J. J. G. Lima

An analytical MHD wind model with latitudinal dependences obtained using separation of the variables

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.

COUNTERROTATION IN MAGNETOCENTRIFUGALLY DRIVEN JETS AND OTHER WINDS

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 Alfven 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.

Nonradial and nonpolytropic astrophysical outflows - IX. Modeling T Tauri jets with a low mass-accretion rate

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.

The heliolatitudinal gradient of the solar wind during solar minimum conditions modelled by exact hydrodynamic solutions

The heliolatitudinal dependence of observations of the solar wind macroscopic quantities such as the averaged proton speed and density is modelled during solar minimum conditions when the rotational and magnetic axes roughly coincide. Published observations via the technique of interplanetary scintillations for the previous two solar cycles were used, as well as recent data from the plasma experiment aboard the ULYSSES spacecraft, which also refer to the declining phase of the present solar cycle. A class of exact, two-dimensional solutions of the full set of steady HD equations is used which is obtained analytically through a nonlinear separation of the variables. The three parameters which emerge in these solutions are fixed from such observations, as well as from observations of solar rotation. The solutions are consistent with observational inferrences that during solar minimum and the declining phase of the solar activity cycle, there is a strong heliolatitudinal gradient in rotation averaged proton speed between about 400–800 km s−1 from equator to pole. This modelling also agrees with previous findings that the gradient in wind speed with the latitude is offset by a gradient in density such that the mass and momentum flux vary relatively little.

Polar stellar-spots and grazing planetary transits - Possible explanation for the low number of discovered grazing planets

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.

Magnetic braking in young late-type stars The effect of polar spots

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.

Application of a MHD hybrid solar wind model with latitudinal dependences to ULYSSES data at minimum

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.

Nonradial and nonpolytropic astrophysical outflows - VII. Fitting ULYSSES solar wind data during minimum

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.

COUNTER-ROTATION IN RELATIVISTIC MAGNETOHYDRODYNAMIC JETS

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.

Modelling of T Tauri jets with low mass accretion rate

We 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.