arXiv subjects

Jonathan Ferreira

Publications and source records attributed to Jonathan Ferreira.

35 records · Page 2Linked to original sources

arXiv2011

Magnetic Field dragging in accretion discs

Accretion discs are composed of ionized gas in motion around a central object. Sometimes, the disc is the source of powerful bipolar jets along its rotation axis. Theoretical models invoke the existence of a bipolar magnetic field crossing the disc and require two conditions to produce powerful jets: field lines need to be bent enough at the disc surface and the magnetic field needs to be close to equipartition. The work of Petrucci et al (2008) on the variability of X-ray binaries supposes that transitions between pure accretion phases and accretion-ejection phases are due to some variations of the disc magnetization. This rises the problem of the magnetic field dragging in accretion discs. We revisit the method developed by Lubow et al (1994) by including momentum and mass conservation equations in a time-dependent 1D MHD code.

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arXiv2011

Jet launching and field advection in quasi-Keplerian discs

The fact that self-confined jets are observed around black holes, neutron stars and young forming stars points to a jet launching mechanism independent of the nature of the central object, namely the surrounding accretion disc. The properties of Jet Emitting Discs (JEDs) are briefly reviewed. It is argued that, within an alpha prescription for the turbulence (anomalous viscosity and diffusivity), the steady-state problem has been solved. Conditions for launching jets are very stringent and require a large scale magnetic field $B_z$ close to equipartition with the total (gas and radiation) pressure. The total power feeding the jets decreases with the disc thickness: fat ADAF-like structures with $h\sim r$ cannot drive super-Alfvénic jets. However, there exist also hot, optically thin JED solutions that would be observationally very similar to ADAFs. Finally, it is argued that variations in the large scale magnetic $B_z$ field is the second parameter required to explain hysteresis cycles seen in LMXBs (the first one would be $\dot M_a$).

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arXiv2010

Dead zones in protostellar discs: the case of Jet Emitting Discs

Planet formation and migration in accretion discs is a very active topic. Among the many aspects related to that question, dead zones are of particular importance as they can influence both the formation and the migration of planetary embryos. The ionisation level in the disc is the key element in determining the existence and the location of the dead zone. This has been studied either within the Standard Accretion Disc (SAD) framework or using parametric discs. In this paper, we extend this study to the case of Jet Emitting Discs (JED), the structure of which strongly differ from SADs because of the new energy balance and angular momentum extraction imposed by the jets. We make use of the (r,z) density distributions provided by self-similar accretion-ejection models, along with the JED thermal structure derived in a previous paper, to create maps of the ionisation structure of JEDs. We compare the ionisation rates we obtain to the critical value required to trigger the magneto-rotational instability. It is found that JEDs have a much higher ionisation degree than SADs which renders very unlikely the presence of a dead zone in these discs. As JEDs are believed to occupy the inner regions of accretion discs, the extension of the dead zones published in the literature should be re-considered for systems in which a jet is present. Moreover, since JEDs require large scale magnetic fields close to equipartition, our findings raise again the question of magnetic field advection in circumstellar accretion discs.

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arXiv2010

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

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. We investigate the time evolution of an 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. 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). The large scale magnetic field introduces only a small perturbation to the disk structure, with accretion driven by the dominant viscous torque. A super fast magnetosonic jet is observed to be launched from the innermost regions and remains stationary over more than 953 Keplerian orbits. 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 due to the negligible power carried away by the jet. 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.

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arXiv2006

MHD disc winds

This is a doctorate level lecture on the physics of accretion discs driving magnetically self-confined jets, usually referred to in the literature as disc winds. I will first review the governing magnetohydrodynamic equations and then discuss their physical content. At that level, necessary conditions to drive jets from keplerian accretion discs can already be derived. These conditions are validated with self-similar calculations of accretion-ejection structures. In a second part, I will critically discuss the biases introduced when using self-similarity as well as some other questions such as: Are these systems really unstable? Can a standard accretion disc provide the conditions to launch jets in its innermost parts? What is the difference between X-winds and disc-winds? Finally, the magnetic interaction between a protostar and its circumstellar disc will be discussed with a focus on stellar spin down.

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arXiv2006

Which jet launching mechanism(s) in TTauri stars?

