Magnetized Accretion-Ejection Structures: 2.5D MHD simulations of continuous Ideal Jet launching from resistive accretion disks

We present numerical magnetohydrodynamic (MHD) simulations of a magnetized accretion disk launching trans-Alfvenic jets. These simulations, performed in a 2.5 dimensional time-dependent polytropic resistive MHD framework, model a resistive accretion disk threaded by an initial vertical magnetic field. The resistivity is only important inside the disk, and is prescribed as eta = alpha_m V_AH exp(-2Z^2/H^2), where V_A stands for Alfven speed, H is the disk scale height and the coefficient alpha_m is smaller than unity. By performing the simulations over several tens of dynamical disk timescales, we show that the launching of a collimated outflow occurs self-consistently and the ejection of matter is continuous and quasi-stationary. These are the first ever simulations of resistive accretion disks launching non-transient ideal MHD jets. Roughly 15% of accreted mass is persistently ejected. This outflow is safely characterized as a jet since the flow becomes super-fastmagnetosonic, well-collimated and reaches a quasi-stationary state. We present a complete illustration and explanation of the `accretion-ejection' mechanism that leads to jet formation from a magnetized accretion disk. In particular, the magnetic torque inside the disk brakes the matter azimuthally and allows for accretion, while it is responsible for an effective magneto-centrifugal acceleration in the jet. As such, the magnetic field channels the disk angular momentum and powers the jet acceleration and collimation. The jet originates from the inner disk region where equipartition between thermal and magnetic forces is achieved. A hollow, super-fastmagnetosonic shell of dense material is the natural outcome of the inwards advection of a primordial field.Comment: ApJ (in press), 32 pages, Higher quality version available at http://www-laog.obs.ujf-grenoble.fr/~fcass

Distinguishing an ejected blob from alternative flare models at the Galactic centre with GRAVITY

The black hole at the Galactic centre exhibits regularly flares of radiation, the origin of which is still not understood. In this article, we study the ability of the near-future GRAVITY infrared instrument to constrain the nature of these events. We develop realistic simulations of GRAVITY astrometric data sets for various flare models. We show that the instrument will be able to distinguish an ejected blob from alternative flare models, provided the blob inclination is >= 45deg, the flare brightest magnitude is 14 <= mK <= 15 and the flare duration is >= 1h30.Comment: 11 pages, 9 figures, accepted by MNRA

Stability and structure of analytical MHD jet formation models with a finite outer disk radius

(Abridged) Finite radius accretion disks are a strong candidate for launching astrophysical jets from their inner parts and disk-winds are considered as the basic component of such magnetically collimated outflows. The only available analytical MHD solutions for describing disk-driven jets are those characterized by the symmetry of radial self-similarity. Radially self-similar MHD models, in general, have two geometrical shortcomings, a singularity at the jet axis and the non-existence of an intrinsic radial scale, i.e. the jets formally extend to radial infinity. Hence, numerical simulations are necessary to extend the analytical solutions towards the axis and impose a physical boundary at finite radial distance. We focus here on studying the effects of imposing an outer radius of the underlying accreting disk (and thus also of the outflow) on the topology, structure and variability of a radially self-similar analytical MHD solution. The initial condition consists of a hybrid of an unchanged and a scaled-down analytical solution, one for the jet and the other for its environment. In all studied cases, we find at the end steady two-component solutions.Comment: 14 pages, 15 figures, accepted for publication in A &

Radial and vertical angular momentum transport in protostellar discs

Angular momentum in protostellar discs can be transported either radially, through turbulence induced by the magnetorotational instability (MRI), or vertically, through the torque exerted by a large-scale magnetic field. We present a model of steady-state discs where these two mechanisms operate at the same radius and derive approximate criteria for their occurrence in an ambipolar diffusion dominated disc. We obtain "weak field'' solutions - which we associate with the MRI channel modes in a stratified disc - and transform them into accretion solutions with predominantly radial angular-momentum transport by implementing a turbulent-stress prescription based on published results of numerical simulations. We also analyze "intermediate field strength'' solutions in which both radial and vertical transport operate at the same radial location. Our results suggest, however, that this overlap is unlikely to occur in real discs.Comment: 5 pages, 2 figures, 1 table, aastex.cls. Accepted for publication in Astrophysics & Space Scienc

