Jonathan Ferreira

Direct detection of a magnetic field in the innermost regions of an accretion disk

Models predict that magnetic fields play a crucial role in the physics of astrophysical accretion disks and their associated winds and jets. For example, the rotation of the disk twists around the rotation axis the initially vertical magnetic field, which responds by slowing down the plasma in the disk and by causing it to fall towards the central star. The magnetic energy flux produced in this process points away from the disk, pushing the surface plasma outwards, leading to a wind from the disk and sometimes a collimated jet. But these predictions have hitherto not been supported by observations. Here we report the direct detection of the magnetic field in the core of the protostellar accretion disk FU Orionis. The surface field reaches strengths of about 1 kG close to the centre of the disk, and it includes a significant azimuthal component, in good agreement with recent models. But we find that the field is very filamentary and slows down the disk plasma much more than models predict, which may explain why FU Ori fails to collimate its wind into a jet.

Accretion funnels onto weakly magnetized young stars

Aims. We re-examine the conditions required to steadily deviate an accretion flow from a circumstellar disc into a magnetospheric funnel flow onto a slow rotating young forming star. Methods. New analytical constraints on the formation of accretion funnels flows due to the presence of a dipolar stellar magnetic field disrupting the disc are derived. The Versatile Advection Code is used to confirm these constraints numerically. Axisymmetric MHD simulations are performed, where a stellar dipole field enters the resistive accretion disc, whose structure is self-consistently computed. Results. The analytical criterion derived allows to predict a priori the position of the truncation radius from a non perturbative accretion disc model. Accretion funnels are found to be robust features which occur below the co-rotation radius, where the stellar poloidal magnetic pressure becomes both at equipartition with the disc thermal pressure and is comparable to the disc poloidal ram pressure. We confirm the results of Romanova et al. (2002, ApJ, 578, 420) and find accretion funnels for stellar dipole fields as low as 140 G in the low accretion rate limit of 10(-9) M-circle dot yr(-1). With our present numerical setup with no disc magnetic field, we found no evidence of winds, neither disc driven nor X-winds, and the star is only spun up by its interaction with the disc. Conclusions. Weak dipole fields, similar in magnitude to those observed, lead to the development of accretion funnel flows in weakly accreting T Tauri stars. However, the higher accretion observed for most T Tauri stars ((M)overdot similar to 10(-8) M-circle dot yr(-1)) requires either larger stellar field strength and/or different magnetic topologies to allow for magnetospheric accretion.

Atomic T Tauri disk winds heated by ambipolar diffusion - I. Thermal structure

Motivated by recent subarcsecond resolution observations of jets from T Tauri stars, we extend the work of Saer (1993a,b) by computing the thermal and ionization structure of self-similar, magnetically-driven, atomic disk winds heated by ambipolar diusion. Improvements over his work include: (1) new magnetized cold jet solutions consistent with the underlying accretion disk (Ferreira 1997); (2) a more accurate treatment of ionization and ion-neutral momentum exchange rates; and (3) predictions for spatially resolved forbidden line emission (maps, long-slit spectra, and line ratios), presented in a companion paper, Garcia et al. (2001). As in Saer (1993a), we obtain jets with a temperature plateau around 10 4 K, but ionization fractions are revised downward by a factor of 10{100. This is due to previous omission of thermal speeds in ion-neutral momentum-exchange rates and to dierent jet solutions. The physical origin of the hot temperature plateau is outlined. In particular we present three analytical criteria for the presence of a hot plateau, applicable to any given MHD wind solution where ambipolar diusion and adiabatic expansion are the dominant heating and cooling terms. We nally show that, for solutions favored by observations, the jet thermal structure remains consistent with the usual approximations used for MHD jet calculations (thermalized, perfectly conducting, single hydromagnetic cold fluid calculations).

STATIONARY ACCRETION DISKS LAUNCHING SUPER-FAST-MAGNETOSONIC MAGNETOHYDRODYNAMIC JETS

We present self-similar models of resistive viscous Keplerian disks driving nonrelativistic MHD jets and becoming super-fast-magnetosonic. We show that in order to obtain such solutions, the thermal pressure must be a sizeable fraction of the poloidal magnetic pressure at the Alfven surface. These steady solutions that undergo a recollimation shock causally disconnected from the driving engine account for structures with a high-temperature plasma in the sub-Alfvenic region. We suggest that only unsteady outflows with typical timescales of several disk dynamical timescales can be produced if the suitable pressure conditions are not fulfilled.

Atomic T Tauri disk winds heated by ambipolar diffusion - II. Observational tests

We summarize results on the thermal and ionization structure of self-similar, magnetically-driven, atomic disk winds heated by ambipolar diffusion. We improve upon earlier work by Safier by considering (1) new MHD solutions consistent with underlying cold keplerian disk equilibrium, (2) a more accurate treatment of the micro-physics, and (3) predictions for spatially resolved forbidden line emission (maps, long-slit spectra). The temperature plateau ≃ 10 4 K found earlier is recovered, but ionization fractions are revised downward by a factor of 10, due to previous omission of thermal speeds in ion-neutral momentum-exchange rates. The physical origin of the temperature plateau is outlined. Predictions are then compared with T Tauri star observations, with emphasis on the necessity of suitable beam convolution. Jet widths and variations in line profiles with distance and line tracer are well reproduced. However, predicted maximum velocities are too high, total densities too low, and the low-velocity [O i] component is too weak. Denser, slower MHD winds from warm disks might resolve these discrepancies.

