arXiv subjects

J. Ferreira

Publications and source records attributed to J. Ferreira.

50 records · Page 3Linked to original sources

arXiv2009

Surface magnetic fields on two accreting T Tauri stars: CV Cha and CR Cha

We have produced brightness and magnetic field maps of the surfaces of CV Cha and CR Cha: two actively accreting G and K-type T Tauri stars in the Chamaeleon I star-forming cloud with ages of 3-5 Myr. Our magnetic field maps show evidence for strong, complex multi-polar fields similar to those obtained for young rapidly rotating main sequence stars. Brightness maps indicate the presence of dark polar caps and low latitude spots -- these brightness maps are very similar to those obtained for other pre-main sequence and rapidly rotating main sequence stars. Only two other classical T Tauri stars have been studied using similar techniques so far: V2129 Oph and BP Tau. CV Cha and CR Cha show magnetic field patterns that are significantly more complex than those recovered for BP Tau, a fully convective T Tauri star. We discuss possible reasons for this difference and suggest that the complexity of the stellar magnetic field is related to the convection zone; with more complex fields being found in T Tauri stars with radiative cores (V2129 Oph, CV Cha and CR Cha). However, it is clearly necessary to conduct magnetic field studies of T Tauri star systems, exploring a wide range of stellar parameters in order to establish how they affect magnetic field generation, and thus how these magnetic fields are likely to affect the evolution of T Tauri star systems as they approach the main sequence.

astro-ph.SR↗

arXiv2008

A Global Energetic Model for Microquasars (GEMM): A rich and consistent disk+jet solution

Based on a dynamical model describing how stationary, powerful and self-collimated jets are being launched from a magnetized disk, we build a consistent disk+jet microquasar picture. Our disk is a new type of disk solution called the Jet Emitting Disk (JED), and whose characteristics are directly constrained by the presence of a jet. We assume a one-temperature plasma with thermal particles only. By solving the radiative equilibrium of the disk, we obtain three branches of solutions, a hot and a cold ones (both thermally stable), and an intermediate one, thermally unstable. The hot solution possess the global observed characteristics of what has been often called a "corona" located above the inner disk region. We present this new disk solution, and how the radiative equilibrium is computed. We discuss the richness of the solution, and show the ability of the model to reproduce an observed spectral energy distribution of XTE J1118+480 with reasonable parameters. We finally outline some perspectives of the model.

astro-ph↗

arXiv2008

A Global Energetic Model for Microquasars: preliminary results and spectral energy distributions

We present preliminary results and observables from a model of microquasar based on a theoretical framework where stationary, powerful, compact jets are launched and then accelerated from an inner magnetized disk. This model aim at providing a consistent picture of microquasars in all their spectral states. It is composed of an outer standard accretion disk down to a variable transition radius where it changes to a magnetized disk, called the Jet Emitting Disk (JED). The theoretical framework providing the heating, we solve the radiative equilibrium and obtain the JED structure. Our JED solutions are rich, and reproduce the already known scheme where a cold optically-thick and a hot optically-thin solutions bracket a thermally unstable one. We present the model and preliminary results, whith a first attempt at reproducing the observed SED of XTE J1118+480.

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arXiv2007

The role of the disk magnetization on the hysteresis behavior 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 $μ(r,t) =B_z^2/(μ_o P_{tot})$, where $P_{tot}$ is the total, gas and radiation, pressure. Below a transition radius $r_J$, a jet emitting disk (the JED) is settled and drives self-collimated non relativistic jets. Beyond $r_J$, no jet is produced despite the presence of the magnetic field and a standard accretion disc (the SAD) is established. The radial distribution of the disk magnetization $μ$ adjusts itself to any change of the disk accretion rate $\dot m$, thereby modifying the transition radius $r_J$. We propose that a SAD-to-JED transition occurs locally, at a given radius, in a SAD when $μ=μ_{max} \simeq 1$ while the reverse transition occurs in a JED only when $μ=μ_{min}\simeq 0.1$. This bimodal behavior of the accretion disk provides a promising way to explain the hysteresis cycles followed by X-ray binaries during outbursts.

astro-ph↗

arXiv2007

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 and find accretion funnels for stellar dipole fields as low as 140 G in the low accretion rate limit of $10^{-9} M_\odot.yr^{-1}$. 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 (${\dot M} \sim 10^{-8} M_\odot.yr^{-1}$) requires either larger stellar field strength and/or different magnetic topologies to allow for magnetospheric accretion.

