The properties of ten O-type stars in the low-metallicity galaxies IC 1613, WLM and NGC 3109

Massive stars likely played an important role in the reionization of the Universe, and the formation of the first black holes. Massive stars in low-metallicity environments in the local Universe are reminiscent of their high redshift counterparts. In a previous paper, we reported on indications that the stellar winds of low-metallicity O stars may be stronger than predicted, which would challenge the current paradigm of massive star evolution. In this paper, we aim to extend our initial sample of six O stars in low-metallicity environments by four. We aim to derive their stellar and wind parameters, and compare these to radiation-driven wind theory and stellar evolution models. We have obtained intermediate-resolution VLT/X-Shooter spectra of our sample of stars. We derive the stellar parameters by fitting synthetic fastwind line profiles to the VLT/X-Shooter spectra using a genetic fitting algoritm. We compare our parameters to evolutionary tracks and obtain evolutionary masses and ages. We also investigate the effective temperature versus spectral type calibration for SMC and lower metallicities. Finally, we reassess the wind momentum versus luminosity diagram. The derived parameters of our target stars indicate stellar masses that reach values of up to 50 $M_{\odot}$. The wind strengths of our stars are, on average, stronger than predicted from radiation-driven wind theory and reminiscent of stars with an LMC metallicity. We discuss indications that the iron content of the host galaxies is higher than originally thought and is instead SMC-like. We find that the discrepancy with theory is lessened, but remains significant for this higher metallicity. This may imply that our current understanding of the wind properties of massive stars, both in the local universe as well as at cosmic distances, remains incomplete.Comment: Accepted for publication in Astronomy and Astrophysics. 10 pages, 8 figure

Continuity of Local Time: An applied perspective

Continuity of local time for Brownian motion ranks among the most notable mathematical results in the theory of stochastic processes. This article addresses its implications from the point of view of applications. In particular an extension of previous results on an explicit role of continuity of (natural) local time is obtained for applications to recent classes of problems in physics, biology and finance involving discontinuities in a dispersion coefficient. The main theorem and its corollary provide physical principles that relate macro scale continuity of deterministic quantities to micro scale continuity of the (stochastic) local time.Comment: To appear in: "The fascination of Probability, Statistics and Their Applications. In honour of Ole E. Barndorff-Nielsen on his 80th birthday

The VLT-FLAMES Tarantula Survey

We present a number of notable results from the VLT-FLAMES Tarantula Survey (VFTS), an ESO Large Program during which we obtained multi-epoch medium-resolution optical spectroscopy of a very large sample of over 800 massive stars in the 30 Doradus region of the Large Magellanic Cloud (LMC). This unprecedented data-set has enabled us to address some key questions regarding atmospheres and winds, as well as the evolution of (very) massive stars. Here we focus on O-type runaways, the width of the main sequence, and the mass-loss rates for (very) massive stars. We also provide indications for the presence of a top-heavy initial mass function (IMF) in 30 Dor.Comment: 7 Figures, 8 pages. Invited talk: IAUS 329: "The Lives and Death-Throes of Massive Stars

Thermodynamics of two lattice ice models in three dimensions

In a recent paper we introduced two Potts-like models in three dimensions, which share the following properties: (A) One of the ice rules is always fulfilled (in particular also at infinite temperature). (B) Both ice rules hold for groundstate configurations. This allowed for an efficient calculation of the residual entropy of ice I (ordinary ice) by means of multicanonical simulations. Here we present the thermodynamics of these models. Despite their similarities with Potts models, no sign of a disorder-order phase transition is found.Comment: 5 pages, 7 figure

Novel Approaches towards Highly Selective Self-Powered Gas Sensors

The prevailing design approaches of semiconductor gas sensors struggle to overcome most of their current limitations such as poor selectivity, and high power consumption. Herein, a new sensing concept based on devices that are capable of detecting gases without the need of any external power sources required to activate interaction of gases with sensor or to generate the sensor read out signal. Based on the integration of complementary functionalities (namely; powering and sensing) in a singular nanostructure, self-sustained gas sensors will be demonstrated. Moreover, a rational methodology to design organic surface functionalization that provide high selectivity towards single gas species will also be discussed. Specifically, theoretical results, confirmed experimentally, indicate that precisely tuning of the sterical and electronic structure of sensor material/organic interfaces can lead to unprecedented selectivity values, comparable to those typical of bioselective processes. Finally, an integrated gas sensor that combine both the self-powering and selective detection strategies in one single device will also be presented. © 2015 Published by Elsevier Ltd.Peer ReviewedPostprint (published version

Coulomb suppression of NMR coherence peak in fullerene superconductors

The suppressed NMR coherence peak in the fullerene superconductors is explained in terms of the dampings in the superconducting state induced by the Coulomb interaction between conduction electrons. The Coulomb interaction, modelled in terms of the onsite Hubbard repulsion, is incorporated into the Eliashberg theory of superconductivity with its frequency dependence considered self-consistently at all temperatures. The vertex correction is also included via the method of Nambu. The frequency dependent Coulomb interaction induces the substantial dampings in the superconducting state and, consequently, suppresses the anticipated NMR coherence peak of fullerene superconductors as found experimentally.Comment: 4 pages, Revtex, and 2 figures. Revised and final version to appear in Phys. Rev. Lett. (1998

Estudio anatómico de la amplitud de los discos intervertebrales y de la zona foraminal a nivel toracolumbar en perros adultos

La columna vertebral del perro ha sido utilizada como modelo experimental en medicina humana, y es uno de los animales domésticos más estudiados en medicina veterinaria. Sin embargo, existen limitados datos sobre la amplitud del disco intervertebral (DIV) y el foramen intervertebral (FIV). El propósito de este trabajo es medir cuantitativamente la amplitud de los DIV y FIV, e igualmente, la distancia entre la emergencia de los nervios espinales lumbares y el borde anterior del cuerpo vertebral caudal adyacente o altura del triángulo de seguridad de Kambin (ATK). Se realizó un estudio descriptivo en cinco cadáveres de perros. Se midieron: los diametros discales ventrales, la distancia entre la emergencia de los nervios espinales y el borde anterior del cuerpo vertebral caudal adyacente (CVCA), y los diámetros de los FIV de los niveles toracolumbares T12 – L6. La amplitud del DIV en el borde ventral fue en promedio de 7,34 mm (SD± 2,209), y la distancia entre los nervios espinales y el borde posterior del DIV, de 3,56 mm (SD 1,366). Los diámetros promedio del FIV fueron en su aspecto craneocaudal de 5,24 mm (SD 1,838) y dorsoventral de 4,73 mm (SD 1,098). Se evidenció para esta muestra una correlación gráfica entre la ATK y la ADIV, y una longitud máxima de todas las estructuras medidas en el segmento L4 – L5. Además, se plantea la posibilidad del desarrollo de instrumentales intradiscales y transforaminales con un diámetro de 3 mm, hechos a medida para caninos a partir de los 10 kg de peso. Palabras clave: disco intervertebral, foramen, amplitud, morfometría, modelo experimental, perros