but also its shape and emissi on line luminosity.

Driven multifluid magnetohydrodynamic molecular cloud turbulence

It is believed that turbulence may have a significant impact on star formation and the dynamics and evolution of the molecular clouds in which this occurs. It is also known that non-ideal magnetohydrodynamic (MHD) effects influence the nature of this turbulence. We present the results of a numerical study of four-fluid MHD turbulence in which the dynamics of electrons, ions, charged dust grains and neutrals and their interactions are followed. The parameters describing the fluid being simulated are based directly on observations of molecular clouds. We find that the velocity and magnetic field power spectra are strongly influenced by multifluid effects on length-scales at least as large as 0.05 pc. The probability density functions of the various species in the system are all found to be close to log-normal, with charged species having a slightly less platykurtic (flattened) distribution than the neutrals. We find that the introduction of multifluid effects does not significantly alter the structure functions of the centroid velocity increment.

A Hybridized Forward Backward Method Applied to Electromagnetic Wave Scattering Problems

This communication presents an improved forward backward technique for the solution of electromagnetic wave scattering problems. The forward backward method displays rapid convergence when the eigenvalues of the associated iteration matrix are small. Conversely when the eigenvalues are large it displays poorer convergence. The hybridized method presented in this communication helps to circumvent the poor convergence of the forward backward method in the latter case by introducing an optimally sized correction in the approximate direction of the eigenvector associated with the iteration matrixs dominant eigenvalue. Numerical results are presented in order to demonstrate the convergence of the improved forward backward method.

The role of Kelvin–Helmholtz instability in dusty and partially ionized outflows

We investigate the linear theory of Kelvin-Helmholtz instability at the interface between a partially ionized dusty outflow and the ambient material analytically. We model the interaction as a multifluid system in a planar geometry. The unstable modes are independent from the charge polarity of the dust particles. Although our results show a stabilizing effect for charged dust particles, the growth time-scale of the growing modes gradually becomes independent of the mass or charge of the dust particles when the magnetic-field strength increases. We show that growth time-scale decreases with increasing the magnetic field. Also, as the mass of the dust particles increases, the growth time-scale of the unstable mode increases.

Multifluid Magnetohydrodynamic Turbulent Decay

It is generally believed that turbulence has a significant impact on the dynamics and evolution of molecular clouds and the star formation that occurs within them. Non-ideal magnetohydrodynamic (MHD) effects are known to influence the nature of this turbulence. We present the results of a suite of 5123 resolution simulations of the decay of initially super-Alfv?nic and supersonic fully multifluid MHD turbulence. We find that ambipolar diffusion increases the rate of decay of the turbulence while the Hall effect has virtually no impact. The decay of the kinetic energy can be fitted as a power law in time and the exponent is found to be ?1.34 for fully multifluid MHD turbulence. The power spectra of density, velocity, and magnetic field are all steepened significantly by the inclusion of non-ideal terms. The dominant reason for this steepening is ambipolar diffusion with the Hall effect again playing a minimal role except at short length scales where it creates extra structure in the magnetic field. Interestingly we find that, at least at these resolutions, the majority of the physics of multifluid turbulence can be captured by simply introducing fixed (in time and space) resistive terms into the induction equation without the need for a full multifluid MHD treatment. The velocity dispersion is also examined and, in common with previously published results, it is found not to be power law in nature.