Joao Jose Graca Lima
@astro.up.pt
Department of Physics and Astronomy, Faculty of Sciences
University of Porto
EDUCATION
Ph.D. in Astronomy, University of St. Andrews, 1995
RESEARCH, TEACHING, or OTHER INTERESTS
Astronomy and Astrophysics, Physics and Astronomy, General Physics and Astronomy
Scopus Publications
Scopus Publications
Filipa S. Barros, Paula A. Graça, J.J.G. Lima, Rui F. Pinto, André Restivo, and Murillo Villa
Elsevier BV
C. Sauty, R. M. G. de Albuquerque, V. Cayatte, J. J. G. Lima, and J. F. Gameiro
EDP Sciences
Context. Recent observational evidence has shown that RY Tau may present two different outflow stages, a quiescent and a more active stage. We try to model this phenomenon. Aims. We have performed new 2.5D magnetohydrodynamical simulations of the possible accretion-outflow environment of RY Tau based on analytical solutions with the aim to reduce the relaxation time. Methods. We used the analytical self-similar solution that we used to model the RY Tau microjet as initial conditions. In the closed field line region of the magnetosphere, we reversed the direction of the flow and increased the accretion rate by increasing the density and velocity. We also implemented the heating rate and adjusted it according to the velocity of the flow. The accretion disk was treated as a boundary condition. Results. The simulations show that the stellar jet and the accreting magnetosphere attain a steady state in only a few stellar rotations. This confirms the robustness and stability of self-similar solutions. Additionally, two types of behavior were observed that are similar to the behavior observed in RY Tau. Either the steady stellar outflow and magnetospheric inflow are separated by a low static force-free region or the interaction between the stellar jet and the magnetospheric accretion creates episodic coronal mass ejections that originate from the disk and bounce back onto the star. Conclusions. The ratio of mass-loss rate to mass-accretion rate that coincides with the change in behavior observed in RY Tau lies within the range of ratios that have been measured during the period in which the initial microjet was analyzed.
Mariana Balaton, Jorge Cavadas, Paulo Simeão Carvalho, and J J G Lima
IOP Publishing
Abstract Experimental teaching is essential for a good understanding of science, especially on Physics. Practical activities play an important role for engaging students with science, mainly when they interact directly with equipment, collect experimental data with computers and/or use interactive software for data analysis. In this work, we present the use of low-cost mini-robots as an ‘object-to-think-with’ for teaching and learning with technology. The activity concerns programming the robots to make them run in circular paths, record videos of their trajectories and analyse them with Tracker Software, to boost the study of Astronomy contents. This kind of practical activity develops multiple skills in students and is usually very well accepted because it involves robots, programming, manipulating technology and for raising topics that are difficult to understand in real-life observations, making them cognitively accessible to the vast majority of students. In this practical activity, students are asked to create the robot programming code and make a video recording (with a smartphone) of the robots’ trajectories, mediated by the teacher, who assists in the construction of the experimental activity and analysis of the data obtained. The results will allow students to understand Kepler’s laws of planetary motion and why some planets seem to have an apparent retrograde motion as seen from the Earth, a problem that arose in IV BC and was only officially solved by the Copernicus heliocentric model, published in 1543, the year of his death.
R. M. G. de Albuquerque, J. F. Gameiro, S. H. P. Alencar, J. J. G. Lima, C. Sauty, and C. Melo
EDP Sciences
Context.Although the Orion Nebula Cluster is one of the most studied clusters in the solar neighborhood, the evolution of the very low-mass members (M* < 0.25 M⊙) has not been fully addressed due to their faintness.Aims.Our goal is to verify if some young and very low-mass objects in the Orion Nebula Cluster show evidence of ongoing accretion using broadband VLT/X-shooter spectra.Methods.For each target, we determined the corresponding stellar parameters, veiling, observed Balmer jump, and accretion rates. Additionally, we searched for the existence of circumstellar disks through available on-line photometry.Results.We detected accretion activity in three young stellar objects in the Orion Nebula Cluster, two of them being in the very low-mass range. We also detected the presence of young transition disks with ages between 1 and 3.5 Myr.
