Anomalous Z Cam stars: a response to mass-transfer outbursts

Recent observations of two unusual Z Cam systems, V513 Cas and IW And have shown light curves that seem to contradict the disc-instability model for dwarf novae: outbursts are appearing during standstills of the system when according to the model, the disc is supposed to be in a hot quasi-equilibrium state. We investigate what additional physical processes need to be included in the model to reconcile it with observations of such anomalous Z Cam systems. We used our code for modeling thermal-viscous outbursts of the accretion discs and determined what types of mass-transfer variations reproduce the observed light curves. Outbursts of mass transfer (with a duration of a few days, with a short rise time and an exponential decay) from the stellar companion will account for the observed properties of V513 Cas and IW And, provided they are followed by a short but significant mass-transfer dip. The total mass involved in outbursts is of the order of 10$^{23}$g. We studied the possible origins of these mass transfer outbursts and showed that they most probably result from a giant flare near the secondary star surface, possibly due to the absence of star spots in the $L_1$ region.Comment: 5 pages, 2 figures. Misprints corrected, in particular "inside-out" should have read "outside-in

The disappearance and reformation of the accretion disc during a low state of FO Aquarii

FO Aquarii, an asynchronous magnetic cataclysmic variable (intermediate polar) went into a low-state in 2016, from which it slowly and steadily recovered without showing dwarf nova outbursts. This requires explanation since in a low-state, the mass-transfer rate is in principle too low for the disc to be fully ionized and the disc should be subject to the standard thermal and viscous instability observed in dwarf novae. We investigate the conditions under which an accretion disc in an intermediate polar could exhibit a luminosity drop of 2 magnitudes in the optical band without showing outbursts. We use our numerical code for the time evolution of accretion discs, including other light sources from the system (primary, secondary, hot spot). We show that although it is marginally possible for the accretion disc in the low-state to stay on the hot stable branch, the required mass-transfer rate in the normal state would then have to be extremely high, of the order of 10$^{19}$ gs$^{-1}$ or even larger. This would make the system so intrinsically bright that its distance should be much larger than allowed by all estimates. We show that observations of FO Aqr are well accounted for by the same mechanism that we have suggested as explaining the absence of outbursts during low states of VY Scl stars: during the decay, the magnetospheric radius exceeds the circularization radius, so that the disc disappears before it enters the instability strip for dwarf nova outbursts. Our results are unaffected, and even reinforced, if accretion proceeds both via the accretion disc and directly via the stream during some intermediate stages; the detailed process through which the disc disappears still needs investigations.Comment: 6 pages, 5 figures. Accepted for publication in A&

Design and Analysis of a Task-based Parallelization over a Runtime System of an Explicit Finite-Volume CFD Code with Adaptive Time Stepping

FLUSEPA (Registered trademark in France No. 134009261) is an advanced simulation tool which performs a large panel of aerodynamic studies. It is the unstructured finite-volume solver developed by Airbus Safran Launchers company to calculate compressible, multidimensional, unsteady, viscous and reactive flows around bodies in relative motion. The time integration in FLUSEPA is done using an explicit temporal adaptive method. The current production version of the code is based on MPI and OpenMP. This implementation leads to important synchronizations that must be reduced. To tackle this problem, we present the study of a task-based parallelization of the aerodynamic solver of FLUSEPA using the runtime system StarPU and combining up to three levels of parallelism. We validate our solution by the simulation (using a finite-volume mesh with 80 million cells) of a take-off blast wave propagation for Ariane 5 launcher.Comment: Accepted manuscript of a paper in Journal of Computational Scienc

The zoo of dwarf novae : illumination, evaporation and disc radius variation

In the framework of the dwarf nova thermal-viscous disc instability model, we investigate the combined effects on the predicted dwarf nova lightcurves of irradiating the accretion disc and the secondary star and of evaporating the inner parts of the disc. We assume the standard values of viscosity. We confirm the suggestion by Warner (1998) that the large variety of observed outbursts' behaviour may result from the interplay of these three effects. We are able to reproduce light curves reminiscent of those of systems such as RZ LMi or EG Cnc. We can obtain long lasting outbursts, very similar to superoutbursts, without assuming the presence of a tidal instability.Comment: 10 pages, 7 figures; accepted for publication in Astronomy and Astrophysic

Sustainable rural livelihoods to analyse family farming dynamics: A comparative perspective

The very nature of family farming makes it a complex scientific subject, being at the same time a social form of production and an economic agent. Its nature challenges disciplines that most of the time overlook dimensions that do not fit in with their own framework leading to partial views in anthropology, sociology, political science or economics, just to mention the most common disciplinary focus on rural societies. We suggest exploring the well-known Sustainable Rural Livelihood framework as a comprehensive and open conceptual design to address the evolution of family farming. While the entry point concerns individuals, it also considers the social structures and institutions in which they are embedded. It also contemplates natural, social and human assets in addition to physical and financial ones. The activity system developed by each individual within its social setting goes beyond sectorial approaches; the strategies developed are contextualized and influenced by policies. To illustrate how this framework can be implemented, we developed a case study approach in contrasting rural contexts ranging from Argentina, Brazil or Nicaragua for Latin American situations, to Indonesia, China or India for Asia, or to Mali, Cameroon or Mozambique for African illustrations. The cases will not be extensively presented here as we choose to highlight some of the main findings and crosscutting themes as ways and means of adapting to changing contexts. We also discuss the challenges and perspectives faced by family farming from other forms of production and provide some insight into "blind" issues: the social drawbacks and political dimensions linked to agriculture related to broader territorial and national concerns