VELO Module Production - Laser Test and Noise Analysis

This note describes the algorithms used to detect problems by analyzing datasets taken at different stages of module production using the hybrid readout systems

A simple test for the existence of two accretion modes in Active Galactic Nuclei

By analogy to the different accretion states observed in black-hole X-ray binaries (BHXBs), it appears plausible that accretion disks in active galactic nuclei (AGN) undergo a state transition between a radiatively efficient and inefficient accretion flow. If the radiative efficiency changes at some critical accretion rate, there will be a change in the distribution of black hole masses and bolometric luminosities at the corresponding transition luminosity. To test this prediction, I consider the joint distribution of AGN black hole masses and bolometric luminosities for a sample taken from the literature. The small number of objects with low Eddington-scaled accretion rates mdot < 0.01 and black hole masses Mbh < 10^9 Msun constitutes tentative evidence for the existence of such a transition in AGN. Selection effects, in particular those associated with flux-limited samples, systematically exclude objects in particular regions of the black hole mass-luminosity plane. Therefore, they require particular attention in the analysis of distributions of black hole mass, bolometric luminosity, and derived quantities like the accretion rate. I suggest further observational tests of the BHXB-AGN unification scheme which are based on the jet domination of the energy output of BHXBs in the hard state, and on the possible equivalence of BHXB in the very high (or "steep power-law") state showing ejections and efficiently accreting quasars and radio galaxies with powerful radio jets.Comment: Accepted by ApJ; 14 pages, 4 figures, uses emulateap

The Local Abundance of 3^3He: A Confrontation Between Theory and Observation

Determinations of the \he3 concentrations in Galactic matter serve to impose interesting and important constraints both on cosmological models and on models of Galactic chemical evolution. At present, observations of \he3 in the solar system and in the interstellar medium today suggest that the \he3 abundance has not increased significantly over the history of the Galaxy, while theoretical models of Galactic chemical evolution (utilizing current nucleosynthesis yields from stellar evolution and supernova models) predict a rather substantial increase in \he3. We consider the possibility that the solar \he3 abundance may have been affected by stellar processing in the solar neighborhood prior to the formation of the solar system. Such a discrepancy between solar abundances and average galactic abundances by as much as a factor of two, may be evidenced by several isotopic anomalies. Local destruction of \he3 by a similar amount could serve to help to reconcile the expected increase in the \he3 abundance predicted by models of galactic chemical evolution. We find however, that the production of heavier elements, such as oxygen, places a strong constraint on the degree of \he3 destruction. We also explore the implications of both alternative models of Galactic chemical evolution and the stellar yields for \he3 in low mass stars, which can explain the history of the \he3 concentration in the Galaxy.Comment: 27 pages, latex, 5 figure

Magnetohydrodynamic jets from different magnetic field configurations

Using axisymmetric MHD simulations we investigate how the overall jet formation is affected by a variation in the disk magnetic flux profile and/or the existence of a central stellar magnetosphere. Our simulations evolve from an initial, hydrostatic equilibrium state in a force-free magnetic field configuration. We find a unique relation between the collimation degree and the disk wind magnetization power law exponent. The collimation degree decreases for steeper disk magnetic field profiles. Highly collimated outflows resulting from a flat profile tend to be unsteady. We further consider a magnetic field superposed of a stellar dipole and a disk field in parallel or anti-parallel alignment. Both stellar and disk wind may evolve in a pair of outflows, however, a reasonably strong disk wind component is essential for jet collimation. Strong flares may lead to a sudden change in mass flux by a factor two. We hypothesize that such flares may eventually trigger jet knots.Comment: 5 pages, 4 figures; proceedings from conference: Protostellar Jets in Context, held in Rhodes, July 7-12, 200