R. M. G. de Albuquerque, V. Cayatte, J. F. Gameiro, J. J. G. Lima, C. Sauty, and S. Ulmer-Moll
Springer International Publishing
J. J. G. Lima, C. Sauty, and N. Vlahakis
Springer International Publishing
M. Oshagh, N. C. Santos, P. Figueira, V. Zh. Adibekyan, A. Santerne, S. C. C. Barros, and J. J. G. Lima
EDP Sciences
We assess a physically feasible explanation for the low number of discovered (near-)grazing planetary transits through all ground and space based transit surveys. We performed simulations to generate the synthetic distribution of detectable planets based on their impact parameter, and found that a larger number of (near-)grazing planets should have been detected than have been detected. Our explanation for the insufficient number of (near-)grazing planets is based on a simple assumption that a large number of (near-)grazing planets transit host stars which harbor dark giant polar spot, and thus the transit light-curve vanishes due to the occultation of grazing planet and the polar spot. We conclude by evaluating the properties required of polar spots in order to make disappear the grazing transit light-curve, and we conclude that their properties are compatible with the expected properties from observations.
V. Cayatte, N. Vlahakis, T. Matsakos, J. J. G. Lima, K. Tsinganos, and C. Sauty
American Astronomical Society
Young stellar object observations suggest that some jets rotate in the opposite direction with respect to their disk. In a recent study, Sauty et al. showed that this does not contradict the magnetocentrifugal mechanism that is believed to launch such outflows. Motion signatures that are transverse to the jet axis, in two opposite directions, have recently been measured in M87. One possible interpretation of this motion is that of counter-rotating knots. Here, we extend our previous analytical derivation of counter-rotation to relativistic jets, demonstrating that counter-rotation can indeed take place under rather general conditions. We show that both the magnetic field and a non-negligible enthalpy are necessary at the origin of counter-rotating outflows, and that the effect is associated with a transfer of energy flux from the matter to the electromagnetic field. This can be realized in three cases: if a decreasing enthalpy causes an increase of the Poynting flux, if the flow decelerates, or if strong gradients of the magnetic field are present. An illustration of the involved mechanism is given by an example of a relativistic magnetohydrodynamic jet simulation.
A.R.G. Santos, M.S. Cunha, and J.J.G. Lima
Wiley
AbstractSmall cyclic variations in the frequencies of acoustic modes are expected to be a common phenomenon in solar‐like pulsators, as a result of stellar magnetic activity cycles. The frequency variations observed throughout the solar and stellar cycles contain information about structural changes that take place inside the stars as well as about variations in magnetic field structure and intensity. The task of inferring and disentangling that information is, however, not a trivial one. In the sun and solar‐like pulsators, the direct effect of the magnetic field on the oscillations might be significantly important in regions of strong magnetic field (such as solar/stellar spots), where the Lorentz force can be comparable to the gas‐pressure gradient. Our aim is to determine the sun‐/starspot effect on the oscillation frequencies and attempt to understand if this effect contributes strongly to the frequency changes observed along the magnetic cycle. The total contribution of the spots to the frequency shifts results from a combination of direct and indirect effects of the magnetic field on the oscillations. In this first work we considered only the indirect effect associated with changes in the stratification within the starspot. Based on the solution of the wave equation and the variational principal we estimated the impact of these stratification changes on the oscillation frequencies of global modes in the sun and found that the induced frequency shifts are about two orders of magnitude smaller than the frequency shifts observed over the solar cycle (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
C. Sauty, V. Cayatte, J. J. G. Lima, T. Matsakos, and K. Tsinganos
American Astronomical Society
Rotation measurement in jets from T Tauri stars is a rather difficult task. Some jets seem to be rotating in a direction opposite to that of the underlying disk, although it is not yet clear if this affects the totality or part of the outflows. On the other hand, Ulysses data also suggest that the solar wind may rotate in two opposite ways between the northern and southern hemispheres. We show that this result is not as surprising as it may seem and that it emerges naturally from the ideal MHD equations. Specifically, counterrotating jets neither contradict the magnetocentrifugal driving of the flow nor prevent extraction of angular momentum from the disk. The demonstration of this result is shown by combining the ideal MHD equations for steady axisymmetric flows. Provided that the jet is decelerated below some given threshold beyond the Alfvén surface, the flow will change its direction of rotation locally or globally. Counterrotation is also possible for only some layers of the outflow at specific altitudes along the jet axis. We conclude that the counterrotation of winds or jets with respect to the source, star or disk, is not in contradiction with the magnetocentrifugal driving paradigm. This phenomenon may affect part of the outflow, either in one hemisphere, or only in some of the outflow layers. From a time-dependent simulation, we illustrate this effect and show that it may not be permanent.
