Numerical simulations of mass loading in the tails of Bow Shock Pulsar Wind Nebulae

When a pulsar is moving through a partially ionized medium, a fraction of neutral Hydrogen atoms penetrate inside the pulsar wind and can be photo-ionized by the nebula UV radiation. The resulting protons remains attached to the magnetic field of the light leptonic pulsar wind enhancing its inertia and changing the flow dynamics of the wind. We present here the first numerical simulations of such effect in the tails of bow shock nebulae. We produce a set of different models representative of pulsars moving in the interstellar medium with different velocities, from highly subsonic to supersonic, by means of 2D hydrodynamic relativistic simulations. We compare the different tail morphologies with results from theoretical models of mass loading in bow shocks. As predicted by analytical models we observe a fast sideways expansion of the tail with the formation of secondary shocks in the ISM. This effect could be at the origin of the head-and-shoulder morphology observed in many BSPWNe.Comment: 7 pages, 5 figures, 1 tabl

The long-term evolution of photoevaporating transition discs with giant planets

Photo-evaporation and planet formation have both been proposed as mechanisms responsible for the creation of a transition disc. We have studied their combined effect through a suite of 2d simulations of protoplanetary discs undergoing X-ray photoevaporation with an embedded giant planet. In a previous work we explored how the formation of a giant planet triggers the dispersal of the inner disc by photo-evaporation at earlier times than what would have happened otherwise. This is particularly relevant for the observed transition discs with large holes and high mass accretion rates that cannot be explained by photo-evaporation alone. In this work we significantly expand the parameter space investigated by previous simulations. In addition, the updated model includes thermal sweeping, needed for studying the complete dispersal of the disc. After the removal of the inner disc the disc is a non accreting transition disc, an object that is rarely seen in observations. We assess the relative length of this phase, to understand if it is long lived enough to be found observationally. Depending on the parameters, especially on the X-ray luminosity of the star, we find that the fraction of time spent as a non-accretor greatly varies. We build a population synthesis model to compare with observations and find that in general thermal sweeping is not effective enough to destroy the outer disc, leaving many transition discs in a relatively long lived phase with a gas free hole, at odds with observations. We discuss the implications for transition disc evolution. In particular, we highlight the current lack of explanation for the missing non-accreting transition discs with large holes, which is a serious issue in the planet hypothesis.Comment: 11 pages, 5 figures; accepted by MNRA

Chained activation of the motor system during language understanding

Two experiments were carried out to investigate whether and how one important characteristic of the motor system, that is its goal-directed organization in motor chains, is reflected in language processing. This possibility stems from the embodied theory of language, according to which the linguistic system re-uses the structures of the motor system. The participants were presented with nouns of common tools preceded by a pair of verbs expressing grasping or observational motor chains (i.e., grasp-to-move, grasp-to-use, look-at-to-grasp, and look-at-to-stare). They decided whether the tool mentioned in the sentence was the same as that displayed in a picture presented shortly after. A primacy of the grasp-to-use motor chain over the other motor chains in priming the participants' performance was observed in both the experiments. More interestingly, we found that the motor information evoked by the noun was modulated by the specific motor-chain expressed by the preceding verbs. Specifically, with the grasping chain aimed at using the tool, the functional motor information prevailed over the volumetric information, and vice versa with the grasping chain aimed at moving the tool (Experiment 2). Instead, the functional and volumetric information were balanced for those motor chains that comprise at least an observational act (Experiment 1). Overall our results are in keeping with the embodied theory of language and suggest that understanding sentences expressing an action directed toward a tool drives a chained activation of the motor system

The interplay between X-ray photoevaporation and planet formation

We assess the potential of planet formation instigating the early formation of a photoevaporation driven gap, up to radii larger than typical for photoevaporation alone. For our investigation we make use of hydrodynamics models of photoevaporating discs with a giant planet embedded. We find that, by reducing the mass accretion flow onto the star, discs that form giant planets will be dispersed at earlier times than discs without planets by X-ray photoevaporation. By clearing the portion of the disc inner of the planet orbital radius, planet formation induced photoevaporation (PIPE) is able to produce transition disc that for a given mass accretion rate have larger holes when compared to standard X-ray photoevaporation. This constitutes a possible route for the formation of the observed class of accreting transition discs with large holes, which are otherwise difficult to explain by planet formation or photoevaporation alone. Moreover, assuming that a planet is able to filter dust completely, PIPE produces a transition disc with a large hole and may provide a mechanism to quickly shut down accretion. This process appears to be too slow however to explain the observed desert in the population of transition disc with large holes and low mass accretion rates.Comment: 11 pages, 10 figures, accepted by MNRAS on 31/12/201