A 3D MHD model of astrophysical flows: algorithms, tests and parallelisation

In this paper we describe a numerical method designed for modelling different kinds of astrophysical flows in three dimensions. Our method is a standard explicit finite difference method employing the local shearing-box technique. To model the features of astrophysical systems, which are usually compressible, magnetised and turbulent, it is desirable to have high spatial resolution and large domain size to model as many features as possible, on various scales, within a particular system. In addition, the time-scales involved are usually wide-ranging also requiring significant amounts of CPU time. These two limits (resolution and time-scales) enforce huge limits on computational capabilities. The model we have developed therefore uses parallel algorithms to increase the performance of standard serial methods. The aim of this paper is to report the numerical methods we use and the techniques invoked for parallelising the code. The justification of these methods is given by the extensive tests presented herein.Comment: 17 pages with 21 GIF figures. Accepted for publication in A&

Modeling and comparison of candidate selection algorithms in opportunistic routing

Opportunistic Routing (OR) has been investigated in recent years as a way to increase the performance of multihop wireless networks by exploiting its broadcast nature. In contrast to traditional routing, where traffic is sent along pre-determined paths, in OR an ordered set of candidates is selected for each next-hop. Upon each transmission, the candidates coordinate such that the most priority one receiving the packet actually forwards it. Most of the research in OR has been addressed to investigate candidate selection algorithms. In this paper we propose a discrete time Markov chain to assess the improvement that may be achieved using opportunistic routing. We use our model to compare a selected group of candidate selection algorithms that have been proposed in the literature. Our main conclusion is that optimality is obtained at a high computational cost, with a performance gain very similar to that of much simpler but non-optimal algorithms. Therefore, we conclude that fast and simple OR candidate selection algorithms may be preferable in dynamic networks, where the candidates sets are likely to be updated frequently. © 2011 Elsevier B.V. All rights reserved.This work was supported by the Spanish government through projects TIN2010-21378-C02-01 and TIN2010-21378-C02-02, by the Generalitat de Catalunya through project 2009-SGR-1167, and by the European Commission through the NoE EuroNF.Darehshoorzadeh, A.; Cerdà-Alabern, L.; Pla, V. (2011). Modeling and comparison of candidate selection algorithms in opportunistic routing. Computer Networks. 55(13):2886-2898. https://doi.org/10.1016/j.comnet.2011.06.009S28862898551

Modeling the X-rays Resulting from High Velocity Clouds

With the goal of understanding why X-rays have been reported near some high velocity clouds, we perform detailed 3 dimensional hydrodynamic and magnetohydrodynamic simulations of clouds interacting with environmental gas like that in the Galaxy's thick disk/halo or the Magellanic Stream. We examine 2 scenarios. In the first, clouds travel fast enough to shock-heat warm environmental gas. In this scenario, the X-ray productivity depends strongly on the speed of the cloud and the radiative cooling rate. In order to shock-heat environmental gas to temperatures of > or = 10^6 K, cloud speeds of > or = 300 km/s are required. If cooling is quenched, then the shock-heated ambient gas is X-ray emissive, producing bright X-rays in the 1/4 keV band and some X-rays in the 3/4 keV band due to O VII and other ions. If, in contrast, the radiative cooling rate is similar to that of collisional ionizational equilibrium plasma with solar abundances, then the shocked gas is only mildly bright and for only about 1 Myr. The predicted count rates for the non-radiative case are bright enough to explain the count rate observed with XMM-Newton toward a Magellanic Stream cloud and some enhancement in the ROSAT 1/4 keV count rate toward Complex C, while the predicted count rates for the fully radiative case are not. In the second scenario, the clouds travel through and mix with hot ambient gas. The mixed zone can contain hot gas, but the hot portion of the mixed gas is not as bright as those from the shock-heating scenario.Comment: 15 pages, 9 figures, 1 table. Accepted for publication in the Astrophysical Journa

Visibility of Old Supernova Remnants in HI 21-cm Emission Line

We estimate the number of old, radiative supernova remnants (SNRs) detectable in HI 21-cm emission line in the Galaxy. According to our result, the contamination due to the Galactic background HI emission limits the number of visible SNRs to 270, or 9% of the total HI SNRs. The telescope sensitivity further limits the number. We compare the result with observations to find that the observed number (< 25) of HI SNRs is much less than the expected. A plausible explanation is that previous observational studies, which were made toward the SNRs identified mostly in radio continuum, missed most of the HI SNRs because they are too faint to be visible in radio continuum. We propose that the faint, extended HI 21-cm emission line wings protruding from the Galactic background HI emission in large-scale (l, v) diagrams could be possible candidates for HI SNRs, although our preliminary result shows that their number is considerably less than the expected in the inner Galaxy. We conclude that a possible explanation for the small number of \schi SNRs in the inner Galaxy is that the interstellar space there is largely filled with a very tenuous gas as in the three-phase ISM model.Comment: 10 pages, 9 figures, accepted for publication in MNRA

Optimum Node Placement in Wireless Opportunistic Routing Networks

In recent years there has been a growing interest in opportunistic routing as a way to increase the capacity of wireless networks by exploiting its broadcast nature. In contrast to traditional uni-path routing, in opportunistic routing the nodes overhearing neighbor¿s transmissions can become candidates to forward the packets towards the destination. In this paper we address the question: What is the maximum performance that can be obtained using opportunistic routing? To answer this question we use an analytical model that allows to compute the optimal position of the nodes, such that the progress towards the destination is maximized. We use this model to compute bounds to the minimum expected number of transmissions that can be achieved in a network using opportunistic routing.This work was supported by the Spanish Ministerio de Ciencia e Innovacion through the Projects TIN2010-21378-C02-01 and TIN2010-21378-C02-02 and by the Generalitat de Catalunya through Project 2009-SGR-1167.Cerdá Alabern, L.; Darehshoorzadeh, A.; Pla, V. (2013). Optimum Node Placement in Wireless Opportunistic Routing Networks. Ad Hoc Networks. 11(8):2273-2287. https://doi.org/10.1016/j.adhoc.2013.05.010S2273228711

