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

Abstract

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.

The role of magnetic reconnection on jet/accretion disk systems

“…We note that previous numerical studies of star-disk interactions have shown that differential rotation between the star and the inner disk regions where the stellar magnetosphere is anchored may lead to field lines opening and reconnection, which eventually restores the initial magnetospheric configuration (e.g., Goodson & Winglee 1999;Matt et al 2002;Uzdensky et al 2002;Romanova et al 2004;von Rekowski & Brandenburg 2004;Zanni 2009; see also Alencar 2007, for a review). Magnetospheric reconnection cycles are expected by most numerical models to develop in a few Keplerian periods at the inner disk, and be accompanied by time dependent accretion onto the star and by episodic outflow events as reconnection takes place.…”

Propeller-driven Outflows and Disk Oscillations

“…We observed that the disk oscillates between "high" and "low" states and expels matter to conical outflows quasi-periodically. The quasi-periodic outbursts associated with the disk-magnetosphere interaction were discussed by Aly & Kuijpers (1990) and observed in simulations by Goodson et al (1997Goodson et al ( , 1999, Matt et al (2002), Romanova et al (2002, hereafter RUKL02), Kato et al (2004), von Rekowski & Brandenburg (2004 and RUKL04. However, none of the earlier simulations concentrated on the propeller stage, and only a few oscillation periods were obtained in earlier simulations.…”

Two-component jet simulations

“…Since it is believed that a sporadic outflow is driven by the star-disk magnetic interaction (Ferreira et al 2000;Matt et al 2002), we examine such cases as well. In this case we adopt a similar function:…”