C. Sauty, Z. Meliani, J. J. G. Lima, K. Tsinganos, V. Cayatte, and N. Globus
EDP Sciences
Context. A large sample of T Tauri stars exhibits optical jets, approximately half of which rotate slowly, only at ten per cent of their breakup velocity. The disk-locking mechanism has been shown to be inefficient to explain this observational fact. Aims. We show that low mass accreting T Tauri stars may have a strong stellar jet component that can effectively brake the star to the observed rotation speed. Methods. By means of a nonlinear separation of the variables in the full set of the MHD equations we construct semi-analytical solutions describing the dynamics and topology of the stellar component of the jet that emerges from the corona of the star. Results. We analyze two typical solutions with the same mass loss rate but different magnetic lever arms and jet radii. The first solution with a long lever arm and a wide jet radius effectively brakes the star and can be applied to the visible jets of T Tauri stars such as RY Tau. The second solution with a shorter lever arm and a very narrow jet radius may explain why similar stars, either weak line T Tauri stars (WTTS) or classical T Tauri stars (CTTS) do not all have visible jets. For instance, RY Tau itself seems to have different phases that probably depend on the activity of the star. Conclusions. First, stellar jets seem to be able to brake pre-main sequence stars with a low mass accreting rate. Second, jets may be visible only part time owing to changes in their boundary conditions. We also suggest a possible scenario for explaining the dichotomy between CTTS and WTTS, which rotate faster and do not have visible jets.
N. Globus, C. Sauty, V. Cayatte, Z. Meliani, J. J. G. Lima, K. Tsinganos, and C. Michaut
Cambridge University Press (CUP)
AbstractWe show that low mass accreting T Tauri stars may have a strong stellar jet component which can effectively brake the star to the observed rotation speed. By means of meridional self similarity, we construct semi analytical solutions describing the complete dynamics and topology of the stellar component of the jet emerging from the corona of the star. We show two typical solutions with the same mass loss rate but different magnetic lever arms and jet radius, corresponding to differente phases of T Tauri star activity.
A. Aibéo, J. M. Ferreira, and J. J. G. Lima
EDP Sciences
Context. The existence of rapidly rotating cool stars in young clusters implies a reduction of angular momentum loss rate for a certain period of the star’s early life. Recently, the concentration of magnetic flux near the poles of these stars has been proposed as an alternative mechanism to dynamo saturation in order to explain the saturation of angular momentum loss. Aims. In this work we study the effect of magnetic surface flux distribution on the coronal field topology and angular momentum loss rate. We investigate if magnetic flux concentration towards the pole is a reasonable alternative to dynamo saturation. Methods. We construct a 1D wind model and also apply a 2-D self-similar analytical model, to evaluate how the surface field distribution affects the angular momentum loss of the rotating star. Results. From the 1D model we find that, in a magnetically dominated low corona, the concentrated polar surface field rapidly expands to regions of low magnetic pressure resulting in a coronal field with small latitudinal variation. We also find that the angular momentum loss rate due to a uniform field or a concentrated field with equal total magnetic flux is very similar. From the 2D wind model we show that there are several relevant factors to take into account when studying the angular momentum loss from a star. In particular, we show that the inclusion of force balance across the field in a wind model is fundamental if realistic conclusions are to be drawn from the effect of non-uniform surface field distribution on magnetic braking. This model predicts that a magnetic field concentrated at high latitudes leads to larger Alfven radii and larger braking rates than a smoother field distribution. Conclusions. From the results obtained, we argue that the magnetic surface field distribution towards the pole does not directly limit the braking efficiency of the wind.
