Anthony P. Goodson

TIME-DEPENDENT ACCRETION BY MAGNETIC YOUNG STELLAR OBJECTS AS A LAUNCHING MECHANISM FOR STELLAR JETS

A time-dependent jet launching and collimating mechanism is presented. Initial results of numerical simulations of the interaction between an aligned dipole rotator and a conducting circumstellar accretion disk that is initially threaded by the dipole field show that differential rotation between the disk and the star leads to the rapid expansion of the magnetic loops that connect the star to the disk. The expansion of these magnetic loops above and below the disk produces a two-component outflow. A hot, well-collimated outflow is generated by the convergent flow of attached plasma toward the rotation axis, while a cool, slower outflow is produced on the disk side of the expanding loop. The expanding loop, which later forms a plasmoid, defines the boundary between the jetlike flow and the disk wind. Episodic magnetic reconnection above and below the disk releases the jet plasma from the system and allows the process to repeat, reinforcing the hot, well-collimated outflow.

Jets from Accreting Magnetic Young Stellar Objects. I. Comparison of Observations and High-Resolution Simulation Results

High-resolution numerical magnetohydrodynamic (MHD) simulations of a new model for the formation of jets from magnetic accreting young stellar objects (YSOs) are presented and compared with observations. The simulation results corroborate a previously laid out conceptual mechanism for forming jets, wherein the interaction of the stellar magnetosphere with a surrounding accretion disk leads to an outflow. The high resolution of the numerical simulation allows optical condensations, which form in the region close to the star to be seen. The optical condensations and the episodic behavior of the jet are effects that are inherent to the jet-launching mechanism itself. A disk wind arises as well. The simulated outflow is compared with observations, and it is shown that simulated images in the forbidden lines [S II] λλ6716+6731 have morphology consistent with recent observations of the jet source HH 30. Furthermore, velocity spectra of the simulated outflow in [S II] λλ6716+6731 and mass weighted by n clearly show a two-component outflow, in agreement with observed outflows from T Tauri stars such as DG Tauri. The mechanism produces a highly collimated fast jet and a slower disk wind. While the match between existing observations and the simulated system are not perfect (the time- and size scales of the jet differ from those in HH 30 by an order of magnitude), the morphology associated with both imagery and velocity spectra of the jet are matched well. A companion paper lays out the physics that control the timescale for knot production and defines the controlling parameters of the jet-launching mechanism in general.

Jets from Accreting Magnetic Young Stellar Objects. II. Mechanism Physics

This paper addresses the physical principles underpinning a new jet-launching mechanism described in a companion paper. In this new jet formation model, magnetic loops that connect the star to the disk become twisted and expand via helicity injection. This expansion drives an outflow, with the axial symmetry of the disk leading to a concentration of outflowing plasma along the rotation axis, forming the jet. In the companion paper, it is found that the radial location of the inner edge of the disk undergoes oscillations. In this paper, the physical causes of the disk oscillations are investigated. This investigation leads to the conclusion that there are three classes of flows that can arise, depending on the role of diffusive instabilities. The most diffusive flows allow the stellar magnetic field to slip through the accretion disk and yield steady accretion flows. Such configurations are unlikely to produce outflows. The flows with intermediate diffusivity have been described by Lovelace, Romanova, & Bisnovatyi-Kogan and represent conditions in which the field is effectively frozen into the accretion disk azimuthally but slips radially. In the absence of magnetic reconnection, such configurations are predicted to produce steady flows with logarithmically collimated disk winds. The least diffusive flows, in which the bulk radial disk velocities are greater at times than the speeds with which magnetic field lines can diffuse into the disks, lead to the formation of the collimated unsteady jets described in the companion paper and are the primary interest of this paper. The jet velocity is also addressed.

Disk Formation by Asymptotic Giant Branch Winds in Dipole Magnetic Fields

We present a simple, robust mechanism by which an isolated star can produce an equatorial disk. The mechanism requires that the star have a simple dipole magnetic field on the surface and an isotropic wind acceleration mechanism. The wind couples to the field, stretching it until the field lines become mostly radial and oppositely directed above and below the magnetic equator, as occurs in the solar wind. The interaction between the wind plasma and magnetic field near the star produces a steady outflow in which magnetic forces direct plasma toward the equator, constructing a disk. In the context of a slow (10 km s-1) outflow (10-5 M☉ yr-1) from an asymptotic giant branch star, MHD simulations demonstrate that a dense equatorial disk will be produced for dipole field strengths of only a few Gauss on the surface of the star. A disk formed by this model can be dynamically important for the shaping of planetary nebulae.

Simulation‐based Investigation of a Model for the Interaction between Stellar Magnetospheres and Circumstellar Accretion Disks

We examine, parametrically, the interaction between the magnetosphere of a rotating young stellar object and a circumstellar accretion disk using 2.5-dimensional (cylindrically symmetric) numerical magnetohydrodynamic simulations. The interaction drives a collimated outflow, and we find that the jet formation mechanism is robust. For variations in initial disk density of a factor of 16, variations of stellar dipole strength of a factor of 4, and various initial conditions with respect to the disk truncation radius and the existence of a disk field, outflows with similar morphologies were consistently produced. Second, the system is self-regulating, where the outflow properties depend relatively weakly on the parameters above. The large-scale magnetic field structure rapidly evolves to a configuration that removes angular momentum from the disk at a rate that depends most strongly on the field and weakly on the rotation rate of the footpoints of the field in the disk and the mass outflow rate. Third, the simulated jets are episodic, with the timescale of jet outbursts identical to the timescale of magnetically induced oscillations of the inner edge of the disk. To better understand the physics controlling these disk oscillations, we present a semianalytical model and confirm that the oscillation period is set by the spin-down rate of the disk inner edge. Finally, our simulations offer strong evidence that it is indeed the interaction of the stellar magnetosphere with the disk, rather than some primordial field in the disk itself, that is responsible for the formation of jets from these systems.

Mini-Magnetospheric Plasma Propulsion (M2P2): High Speed Propulsion Sailing the Solar Wind

Mini-Magnetospheric Plasma Propulsion (M2P2) seeks the creation of a magnetic wall or bubble (i.e. a magnetosphere) that will intercept the supersonic solar wind which is moving at 300–800 km/s. In so doing, a force of about 1 N will be exerted on the spacecraft by the spacecraft while only requiring a few mN of force to sustain the mini-magnetosphere. Equivalently, the incident solar wind power is about 1 MW while about 1 kW electrical power is required to sustain the system, with about 0.25–0.5 kg being expended per day. This nominal configuration utilizing only solar electric cells for power, the M2P2 will produce a magnetic barrier approximately 15–20 km in radius, which would accelerate a 70–140 kg payload to speeds of about 50–80 km/s. At this speed, missions to the heliopause and beyond can be achieved in under 10 yrs. Design characteristics for a prototype are also described.

Time dependent outflows from accreting magnetic YSOs

We present high resolution numerical magnetohydrodynamic (MHD) simulation results of the interaction between a magnetic young stellar object (YSO) and a surrounding conducting accretion disk. A two component outflow arises, consisting of a high velocity, collimated jet and a slower, poorly collimated disk wind. The stellar magnetosphere undergoes forced oscillations in radius, with each oscillation correlating with the formation of an optical knot in the jet and with major magnetic reconnection and accretion events.