The radial structure of protostellar accretion disks : influence of jets

Context. The radial structure of accretion disks is a fundamental issue regarding star and planet formation. Many theoretical studies, focussing on different aspects such as e.g. disk emissivity or ionisation, have been conducted in the context of the standard accretion disk (SAD) model, where no jet is present. Aims. We wish to calculate the structure of young stellar object (YSO) accretion disks in an approach that takes into account the presence of the protostellar jets. The radial structure of these jet emitting disks (JED) should then be compared to that of SADs. Methods. The analytical treatment used in this work is similar to standard modelling of accretion disks but uses the parameter space of magnetised accretion-ejection structures that include the jet torque on the underlying disk. In this framework, the analytical expressions of key quantities are derived, such as mid-plane temperatures, surface densities or disk aspect ratios. Results. We find that JEDs present a structure different from the SADs, which can be observationally tested. The implications on planet formation in the inner regions of accretion disks are briefly discussed. We also supply sets of analytical formulae, valid in different opacity regimes, for the disk quantities. These expressions can be readily used for any work where the disk structure is needed as an input for the model.

Large scale magnetic fields in viscous resistive accretion disks - I. Ejection from weakly magnetized disks

Aims. Cold steady-state disk wind theory from near Keplerian accretion disks requires a large scale magnetic field at near equipartition strength. However the minimum magnetization has never been tested with time dependent simulations. We investigate the time evolution of a Shakura-Sunyaev accretion disk threaded by a weak vertical magnetic field. The strength of the field is such that the disk magnetization falls off rapidly with radius. Methods. Four 2.5D numerical simulations of viscous resistive accretion disk are performed using the magnetohydrodynamic code PLUTO. In these simulations, a mean field approach is used and turbulence is assumed to give rise to anomalous transport coefficients (alpha prescription). Results. The large scale magnetic field introduces only a small perturbation to the disk structure, with accretion driven by the dominant viscous torque. However, a super fast magnetosonic jet is observed to be launched from the innermost regions and remains stationary over more than 953 Keplerian orbits. This is the longest accretion-ejection simulation ever carried out. The self-confined jet is launched from a finite radial zone in the disk which remains constant over time. Ejection is made possible because the magnetization reaches unity at the disk surface, due to the steep density decrease. However, no ejection is reported when the midplane magnetization becomes too small. The asymptotic jet velocity remains nevertheless too low to explain observed jets. This is because of the negligible power carried away by the jet. Conclusions. Astrophysical disks with superheated surface layers could drive analogous outflows even if their midplane magnetization is low. Sufficient angular momentum would be extracted by the turbulent viscosity to allow the accretion process to continue. The magnetized outflows would be no more than byproducts, rather than a fundamental driver of accretion. However, if the midplane magnetization increases towards the center, a natural transition to an inner jet dominated disk could be achieved.

The role of the disc magnetization on the hysteresis behaviour of X-ray binaries

We present a framework for understanding the dynamical and spectral properties of X-ray Binaries, where the presence of an organized large scale magnetic field plays a major role. Such a field is threading the whole accretion disk with an amplitude measured by the disk magnetization � (r, t) = B 2 z /(� oPtot), where Ptot is the total, gas and radiation, pressure. Below a transition radius rJ, a jet emitting disk (the JED) is settled and drives self-collimated non relativistic jets. Beyond rJ, no jet is produced despite the presence of the magnetic field and a standard accretion disc (the SAD) is

Jets from Young Stars

Observational Constraints.- The First Three Million Years.- Jets from Young Stars: The Need for MHD Collimation and Acceleration Processes.- Star-disk Interaction in Classical T Tauri Stars.- Magneto-Hydrodynamic Models.- to Magneto-Hydrodynamics.- Theory and Models of Standard Accretion Disks.- Theory of MHD Jets and Outflows.- Transit Flows and Jet Asymptotics.- MHD Disc Winds.- Stellar Wind Models.

The Origin of Jets from Young Stars: Steady State Disk Wind Models Confronted to Observations

We discuss in this contribution constraints on the origin of mass-loss from young stars brought by recent observations at high angular resolution (0.1″ = 14 AU) of the inner regions of winds from T Tauri stars. Jet widths and collimation scales, the large extent of the velocity profile as well as the detection of rotation signatures agree with predictions from extended (Re ≥ 1 AU) magneto-centrifugal disk wind ejection models. Detected poloidal and toroidal velocities imply large ejection efficiencies (ξ ≃ 0.05, λ ≃ 10), suggesting that thermal gradients (originating in an accretion heated disk corona for example) play an important role in accelerating the flow.