C. Sauty, J. J. G. Lima, K. Tsinganos, A. Aibeo, Z. Meliani, and N. Vlahakis
AIP
In parallel to the development of numerical simulations, analytical solutions for modelling the acceleration and the collimation of winds and jets have been proposed. We present here how meridionally self‐similar solutions can be used to model the solar wind using Ulysses data at solar minimum. Such solutions may also be adapted to explain the formation core or spine jets in classical and weak TTauri stars (class II and III young stellar jets) as well as relativistic jet cores from AGN. The criterion for collimation explains how the jet evolves towards a wind as the star approaches the main sequence. A similar scenario could explain the winds from Seyfert galaxies by opposition to the powerful jets from Fanaroff Riley sources.
A. Aibéo, J. J. G. Lima, and C. Sauty
EDP Sciences
Aims. In a previous work, ULYSSES data was analyzed to build a complete axisymmetric MHD solution for the solar wind at minimum including rotation and the initial flaring of the solar wind in the low corona. This model has some problems in reproducing the values of magnetic field at 1 AU despite the correct values of the velocity. Here, we intend to extend the previous analysis to another type of solutions and to improve our modelling of the wind from the solar surface to 1 AU. Methods. We compare the previous results to those obtained with a fully helicoidal model and construct a hybrid model combining both previous solutions, keeping the flexibility of the parent models in the appropriate domain. From the solar surface to the Alfven point, a three component solution for velocity and magnetic field is used, reproducing the complex wind geometry and the well-known flaring of the field lines observed in coronal holes. From the Alfven radius to I AU and further, the hybrid model keeps the latitudinal dependences as flexible as possible, in order to deal with the sharp variations near the equator and we use the helicoidal solution, turning the poloidal streamlines into radial ones. Results. Despite the absence of the initial flaring, the helicoidal model and the first hybrid solution suffer from the same low values of the magnetic field at 1 AU. However, by adjusting the parameters with a second hybrid solution, we are able to reproduce both the velocity and magnetic profiles observed by ULYSSES and a reasonable description of the low corona, provided that a certain amount of energy deposit exists along the flow. Conclusions. The present paper shows that analytical axisymmetric solutions can be constructed to reproduce the solar structure and dynamics from 1 solar radius up to 1 AU.
J. M. FERREIRA, A. AIBÉO, and J. LIMA
WORLD SCIENTIFIC
C. Sauty, J. J. G. Lima, N. Iro, and K. Tsinganos
EDP Sciences
Exact axisymmetric analytical solutions of the governing MHD equations for magnetized and rotating outflows are applied to the solar wind during solar minimum as observed by ULYSSES. Using the spacecraft data, the latitudinal dependences of physical quantities such as the density, velocity, magnetic field and temperature are analytically described. The self-similar solutions are then compared to the global structure of the wind from one solar radius to 5 AU and beyond, including consistently the rotation of the outflow. The model makes it possible to describe the initial flaring of the magnetic dipolar structure, repro- ducing in a satisfactory way the observed profiles of the velocity, density and temperature with heliocentric distance. Finally, this model is in agreement with the conjecture that the solar wind should not be collimated at large distances, even close to its rotational axis.
K. Tsinganos, N. Vlahakis, S. Bogovalov, C. Sauty, E. Trussoni, and J.J.G. Lima
Springer Science and Business Media LLC
L.J.R. Machado, J.J.G. Lima, and M.T.V.T. Lago
Springer Science and Business Media LLC
J. J. G. Lima, E. R. Priest, and K. Tsinganos
EDP Sciences
A new class of analytical 2-D solutions of the full set of the steady magnetohydrodynamic (MHD) equations, describing an axisymmetric helicoidal magnetized outflow originating from a rotating central object, is presented. The solutions are systematically obtained via a nonlinear separation of the variables in the momentum equation. The analysis yields three parameters which measure the anisotropy in the latitudinal distribution of various flow quantities. Topologically, the wind speed is controlled by an X-type critical point that acts to lter out a single wind-type branch and the Alfv en singularity. The solutions can be regarded as an extension outside the equatorial plane of the Weber & Davis (1967) model of magnetized winds but with a variable polytropic index.