MHD simulations of accretion onto a dipolar magnetosphere. II. Magnetospheric ejections and stellar spin-down

This paper examines the outflows associated with the interaction of a stellar magnetosphere with an accretion disk. In particular, we investigate the magnetospheric ejections (MEs) due to the expansion and reconnection of the field lines connecting the star with the disk. Our aim is to study the dynamical properties of the outflows and evaluate their impact on the angular momentum evolution of young protostars. Our models are based on axisymmetric time-dependent magneto-hydrodynamic simulations of the interaction of the dipolar magnetosphere of a rotating protostar with a viscous and resistive disk, using alpha prescriptions for the transport coefficients. Our simulations are designed in order to model: the accretion process and the formation of accretion funnels; the periodic inflation/reconnection of the magnetosphere and the associated MEs; the stellar wind. Similarly to a magnetic slingshot, MEs can be powered by the rotation of both the disk and the star so that they can efficiently remove angular momentum from both. Depending on the accretion rate, MEs can extract a relevant fraction of the accretion torque and, together with a weak but non-negligible stellar wind torque, can balance the spin-up due to accretion. When the disk truncation approaches the corotation radius, the system enters a "propeller" regime, where the torques exerted by the disk and the MEs can even balance the spin-up due to the stellar contraction. The MEs spin-down efficiency can be compared to other scenarios, such as the Ghosh & Lamb, X-wind or stellar wind models. Nevertheless, for all scenarios, an efficient spin-down torque requires a rather strong dipolar component, which has been seldom observed in classical T Tauri stars. A better analysis of the torques acting on the protostar must take into account non-axisymmetric and multipolar magnetic components consistent with observations.Comment: 21 pages, 16 figures, accepted for publication in Astronomy & Astrophysic

A new steplength selection for scaled gradient methods with application to image deblurring

Gradient methods are frequently used in large scale image deblurring problems since they avoid the onerous computation of the Hessian matrix of the objective function. Second order information is typically sought by a clever choice of the steplength parameter defining the descent direction, as in the case of the well-known Barzilai and Borwein rules. In a recent paper, a strategy for the steplength selection approximating the inverse of some eigenvalues of the Hessian matrix has been proposed for gradient methods applied to unconstrained minimization problems. In the quadratic case, this approach is based on a Lanczos process applied every m iterations to the matrix of the most recent m back gradients but the idea can be extended to a general objective function. In this paper we extend this rule to the case of scaled gradient projection methods applied to non-negatively constrained minimization problems, and we test the effectiveness of the proposed strategy in image deblurring problems in both the presence and the absence of an explicit edge-preserving regularization term

Large scale magnetic fields in viscous resistive accretion disks. I. Ejection from weakly magnetized disks

Cold steady-state disk wind theory from near Keplerian accretion disks requires a large scale magnetic field at near equipartition strength. However the minimum magnetization has never been tested. We investigate the time evolution of an accretion disk threaded by a weak vertical magnetic field. The strength of the field is such that the disk magnetization falls off rapidly with radius. Four 2.5D numerical simulations of viscous resistive accretion disk are performed using the magnetohydrodynamic code PLUTO. In these simulations, a mean field approach is used and turbulence is assumed to give rise to anomalous transport coefficients (alpha prescription). The large scale magnetic field introduces only a small perturbation to the disk structure, with accretion driven by the dominant viscous torque. A super fast magnetosonic jet is observed to be launched from the innermost regions and remains stationary over more than 953 Keplerian orbits. The self-confined jet is launched from a finite radial zone in the disk which remains constant over time. Ejection is made possible because the magnetization reaches unity at the disk surface, due to the steep density decrease. However, no ejection is reported when the midplane magnetization becomes too small. The asymptotic jet velocity remains nevertheless too low to explain observed jets due to the negligible power carried away by the jet. Astrophysical disks with superheated surface layers could drive analogous outflows even if their midplane magnetization is low. Sufficient angular momentum would be extracted by the turbulent viscosity to allow the accretion process to continue. The magnetized outflows would be no more than byproducts, rather than a fundamental driver of accretion. However, if the midplane magnetization increases towards the center, a natural transition to an inner jet dominated disk could be achieved.Comment: Accepted by Astronomy and Astrophysic

