Hydrodynamic simulations of rotating molecular jets

Molecular outflows and the jets which may drive them can be expected to display signatures associated with rotation if they are the channels through which angular momentum is extracted from material accreting on to protostars. Here, we determine some basic signatures of rapidly rotating flows through three-dimensional numerical simulations of hydrodynamic jets with molecular cooling and chemistry. We find that these rotating jets generate a broad advancing interface which is unstable and develops into a large swarm of small bow features. In comparison to precessing jets, there is no stagnation point along the axis. The greater the rotation rate, the greater the instability. On the other hand, velocity signatures are only significant close to the jet inlet since jet expansion rapidly reduces the rotation speed. We present predictions for atomic, H₂ and CO submillimetre images and spectroscopy including velocity channel maps and position–velocity diagrams. We also include simulated images corresponding to Spitzer IRAC band images and CO emission, relevant for APEX and eventual ALMA observations. We conclude that protostellar jets often show signs of slow precession but only a few sources display properties which could indicate jet rotation.     

The influence of the environment on the propagation of protostellar outflows

The properties of bipolar outflows depend on the structure in the environment as well as the nature of the jet. To help distinguish between the two, we investigate here the properties pertaining to the ambient medium. We execute axisymmetric hydrodynamic simulations, injecting continuous atomic jets into molecular media with density gradients (protostellar cores) and density discontinuities (thick swept-up sheets). We determine the distribution of outflowing mass with radial velocity (the mass spectrum) to quantify our approach and to compare to observationally determined values. We uncover a sequence from clump entrainment in the flanks to bow shock sweeping as the density profile steepens. We also find that the dense, highly supersonic outflows remain collimated but can become turbulent after passing through a shell. The mass spectra vary substantially in time, especially at radial speeds exceeding 15 km s⁻¹. The mass spectra also vary according to the conditions: both envelope-type density distributions and the passage through dense sheets generate considerably steeper mass spectra than a uniform medium. The simulations suggest that observed outflows penetrate highly non-uniform media.     

Near-infrared imaging of H2 in molecular outflows from young stars

In an attempt to identify the molecular shocks associated with the entrainment of ambient gas by collimated stellar winds from young stars, we have imaged a number of known molecular outflows in H₂ v=1-0 S(1) and wide-band K. In each flow, the observed H₂ features are closely associated with peaks in the CO outflow maps. We therefore suggest that the H₂ results from shocks associated with the acceleration or entrainment of ambient, molecular gas. This molecular material may be accelerated either in a bow shock at the head of the flow, or along the length of the flow through a turbulent mixing layer.     

H2 emission from shocks in molecular outflows: the significance of departures from a stationary state

We have computed the time dependence of the H₂ rovibrational emission spectrum from molecular outflows. This emission arises in shock waves generated by the impact of jets, associated with low-mass star formation, on molecular gas. The shocks are unlikely to have attained a state of equilibrium, and so their structure will exhibit both C- and J-type characteristics. The rotational excitation diagram is found to provide a measure of the age of the shock; in the case of the outflow observed in Cepheus A West by the ISO satellite, the shock age is found to be approximately 1.5×10³ yr. Emission by other species, such as NH₃ and SiO, is also considered, as are the intensities of the fine-structure transitions of atoms and ions.     

3D numerical simulations of a radiative Herbig–Haro jet in a supersonic side wind

We have performed 3D numerical simulations of an over-pressurized Herbig–Haro-type jet which propagates into a sidestreaming environment. The interaction between the jet and the sidewind results in a perpendicular acceleration of the jet material, and a consequent curvature of the jet as it moves into the anisotropic medium. We find that an approximately steady configuration is achieved both for a sidewind that is perpendicular to the jet and for a sidewind inclined at 45° towards the jet source. The curvature obtained in both these models is consistent with analytic models of the jet/sidewind problem.   We have also calculated Hα maps, which show an emitting sheath around the upwind (with respect to the sidewind) side of the jet beam. This emitting sheath may explain part of the observed emission from curved stellar jets.     

Time-dependent simulations of steady C-type shocks

Using a time-dependent multifluid, magnetohydrodynamic code, we calculated the structure of steady perpendicular and oblique C-type shocks in dusty plasmas. We included relevant processes to describe mass transfer between the different fluids, radiative cooling by emission lines and grain charging, and studied the effect of single- and multiple-sized grains on the shock structure. Our models are the first of oblique fast-mode molecular shocks in which such a rigorous treatment of the dust grain dynamics has been combined with a self-consistent calculation of the thermal and ionization structures including appropriate microphysics. At low densities, the grains do not play any significant rôle in the shock dynamics. At high densities, the ionization fraction is sufficiently low that dust grains are important charge and current carriers and, thus, determine the shock structure. We find that the magnetic field in the shock front has a significant rotation out of the initial upstream plane. This is most pronounced for single-sized grains and small angles of the shock normal with the magnetic field. Our results are similar to previous studies of steady C-type shocks showing that our method is efficient, rigorous and robust. Unlike the method employed in the previous most detailed treatment of dust in steady oblique fast-mode shocks, ours allow a reliable calculation even when chemical or other conditions deviate from local statistical equilibrium. We are also able to model transient phenomena.     

Numerical simulations of astrophysical jets from Keplerian discs – III. The effects of mass loading

We present 2.5D time-dependent simulations of the non-linear evolution of non-relativistic outflows from the surface of Keplerian accretion discs. The gas is accelerated from the surface of the disc (which is a fixed platform in these simulations) into a cold corona in stable hydrostatic equilibrium. We explore the dependence of the resulting jet characteristics upon the mass loading of the winds. Two initial configurations of the threading disc magnetic field are studied: a potential field and a uniform vertical field configuration.
We show that the nature of the resulting highly collimated, jet-like outflows (steady or episodic) is determined by the mass load of the disc wind. The mass load controls the interplay between the collimating effects of the toroidal field and the kinetic energy density in the outflow. In this regard, we demonstrate that the onset of episodic behaviour of jets appears to be determined by the quantity     which compares the speed for a toroidal Alfvén wave to cross the diameter of the jet, with the flow speed v _p along the jet. This quantity decreases with increasing load. For sufficiently large N (small mass loads), disturbances appear to grow leading to instabilities and shocks. Knots are then generated and the outflow becomes episodic. These effects are qualitatively independent of the initial magnetic configuration that we employed and are probably generic to a wide variety of magnetized accretion disc models.