AIMS: We examine whether ejection phenomena from accreting T Tauri stars can be described by only one type of self-collimated jet model. METHODS: We present analytical kinematic predictions valid soon after the Alfvén surface for all types of steady magnetically self-confined jets. RESULTS: We show that extended disc winds, X-winds, and stellar winds occupy distinct regions in the poloidal speed vs. specific angular momentum plane. Comparisons with current observations of T Tauri jets yield quantitative constraints on the range of launching radii, magnetic lever arms, and specific energy input in disc and stellar winds. Implications on the origin of jet asymmetries and disc magnetic fields are outlined. CONCLUSIONS: We argue that ejection phenomena from accreting T Tauri stars most likely include three dynamical components: (1) an outer self-collimated steady disc wind carrying most of the mass-flux in the optical jet (when present), confining (2) a pressure-driven coronal stellar wind and (3) a hot inner flow made of blobs sporadically ejected from the magnetopause. If the stellar magnetic moment is parallel to the disc magnetic field, then the highly variable inner flow resembles a "Reconnection X-wind", that has been proven to efficiently brake down an accreting and contracting young star. If the magnetic moment is anti-parallel, then larger versions of the solar coronal mass ejections are likely to occur. The relative importance of these three components in the observed outflows and the range of radii involved in the disc wind are expected to vary with time, from the stage of embedded source to the optically revealed T Tauri star phase.

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arXiv2005

Direct magnetic field detection in the innermost regions of an accretion disc

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.

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arXiv2003

Stationary accretion disks launching super fast-magnetosonic MHD jets

We present self-similar models of resistive viscous Keplerian disks driving non-relativistic magnetohydrodynamics (MHD) jets 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 which 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 time-scales of several disk dynamical time-scales can be produced if the suitable pressure conditions are not fulfilled.

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arXiv2003

Theory of magnetized accretion discs driving jets

In this lecture I review the theory of magnetized accretion discs driving jets, with a focus on Young Stellar Objects (YSOs). I first introduce observational and theoretical arguments in favor of the ``disc wind'' paradigm. There, accretion and ejection are interdependent, requiring therefore to revisit the standard picture of accretion discs. A simple magnetostatic approach is shown to already provide some insights of the basic phenomena. The magnetohydrodynamic equations as well as all usual assumptions are then clearly listed. The relevant physical mechanisms of steady-state accretion and ejection from Keplerian discs are explained in a model independent way. The results of self-similar calculations are shown and critically discussed, for both cold and warm jet configurations. I finally provide observational predictions and the physical conditions required in YSOs discs. The necessity of introducing a magnetospheric interaction between the disc and the protostar is briefly discussed.

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arXiv2003

Accretion--ejection phenomena from young stars

In the current paradigm of star formation magnetic fields play a very central role. Indeed, they probably help or even channel the initial gravitational collapse of the parent molecular cloud. But their most spectacular effect is certainly the production of ubiquitous, powerful, bipolar self-collimated jets. Also, in the last decade, evidence has come that in such systems a magnetosphere links the protostar to its surrounding accretion disc. I will first briefly review the magnetohydrodynamic models of jets from young stars. Then, I will discuss some constraints on the magnetospheric interaction since accretion must proceed despite strong stellar magnetic fields. Finally, I will introduce the "Reconnection X-wind" scenario that leads to episodic jet formation with efficient removal of protostellar angular momentum.

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arXiv2001

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 \citep{Safier93a, Safier93b} by computing the thermal and ionization structure of self-similar, magnetically-driven, atomic disk winds heated by ambipolar diffusion. Improvements over his work include: (1) new magnetized cold jet solutions consistent with the underlying accretion disk \citep{Ferreira97}; (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, \cite{Garcia2001b}. As in \citep{Safier93a}, 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 different 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 diffusion and adiabatic expansion are the dominant heating and cooling terms. We finally 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).

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arXiv2001

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

Thermal and ionization structures of self-similar, magnetically-driven disk winds obtained in a companion paper \cite{Garcia2001a} are used to compute a series of jet synthetic observations. These include spatially resolved forbidden line emission maps, long-slit spectra, as well as line ratios. Line profiles and jet widths appear to be good tracers of the wind dynamics and collimation, whereas line ratios essentially trace gas excitation conditions. All the above diagnostics are confronted to observations of T Tauri star microjets. Convolution by the observing beam is shown to be essential for a meaningful test of the models. We find that jet widths and qualitative variations in line profiles with both distance and line tracers are well reproduced. A low-velocity [\ion{O}{i}] component is also obtained, originating from the disk wind base. However, this component is too weak, predicted maximum velocities are too high and electronic and total densities are too low. Denser and slower magnetized winds, launched from disks with warm chromospheres, might resolve these discrepancies.