Una breve presentazione

Si tratta della edizione aggiornata della guida rapida alla pinacoteca dell'ateneo di Triest

La vecchia Pescheria di Trieste e la mostra di Kounellis

An exhibition by Jannis Kounellis was organized in Trieste (September, 7th 2013 – February, 2nd 2014) in a building designed and constructed by the architect Giorgio Polli (Trieste, 1862-1942). The building was opened as a Fish-market in 1913, beside the sea and in front of the Neoclassical buildings of the town, but now it is used solely for exhibition purposes. An example of industrial archaeology, the interior has become in Kounellis’s installation an integral part of a sacred epic of the Sea. The artist shows that the value of the building can increase through its technological aspects, the specific relation between its interior and exterior, and also its quality of space and light, and become a sort of “eco-museum”, an expression of the history of Trieste and of that relationship with the sea that determines its cultural identity

Angular momentum evolution of young low-mass stars and brown dwarfs: observations and theory

This chapter aims at providing the most complete review of both the emerging concepts and the latest observational results regarding the angular momentum evolution of young low-mass stars and brown dwarfs. In the time since Protostars & Planets V, there have been major developments in the availability of rotation period measurements at multiple ages and in different star-forming environments that are essential for testing theory. In parallel, substantial theoretical developments have been carried out in the last few years, including the physics of the star-disk interaction, numerical simulations of stellar winds, and the investigation of angular momentum transport processes in stellar interiors. This chapter reviews both the recent observational and theoretical advances that prompted the development of renewed angular momentum evolution models for cool stars and brown dwarfs. While the main observational trends of the rotational history of low mass objects seem to be accounted for by these new models, a number of critical open issues remain that are outlined in this review.Comment: 22 pages, 8 figures, accepted for publication in Protostars & Planets VI, 2014, University of Arizona Press, eds. H. Beuther, R. Klessen, K. Dullemond, Th. Hennin

On the filtering effect of iterative regularization algorithms for linear least-squares problems

Many real-world applications are addressed through a linear least-squares problem formulation, whose solution is calculated by means of an iterative approach. A huge amount of studies has been carried out in the optimization field to provide the fastest methods for the reconstruction of the solution, involving choices of adaptive parameters and scaling matrices. However, in presence of an ill-conditioned model and real data, the need of a regularized solution instead of the least-squares one changed the point of view in favour of iterative algorithms able to combine a fast execution with a stable behaviour with respect to the restoration error. In this paper we want to analyze some classical and recent gradient approaches for the linear least-squares problem by looking at their way of filtering the singular values, showing in particular the effects of scaling matrices and non-negative constraints in recovering the correct filters of the solution

Economic Effects of Legislative Framework Changes in Groundwater Use Rights for Irrigation

In most countries, groundwater resource is a public good, and the entitlement of use rights by the public authority to final users differs according to a country-specific legislative framework. In Italy, groundwater extraction has been regulated through non-tradable private licenses. At present, the public authority needs to reform the current legislative framework, in order to comply with the Water Framework Directive, aimed at the enhancement of the efficiency of the resource use. This research analyzes the effects of reforming the current framework based on non-tradable use rights, by comparing two different liberalization scenarios: an intra-sector market, and a regional market. Although positive economic benefits are generally expected from the liberalization of use rights at aggregated level, we want to analyze whether effects of the legislative framework causes uneven changes on some farm groups. The empirical case study refers to the Fortore river basin (South of Italy), where groundwater covers about 50–80% of current needs, and informal (though illegal) water markets across neighbor farmers already exist. From the findings, there is no evidence that the exchange liberalization of groundwater use rights leads to gains in terms of the value added and the farmer’s revenue. In addition, in the case of an auction system regulated by the public authority, farmers whose water productivity is higher may be able to gain, while others may suffer some losses. In this case, resistances from farmers’ associations towards the legislative framework reform may aris