Launching of conical winds and axial jets from the disc���magnetosphere boundary: axisymmetric and 3D simulations

Abstract

We investigate the launching of outflows from the disc–magnetosphere boundary of slowly and rapidly rotating magnetized stars using axisymmetric and exploratory 3D magnetohydrodynamic simulations. We find long‐lasting outflows in the following cases. (1) In the case of slowly rotating stars, a new type of outflow, a conical wind, is found and studied in simulations. The conical winds appear in cases where the magnetic flux of the star is bunched up by the disc into an X‐type configuration. The winds have the shape of a thin conical shell with a half‐opening angle θ∼ 30°–40°. About 10–30 per cent of the disc matter flows from the inner disc into the conical winds. The conical winds may be responsible for episodic as well as long‐lasting outflows in different types of stars. There is also a low‐density, higher velocity component (a jet) in the region inside the conical wind. (2) In the case of rapidly rotating stars (the ‘propeller regime’), a two‐component outflow is observed. One component is similar to the conical winds. A significant fraction of the disc matter may be ejected into the winds. The second component is a high‐velocity, low‐density magnetically dominated axial jet where matter flows along the opened polar field lines of the star. The jet has a mass flux of about 10 per cent of that of the conical wind, but its energy flux (dominantly magnetic) can be larger than the energy flux of the conical wind. The jet's angular momentum flux (also dominantly magnetic) causes the star to spin down rapidly. Propeller‐driven outflows may be responsible for the jets in protostars and for their rapid spin‐down. The jet is collimated by the magnetic force while the conical winds are only weakly collimated in the simulation region. Exploratory 3D simulations show that conical winds are axisymmetric about the rotational axis (of the star and the disc), even when the dipole field of the star is significantly misaligned.

Sub-arcsecond [Fe ii] spectro-imaging of the DG Tauri jet

“…A strong transverse velocity gradient is also present, with faster matter from the stellar magnetosphere on the inside and slow matter from the disk on the outside. However, most of the wind mass reaches a low poloidal speed of about 40 km s −1 for the CTTS dimensional scalings appropriate in DG Tau: v p 0.2v 0 with v 0 = 197 km s −1 in Table 1 of Romanova et al (2009). And the half-opening angle of 30 • −40 • is much wider than the 4−15 • observed in [Fe ii].…”

“…Recent numerical simulations of the interaction between a stellar dipole and a disk with no net magnetic field yield a situation somewhat different from the (semi-analytical) X-wind model (Romanova et al 2009). As the stellar fieldlines are bunched up by accretion and stretched, a narrow conical wind is formed along the neutral line (rather than in the angular sector below it), and powered by the magnetic pressure force (rather than by the centrifugal force as in the X-wind).…”

Disc reflection and a possible disc wind during a soft X-ray state in the neutron star low-mass X-ray binary 1RXS J180408.9–342058

“…MHD simulations of Romanova et al (2009) reveal both a jet-like and a wind-like outflow for an active propeller. In these simulations the wind component is a thin con-ical shell with a half-opening angle of ≃30 • -40 • and a velocity up to ≃0.1c.…”

Accretion, ablation and propeller evolution in close millisecond pulsar binary systems

“…This evolutionary phase is particularly dominant at very low companion mass, M d < 0.016 M ⊙ , and is, perhaps, the reason for the observed lack of systems in this region of parameter space (although lower masses lead to lower ablated material surrounding the system). In addition, the possible occurrence of intermittent accretion during the propeller and spin equilibrium phase, as shown in numerical models of Romanova et al (2009), may further hinder radio MSP detection.…”