Non-axisymmetric relativistic Bondi-Hoyle accretion onto a Schwarzschild black hole

We present the results of an exhaustive numerical study of fully relativistic non-axisymmetric Bondi-Hoyle accretion onto a moving Schwarzschild black hole. We have solved the equations of general relativistic hydrodynamics with a high-resolution shock-capturing numerical scheme based on a linearized Riemann solver. The numerical code was previously used to study axisymmetric flow configurations past a Schwarzschild hole. We have analyzed and discussed the flow morphology for a sample of asymptotically high Mach number models. The results of this work reveal that initially asymptotic uniform flows always accrete onto the hole in a stationary way which closely resembles the previous axisymmetric patterns. This is in contrast with some Newtonian numerical studies where violent flip-flop instabilities were found. As discussed in the text, the reason can be found in the initial conditions used in the relativistic regime, as they can not exactly duplicate the previous Newtonian setups where the instability appeared. The dependence of the final solution with the inner boundary condition as well as with the grid resolution has also been studied. Finally, we have computed the accretion rates of mass and linear and angular momentum.Comment: 21 pages, 13 figures, Latex, MNRAS (in press

Algebras and non-geometric flux vacua

In this work we classify the subalgebras satisfied by non-geometric Q-fluxes in type IIB orientifolds on T^6/(Z_2 x Z_2) with three moduli (S,T,U). We find that there are five subalgebras compatible with the symmetries, each one leading to a characteristic flux-induced superpotential. Working in the 4-dimensional effective supergravity we obtain families of supersymmetric AdS_4 vacua with all moduli stabilized at small string coupling g_s. Our results are mostly analytic thanks to a judicious parametrization of the non-geometric, RR and NSNS fluxes. We are also able to leave the flux-induced C_4 and C_8 RR tadpoles as free variables, thereby enabling us to study which values are allowed for each Q-subalgebra. Another novel outcome is the appearance of multiple vacua for special sets of fluxes. However, they generically have g_s > 1 unless the net number of O3/D3 or O7/D7 sources needed to cancel the tadpoles is large. We also discuss briefly the issues of axionic shift symmetries and cancellation of Freed-Witten anomalies.Comment: 61 pages, LaTex, v2: added reference

Robustness of a high-resolution central scheme for hydrodynamic simulations in full general relativity

A recent paper by Lucas-Serrano et al. indicates that a high-resolution central (HRC) scheme is robust enough to yield accurate hydrodynamical simulations of special relativistic flows in the presence of ultrarelativistic speeds and strong shock waves. In this paper we apply this scheme in full general relativity (involving {\it dynamical} spacetimes), and assess its suitability by performing test simulations for oscillations of rapidly rotating neutron stars and merger of binary neutron stars. It is demonstrated that this HRC scheme can yield results as accurate as those by the so-called high-resolution shock-capturing (HRSC) schemes based upon Riemann solvers. Furthermore, the adopted HRC scheme has increased computational efficiency as it avoids the costly solution of Riemann problems and has practical advantages in the modeling of neutron star spacetimes. Namely, it allows simulations with stiff equations of state by successfully dealing with very low-density unphysical atmospheres. These facts not only suggest that such a HRC scheme may be a desirable tool for hydrodynamical simulations in general relativity, but also open the possibility to perform accurate magnetohydrodynamical simulations in curved dynamic spacetimes.Comment: 4 pages, to be published in Phys. Rev. D (brief report

D=6, N=1 String Vacua and Duality

We review the structure $D=6, N=1$ string vacua with emphasis on the different connections due to T-dualities and S-dualities. The topics discussed include: Anomaly cancellation; K3 and orbifold $D=6, N=1$ heterotic compactifications; T-dualities between $E_8\times E_8$ and $Spin(32)/Z_2$ heterotic vacua; non-perturbative heterotic vacua and small instantons; N=2 Type-II/Heterotic duality in D=4 ; F-theory/heterotic duality in D=6; and heterotic/heterotic duality in six and four dimensions.Comment: 52 pages, plain Latex. To appear in the proceedings of the APCTP Winter School on Duality, Mt. Sorak (Korea), February 199

Normative influences: how socio-cultural and industrial norms influence the adoption of sustainability practices. A grounded theory of Cretan, small tourism firms

Previous research explains the various factors that motivate or discourage the owner-managers of small firms to behave sustainably. However, it has failed to develop a meaningful understanding of how these factors inter-relate or combine to influence their decisions. This research identifies and explains how socio-cultural and industrial norms influence the intentions and behaviours towards sustainability of owner-managers of small tourism firms. This grounded theory study shows how selective peer association allows the use of norms that match one's values to predict the difficulties, benefits and therefore justification for pro-sustainability (in)action. Locally-held socio-cultural norms determine what is commonly (dis)approved of through reflective and comparative processes. Connectedness to the locality triggers empathy for nature and the local society, but not a corresponding sense of responsibility. This dissonance is managed by allocating responsibility to industry actors perceived as more powerful, particularly tour operators and consumers, and to the widespread greed and short term culture dominating the sector. [Abstract copyright: Crown Copyright © 2018. Published by Elsevier Ltd. All rights reserved.

Type-II surface brightness profiles in edge-on galaxies produced by flares

Previous numerical studies had apparently ruled out the possibility that flares in galaxy discs could give rise to the apparent breaks in their luminosity profiles when observed edge-on. However the studies have not, until now, analyzed this hypothesis systematically using realistic models for the disc, the flare, and the bulge. We revisit this theme by analyzing a series of models which sample a wide range of observationally based structural parameters for these three components. We have considered realistic distributions of bulge to disc ratios, morphological parameters of bulges and discs, vertical scale heights of discs and their radial gradients defining the flare for different morphological types and stellar mass bins, based on observations. The surface brightness profiles for the face-on and edge-on views of each model were simulated to find out whether the flared disc produces a Type-II break in the disc profile when observed edge-on, and if so under what conditions. Contrary to previous claims, we find that discs with realistic flares can produce significant breaks in discs when observed edge-on. Specifically a flare with the parameters of that of the Milky Way would produce a significant break of the disc at a Rbreak of ~8.6 kpc if observed edge-on. Central bulges have no significant effects on the results. These simulations show that flared discs can explain the existence of many Type-II breaks observed in edge-on galaxies, in a range of galaxies with low-to-intermediate break strength values of -0.25<S<-0.1.Comment: Published in Astronomy & Astrophysics. 5 pages, 5 figures. Language corrections by the journal included in this new versio