astro-ph↗

arXiv2007

The radial structure of protostellar accretion disks: influence of jets

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 ionization, have been conducted in the context of the Standard Accretion Disk (SAD) model, where no jet is present. We wish to calculate the structure of 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 standard accretion disks. The analytical treatment used in this work is very similar to that of standard accretion disks but is using 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, such as mid-plane temperatures, surface densities or disk aspect ratio are derived. It is found that JEDs present a structure very different from the SADs and that 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.

astro-ph↗

arXiv2007

Status of QGSJET

Basic physics concepts of the QGSJET model are discussed, starting from the general picture of high energy hadronic interactions and addressing in some detail the treatment of multiple scattering processes, contributions of ``soft'' and ``semihard'' parton dynamics, implementation of non-linear interaction effects. The predictions of the new model version (QGSJET II.03) are compared to selected accelerator data. Future developments are outlined and the expected input from the LHC collider for constraining model predictions is discussed.

hep-ph↗

arXiv2007

Magnetic topologies of cool stars

Stellar magnetic fields can be investigated using several, very complementary approaches. While conventional spectroscopy is capable of estimating the average magnetic strength of potentially complex field configurations thanks to its low sensitivity to the vector properties of the field, spectropolarimetry can be used to map the medium- and large-scale structure of magnetic topologies. In particular, the latter approach allows one to retrieve information about the poloidal and toroidal components of the large-scale dynamo fields in low-mass stars, and thus to investigate the physical processes that produce them. Similarly, this technique can be used to explore how magnetic fields couple young stars to their massive accretion disc and thus to estimate how much mass and angular momentum are transfered to the newly-born low-mass star. We present here the latest results in this field obtained with spectropolarimetry, with special emphasis on the surprising discoveries obtained on very-low mass fully-convective stars and classical T Tauri stars thanks to the ESPaDOnS spectropolarimeter recently installed on the 3.6m Canada-France-Hawaii Telescope.

astro-ph↗

arXiv2006

A unified paradigm for the spectral and temporal evolution of Black Hole X-ray Binaries

We present a unified picture to explain the spectral states of BH binaries based on the two-flow model. In our view, the central regions have a multi-flow configuration consisting in (1) an outer standard accretion disc (SAD) down to a transition radius $r_{tr}$, (2) an inner magnetized jet emitting disc (JED) below $r_{tr}$ driving (3) a self-collimated non relativistic MHD jet surrounding, when conditions for pair creation are met, (4) a ultra relativistic pair beam. Large values of $r_{tr}$ correspond to Hard states while small values correspond to Soft states. In between these extremes, in the high intermediate state, $r_{tr}$ can reach values that switch on a pair cascade process giving birth to ultra-relativistic pair blobs that explain the superluminal events. In this model the accretion rate but also the disc magnetization $μ$ play important roles, the latter being necessarily in the range 0.1-1 for the MHD jet to exist. Then, with simple assumptions on $μ$, we propose an explanation for the hysteresis behavior observed in microquasars during their outburst. We also discuss the nature, in our framework, of the X-ray corona and the origin of the X-ray variability in the hard state. Preliminary results are discussed.

astro-ph↗

arXiv2006

Surface density jumps as planet traps

[Abridged] The tidal torque exerted by a protoplanetary disk with power law surface density and temperature profiles onto an embedded protoplanetary embryo is generally a negative quantity that leads to the embryo inwards migration. Here we investigate how the tidal torque balance is affected at a disk surface density radial jump. The jump has two consequences : - it affects the differential Lindblad torque. In particular if the disk is merely empty on the inner side, the differential Lindblad torque almost amounts to the large negative outer Lindblad torque. - It affects the corotation torque, which is a quantity very sensitive to the local gradient of the disk surface density. In particular if the disk is depleted on the inside and if the jump occurs radially over a few pressure scale-heights, the corotation torque is a positive quantity that is much larger than in a power-law disk. We show by means of customized numerical simulations of low mass planets embedded in protoplanetary nebulae with a surface density jump that the second effect is dominant, that is that the corotation torque largely dominates the differential Lindblad torque on the edge of a central depletion. As a consequence the type I migration of low mass objects reaching the jump should be halted, and all these objects should be trapped there.