Opportunistic routing in wireless mesh networks

Opportunistic Routing (OR) has been proposed as a way to increase the performance of wireless networks by exploiting its broadcast nature. In OR, instead of pre-selecting a single specific node to be the next-hop as a forwarder for a packet, multiple nodes can potentially be selected as the next-hop forwarder. Thus the source can use multiple potential paths to deliver the packets to the destination. More specially, when the current node transmits a packet, all the candidates that receive the packet successfully will coordinate with each other to determine which one would actually forward the packet according to some criteria, while the other nodes will simply discard the packet. In this chapter, we survey the state of the art in OR, then focus on the candidates selection algorithms and carry out a comparative performance evaluation of the most relevant proposals appeared in the literature.Peer ReviewedPostprint (author’s final draft

Catalogue of far-infrared loops in the Galaxy

Aims: An all-sky survey of loop- and arc-like intensity enhancements has been performed in order to investigate the large-scale structure of the diffuse far-infrared emission. Methods: We used maps made of 60 and 100 micrometer processed IRAS data (Sky Survey Atlas and dust infrared emission maps) to identify large-scale structures: loops, arcs or cavities, in the far-infrared emission in the Galaxy. Distances were attributed to a subsample of loops using associated objects. Results: We identified 462 far-infrared loops, analyzed their individual FIR properties and their distribution. This data forms the Catalogue of Far-Infrared Loops in the Galaxy. We obtained observational estimates of f_in~30% and f_out~5% for the hot gas volume filling factor of the inward and outward Galactic neighbourhood of the Solar System. We obtained a slope of the power law size luminosity function {beta}=1.37 for low Galactic latitudes in the outer Milky Way. Conclusions: Deviations in the celestial distribution of far-infrared loops clearly indicate, that violent events frequently overwrite the structure of the interstellar matter in the inner Galaxy. Our objects trace out the spiral arm structure of the Galaxy in the neighbourhood of the Sun and their distribution clearly suggests that there is an efficient process that can generate loop-like features at high Galactic latitudes. Power law indices of size luminosity distributions suggest, that the structure of the ISM is ruled by supernovae and stellar winds at low Galactic latitudes while it is governed by supersonic turbulence above the Galactic plane.Comment: 11 pages - 10 figures (together with Appendix A), accepted for publication in Astronomy & Astrophysics. Appendix B, C and D are available at http://kisag.konkoly.hu/CFIRLG

The age, origin and emplacement of the Tsiknias Ophiolite, Tinos, Greece

The Tsiknias Ophiolite, exposed at the highest structural levels of Tinos, Greece, represents a thrust sheet of Tethyan oceanic crust and upper mantle emplaced onto the Attic‐Cycladic Massif. We present new field observations and a new geological map of Tinos, integrated with petrology, THERMOCALC phase diagram modelling, U–Pb geochronology and whole rock geochemistry, resulting in a tectono‐thermal model that describes the formation and emplacement of the Tsiknias Ophiolite and newly identified underlying metamorphic sole. The ophiolite comprises a succession of partially dismembered and structurally repeated ultramafic and gabbroic rocks that represent the Moho Transition Zone. A plagiogranite dated by U‐Pb zircon at 161.9 ± 2.8 Ma, reveals that the Tsiknias Ophiolite formed in a supra‐subduction zone setting, comparable to the “East‐Vardar Ophiolites”, and was intruded by gabbros at 144.4 ± 5.6 Ma. Strongly sheared metamorphic sole rocks show a condensed and inverted metamorphic gradient, from partially anatectic amphibolites at P–T conditions of ca. 8.5 kbar 850‐600 °C, down‐structural section to greenschist‐facies oceanic metasediments over ~250 m. Leucosomes generated by partial melting of the uppermost sole amphibolite, yielded a U–Pb zircon protolith age of ca. 190 Ma and a high‐grade metamorphic‐anatectic age of 74.0 ± 3.5 Ma associated with ophiolite emplacement. The Tsiknias Ophiolite was therefore obducted ~90 Myrs after it formed during initiation of a NE‐dipping intra‐oceanic subduction zone to the northeast of the Cyclades that coincides with Africa's plate motion changing from transcurrent to convergent. Continued subduction resulted in high‐pressure metamorphism of the Cycladic continental margin ~25 Myrs later

Register Allocation After Classical SSA Elimination is NP-Complete

Abstract. Chaitin proved that register allocation is equivalent to graph coloring and hence NP-complete. Recently, Bouchez, Brisk, and Hack have proved independently that the interference graph of a program in static single assignment (SSA) form is chordal and therefore colorable in linear time. Can we use the result of Bouchez et al. to do register allocation in polynomial time by first transforming the program to SSA form, then performing register allocation, and finally doing the classical SSA elimination that replaces φ-functions with copy instructions? In this paper we show that the answer is no, unless P = NP: register allocation after classical SSA elimination is NP-complete. Chaitin’s proof technique does not work for programs after classical SSA elimination; instead we use a reduction from the graph coloring problem for circular arc graphs.