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arXiv2001

Atomic T Tauri disk winds heated by ambipolar diffusion

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 $\simeq 10^4$ 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 [OI] component is too weak. Denser, slower MHD winds from warm disks might resolve these discrepancies.

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arXiv2000

MHD jets around galactic objects

We present a self-similar, steady-state model describing both a magnetized accretion disc and a magnetohydrodynamic jet. We focus on the role of a hot corona in such a structure. This corona enables the disc to launch various types of jets. By considering the energy conservation, we also present a diagnostic of the luminosity of the magnetized disc, which could explain some observational signatures of galactic objects.

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arXiv2000

Magnetized accretion-ejection structures V. Effects of entropy generation inside the disc

In this paper, steady-state MHD calculations of non-relativistic magnetized accretion discs driving jets are presented. For the first time, an energy equation describing the effects of entropy generation along streamlines is included. Using a simplified approach, we showed that thermal effects have a tremendous influence on the disc capability to feed jets with mass. The disc ejection efficiency is measured by the parameter $ξ= d \ln \dot{M}_{a}/d \ln{r}$, where $ \dot{M}_{a}(r)$ is the local disc accretion rate. While previous self-similar solutions were only able to produce jets with $ξ\sim 0.01$, solutions with a coronal heating display huge efficiencies up to $ξ\sim 0.5$. Moreover, taking thermal effects into account allows to obtain both fast and slow magnetic rotators. Since most of the jet properties (like asymptotic velocity or degree of collimation) depend on the mass load, it arises from this study that any quantitative result requires a detailled analysis of the disc energetics.

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arXiv1999

Magnetized accretion-ejection structures IV. Magnetically-driven jets from resistive, viscous, Keplerian discs

We present steady-state calculations of self-similar magnetized accretion discs driving cold, adiabatic, non-relativistic jets. For the first time, both the magnetic torque due to the jets and a turbulent "viscous" torque are taken into account. This latter torque allows a dissipation of the accretion power as radiation at the disc surfaces, while the former predominantly provides jets with power. The parameter space of these structures has been explored. It is characterized by four free parameters, namely the disc aspect ratio and three MHD turbulence parameters, related to the anomalous magnetic diffusivities and viscosity. It turns out that launching cold jets from thin, dissipative discs implies anisotropic turbulent dissipation. Jets that asymptotically reach a high Alfvenic Mach number are only produced by weakly dissipative discs. We obtained general analytical relations between disc and jet quantities that must be fulfilled by any steady-state model of cold jets, launched from a large radial extension of thin discs. We also show that such discs cannot have a dominant viscous torque. This is because of the chosen geometry, imposing the locus of the Alfven surface. Some observational consequences of these cold magnetized accretion-ejection structures are also briefly discussed.

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arXiv1996

Magnetically-driven jets from Keplerian accretion discs

Non-relativistic, magnetically-driven jets are constructed by taking self-consistently into account the feedback on the underlying accretion disc. It is shown that such jets are mostly described by the ejection index $ξ= \mbox{d} \ln \dot M_a/ \mbox{d}\ln r$, which is a local measure of the disc ejection efficiency. This parameter is found to lie in a very narrow range, due to global disc-jets constraints. This investigation provided two important results. First, the disc vertical equilibrium imposes a minimum mass flux ejected. Second, their asymptotic behaviour critically depends on a fastness parameter $ω_A= Ω_*r_A /V_{Ap,A}$, ratio of the field lines rotation velocity to the poloidal Alfvén velocity at the Alfvén surface. This parameter must be bigger than, but of the order of, unity. Self-similar jets from Keplerian discs, after widening up to a maximum radius whose value increases with $ω_A$, always recollimate towards the jet axis, until the fast-magnetosonic critical point is reached. It is doubtful that such solutions could steadily cross this last point, the jet either ending there or rebouncing. Recollimation takes place because of the increasing effect of magnetic constriction. This systematic behaviour is due to the large opening of the magnetic surfaces, leading to such an efficient acceleration that matter always reaches its maximum poloidal velocity. This ``over-widening'' stems from having the same ejection efficiency $ξ$ in the whole jet. Realistic jets, fed with ejection indices varying from one magnetic surface to the other, would not undergo recollimation, allowing either cylindrical or parabolic asymptotic collimation.

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