astro-ph↗

arXiv2005

A unified accretion-ejection paradigm for Black Hole X-ray Binaries

We present a new picture for the central regions of Black Hole X-ray Binaries. In our view, these central regions have a multi-flow configuration which consists in (1) an outer standard accretion disc down to a transition radius r_J, (2) an inner magnetized accretion disc below r_J driving (3) a non relativistic self-collimated electron-proton jet surrounding, when adequate conditions for pair creation are met, (4) a ultra relativistic electron-positron beam. This accretion-ejection paradigm provides a simple explanation to the canonical spectral states by varying the transition radius r_J and disc accretion rate independently. Large values of r_J correspond to the Quiescent state for low $\dot m$ and the Hard state for larger $\dot m$. These states are characterized by the presence of a steady electron-proton MHD jet emitted by the disc below r_J. The hard X-ray component is expected to form at the jet basis. When r_j becomes smaller than the marginally stable orbit r_i, the whole disc resembles a standard accretion disc, characteristic of the Soft state. Intermediate states correspond to situations where r_J ~ r_i. At large $\dot m$, an unsteady pair cascade process is triggered within the jet axis, giving birth to ejection of relativistic pair blobs. This would correspond to the luminous intermediate state, with its associated superluminal motions. The variation of r_J independently of $\dot m$ is a necessary ingredient in this picture, arising from the presence of a large scale vertical magnetic field threading the disc.

astro-ph↗

arXiv2004

Applying Magnetized Accretion-Ejection Models to Microquasars: a preliminary step

We present in this proceeding some aspects of a model that should explain the spectral state changes observed in microquasars. In this model, ejection is assumed to take place only in the innermost disc region where a large scale magnetic field is anchored. Then, in opposite to conventional ADAF models, the accretion energy can be efficiently converted in ejection and not advected inside the horizon. We propose that changes of the disc physical state (e.g. transition from optically thick to optically thin states) can strongly modify the magnetic accretion-ejection structure resulting in the spectral variability. After a short description of our scenario, we give some details concerning the dynamically self-consistent magnetized accretion-ejection model used in our computation. We also present some preliminary results of spectral energy distribution.

astro-ph↗

arXiv2004

Predicted rotation signatures in MHD disc winds and comparison to DG Tau observations

Motivated by the first detections of rotation signatures in the DG Tau jet (Bacciotti et al. 2002), we examine possible biases affecting the relation between detected rotation signatures and true azimuthal velocity for self-similar MHD disc winds, taking into account projection, convolution as well as excitation gradients effects. We find that computed velocity shifts are systematically smaller than the true underlying rotation curve. When outer slower streamlines dominate the emission, we predict observed shifts increasing with transverse distance to the jet axis, opposite to the true rotation profile. Determination of the full transverse rotation profile thus requires high angular resolution observations (< 5 AU) on an object with dominant inner faster streamlines. Comparison of our predictions with HST/STIS observations of DG Tau clearly shows that self-similar, warm MHD disc wind models with lambda = 13 and an outer radius of the disc ~ 3 AU are able to reproduce detected velocity shifts, while cold disc wind models (lambda > 50) are ruled out for the medium-velocity component in the DG Tau jet.

astro-ph↗

arXiv2003

Near-IR [Fe II] emission diagnostics applied to cold disk winds in young stars

We investigate the emissivity properties of the main near-IR transitions of the Fe+ ion in the conditions prevailing in the inner regions of jets from young stars, based on a simplified 16-level atom model. We present new diagnostic diagrams involving prominent near-IR line ratios that allow us to constrain the electronic density, temperature, and Fe gas phase abundance ratio, independently of the heating process. Comparison with recent near-IR observations of a sample of HH objects indicates gas phase Fe abundances ranging from 15-50 % up to 100 % of the solar value in agreement with the moderate depletions previously derived from optical line ratios or shock models. Hence, it appears that Fe-bearing dust is efficiently destroyed in stellar jets. We then use our Fe+ emissivity model to predict near-IR [Fe II] emission maps for self-similar, cold MHD disk wind models. We show that observations in [Fe II] with AMBER on the VLTI could severely constrain the MHD solution and the inner launch radius of the jet. We also compare theoretical predictions with recent observations in the near-IR [Fe II] lines of the L1551-IRS5 and DG Tau jets. The cold disk wind model reproduces quite well the two velocity components observed at -100 and -300 km/s, although the high velocity component appears overestimated by a factor of 1.5 in the DG Tau jet. However, the model predicts too little emission at intermediate-velocity and insufficient densities. Denser disk winds with stronger heating at the jet base, which have been invoked for optical jets, also appear needed in younger, embedded Class I jet sources.

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