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

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

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

Collaboratory Digital Libraries for Humanities in the Italian context

The study investigates the approach to collaboration in Humanities, within the Italian context, to test the possibility of collaborative digital library for scholars. The research hypothesis is that collaboration can foster innovation and scienti c development: therefore, within Humanities, digital libraries can be the collaborative laboratory for research. Thus, understanding perception of scholars towards collaboration, especially online, and comprehending if wiki systems could be the framework of collaboration were the objectives of the study. A qualitative approach has been adopted, using case study as research method: in-depth, semi-structured interviews to Digital Humanities scholars provide data integrated with interviews with two key informants (one of which is prof. Umberto Eco). The results of the study show that Humanities, within Italian context, do appreciate collaboration and the concept of a collaboratory digital library, though several issues need to be solved. In fact, Humanities are still tied with individual work and collaboration is not easy to pursue, for cultural, technical and political reasons. Great e ort needs to be done at many different levels to eliminate obstacles and facilitate online collaboration for scholars. The study provides a draft model for a collaborative digital library arisen from gathered data

Synchronized-tracing of implicit surfaces

Implicit surfaces are known for their ability to represent smooth objects of arbitrary topology thanks to hierarchical combinations of primitives using a structure called a blobtree. We present a new tile-based rendering pipeline well suited for modeling scenarios, i.e., no preprocessing is required when primitive parameters are updated. When using approximate signed distance fields, we rely on compact, smooth CSG operators - extended from standard bounded operators - to compute a tight volume of interest for all primitives of the blobtree. The pipeline relies on a low-resolution A-buffer storing the primitives of interest of a given screen tile. The A-buffer is then used during ray processing to synchronize threads within a subfrustum. This allows coherent field evaluation within workgroups. We use a sparse bottom-up tree traversal to prune the blobtree on-the-fly which allows us to decorrelate field evaluation complexity from the full blobtree size. The ray processing itself is done using the sphere-tracing algorithm. The pipeline scales well to surfaces consisting of thousands of primitives