Skip to main content
European Commission logo print header

Magnetically driven Accretion and Ejection phenomena in aSTROphysics: a numerical study

Final Report Summary - MAESTRO (Magnetically driven Accretion and Ejection phenomena in aSTROphysics: a numerical study)

Accretion and ejection are two important processes that play a fundamental role in the evolution of many astrophysical settings. It is widely accepted that astrophysical systems as different as active galactic nuclei (AGN), X-ray binaries and young stellar objects (YSO) are powered by the gravitational energy liberated by material accreting onto a central object, most plausibly in the form of an accretion disk. In the case of YSO, the central body is a forming protostar, for a microquasar is either a stellar mass black hole or a compact neutron star, while in the case of an AGN is a supermassive black hole. The accretion power can be released to fuel the radiative emission of the disk and/or to power collimated supersonic outflows, which is a feature commonly observed in all the aforementioned objects. In the case of X-ray binaries, microquasars or active galactic nuclei, the central object is compact and the resulting jets are relativistic. In any case, it is likely that magnetic fields are a fundamental ingredient of the processes taking place in these astrophysical systems. For example, it has been shown that the non-linear evolution of the magneto rotational instability (MRI) in weakly magnetised Keplerian disks leads to an efficient turbulent transport of angular momentum that is extracted radially inside the disk itself. This seems a viable mechanism to drive accretion and to dissipate the magnetic energy inside the disk itself to fuel its radiative emission. The magnetic driving of jets from magnetised accretion disks and stars is considered as a promising mechanism to explain the origin of supersonic collimated flows. According to this scenario, a large- scale magnetic field extracts gravitational and rotational energy from the accretion disk and/or the central object and transfers it to the surrounding plasma that is therefore magnetically and centrifugally accelerated. Besides this, the tension exerted by the twisted field can provide a self-confining mechanism. Finally, strong magnetic fields have been measured in many classes of accreting stars (i.e. classical T Tauri stars (CTTS), accreting X-ray pulsars, cataclysmic variables): these magnetospheres are strong enough to disrupt the inner parts of the disk and control the accretion flow down to the stellar surface.

These magnetohydrodynamic (MHD) processes are strongly non-linear and a detailed study of these phenomena requires direct time-dependent MHD simulations. The main aim of the MAESTRO project is to model different kinds of accretion and ejection phenomena taking place in different astrophysical systems using two- and three-dimensional MHD numerical simulations. The numerical experiments are carried out using the PLUTO code (see online). PLUTO is a modular Godunov-type code aimed at solving the equations of hydrodynamics and magneto-hydrodynamics in both classical and special relativistic regimes. Claudio Zanni, the researcher in charge of the development of the MAESTRO project, actively collabourates with the University of Torino to develop and provide new modules to the PLUTO framework. The MAESTRO project had three main objectives:

1) to develop numerical models of the MHD acceleration and collimation process of cosmic jets, both in the classical and relativistic regimes;
2) to carry out numerical simulations of the interaction of a cirumstellar gaseous disk with the magnetosphere of a central stellar object;
3) to develop the numerical tools that are specifically required to perform the numerical simulations of the first two tasks.

The main results obtained for each task are here summarised.

1) Numerical models of magnetically driven cosmic jets

Collimated jets of ionised plasma are a common phenomenon observed in many astrophysical systems, from newly forming stars similar to our Sun, where Herbig-Haro (HH) jets take their origin, X-ray binaries, in which galactic microquasars are produced, or active galactic nuclei (AGN), where relativistic jets associated with extended radio-galaxies are accelerated. It is widely accepted that all these systems derive their power from the gravitational energy liberated by material accreting onto a central compact object, most plausibly in the form of an accretion disk. One of the most challenging objectives of the MAESTRO projects was to compute the very first three-dimensional models of the acceleration process of classical (non-relativistic) astrophysical jets from magnetised accretion disks that consider at the same time both the vertical structure of the disk and the large-scale propagation of the bipolar jets. The simulations exploited the newly developed adaptive mesh refinement (AMR) version of the PLUTO code (see task 3) and they have been carried out on high-performance computing systems (Cineca - Italy and IDRIS - France) in the framework of the PRACE - DECI EU funded project. The aim of these models was to study the stability of the acceleration process and investigate the dynamical and thermal structure of protostellar jets. The simulations clearly demonstrated the stability of the acceleration mechanism in the sub-Alfènic regime and showed how the jets can survive current-driven instabilities in the super-Alfvènic regime. Besides this, it was possible to produce synthetic observations of the outflows showing how the instabilities can drive waves and shocks that can heat the jets and support their radiative emission. Important results based on two-dimensional axisymmetric numerical simulations have also been obtained. The effects of the magnetic field intensity on the launching process have been studied in Tzeferacos et al. (2009) and Murphy et al. (2010). These works investigated the structure of the accretion disk and the properties of the outflows in strong and weakly magnetised regimes, respectively. Tzeferacos et al. (2012) developed a study of the thermodynamic properties of disks that drive magnetised jets, with a specific focus on the effects of the magnetic energy dissipation inside the disk on the dynamical properties of the outflows. At the same time, C. Zanni has started the study of MHD jet acceleration processes in the relativistic regime, obtaining fundamental results on the efficiency of this mechanism in the case of AGN jets. In particular, it was possible to determine a relationship between the intensity and distribution of the magnetic field through the accretion disk and the asymptotic bulk Lorentz factor of the relativistic jets.

2) Numerical study of the magnetospheric star-disk interaction

The main aim of this task was to perform numerical experiments to investigate the peculiar angular momentum evolution of different accreting stellar systems. In fact, many X-ray pulsars are characterised by several episodes of spin-up / spin-down torque reversal. Young forming protostars like classical T Tauri stars are characterised by slow rotation periods (3 - 10 days), corresponding to about 10 % of their break-up speed. Moreover, the rotation period seems to stay constant during the T Tauri phase, despite the fact that the protostar is still actively accreting and contracting. Based on two-dimensional axisymmetric MHD simulations, different mechanisms to control the spin of the central star have been investigated. Zanni and Ferreira (2009) elucidated the conditions that lead to the disk truncation and the formation of accretion columns on the stellar surface. This work also proved the inefficiency of the Ghosh and Lamb (1979) mechanism, one of the first scenarios that had been proposed to explain the braking of the stellar rotation. The merits and the limits of accretion-powered stellar winds have been presented in Zanni and Ferreira (2011), where the authors provided strong observational limits to the energy available to launch these outflows. A new scenario has been investigated in Zanni and Ferreira (2012), based on a new class of outflows determined by the periodic episodes of inflation and reconnection of the stellar magnetosphere.

3) Development of new numerical tools

The accretion and ejection phenomena investigated within the MAESTRO project are characterised by a large range of temporal and spatial scales. AMR techniques are the best suited to deal with such problems, locally increasing the spatial and temporal resolution where it is necessary. An AMR module was implemented in the PLUTO framework exploiting the CHOMBO library, developed at the Lawrence Berkeley National Laboratory (Mignone et al., 2012). These new features have been extensively used to perform the three-dimensional simulations of the jet acceleration process (task 1). C. Zanni started a fruitful collabouration with LBNL to extend the CHOMBO capabilities to include curvilinear systems of coordinates and more complex geometries, such as a 'cubed sphere' grids. These curvilinear grids will be of the utmost importance to develop fully three-dimensional models of the magnetic star-disk interaction (task 2).