Smoothed Particle Hydrodynamics simulations of the core-degenerate scenario for Type Ia supernovae

The core-degenerate (CD) scenario for type Ia supernovae (SN Ia) involves the merger of the hot core of an asymptotic giant branch (AGB) star and a white dwarf, and might contribute a non-negligible fraction of all thermonuclear supernovae. Despite its potential interest, very few studies, and based on only crude simplifications, have been devoted to investigate this possible scenario, compared with the large efforts invested to study some other scenarios. Here we perform the first three-dimensional simulations of the merger phase, and find that this process can lead to the formation of a massive white dwarf, as required by this scenario. We consider two situations, according to the mass of the circumbinary disk formed around the system during the final stages of the common envelope phase. If the disk is massive enough, the stars merge on a highly eccentric orbit. Otherwise, the merger occurs after the circumbinary disk has been ejected and gravitational wave radiation has brought the stars close to the Roche lobe radius on a nearly circular orbit. Not surprisingly, the overall characteristics of the merger remnants are similar to those found for the double-degenerate (DD) scenario, independently of the very different core temperature and of the orbits of the merging stars. They consist of a central massive white dwarf, surrounded by a hot, rapidly rotating corona and a thick debris region.Comment: 17 pages, 10 figures. Accepted for publication in MNRA

An upper limit to the secular variation of the gravitational constant from white dwarf stars

A variation of the gravitational constant over cosmological ages modifies the main sequence lifetimes and white dwarf cooling ages. Using an state-of-the-art stellar evolutionary code we compute the effects of a secularly varying G on the main sequence ages and, employing white dwarf cooling ages computed taking into account the effects of a running G, we place constraints on the rate of variation of Newton's constant. This is done using the white dwarf luminosity function and the distance of the well studied open Galactic cluster NGC 6791. We derive an upper bound G'/G ~ -1.8 10^{-12} 1/yr. This upper limit for the secular variation of the gravitational constant compares favorably with those obtained using other stellar evolutionary properties, and can be easily improved if deep images of the cluster allow to obtain an improved white dwarf luminosity function.Comment: 15 pages, 4 figures, accepted for publication in JCA

Finding Exogenous Variables in Data with Many More Variables than Observations

Many statistical methods have been proposed to estimate causal models in classical situations with fewer variables than observations (p<n, p: the number of variables and n: the number of observations). However, modern datasets including gene expression data need high-dimensional causal modeling in challenging situations with orders of magnitude more variables than observations (p>>n). In this paper, we propose a method to find exogenous variables in a linear non-Gaussian causal model, which requires much smaller sample sizes than conventional methods and works even when p>>n. The key idea is to identify which variables are exogenous based on non-Gaussianity instead of estimating the entire structure of the model. Exogenous variables work as triggers that activate a causal chain in the model, and their identification leads to more efficient experimental designs and better understanding of the causal mechanism. We present experiments with artificial data and real-world gene expression data to evaluate the method.Comment: A revised version of this was published in Proc. ICANN201

Manual / Issue 12 / On Further Review

Manual, a journal about art and its making. On Further Review. This issue uncovers narratives once central to objects’ histories but that now have been systematically obscured, inadvertently overlooked, or otherwise lost. Softcover, 96 pages. Published 2019 by the RISD Museum.(On Further Review) contributors include Anita N. Bateman, Laurie Anne Brewer, Becci Davis, Jamie Gabbarelli, Bethany Johns, Elon Cook Lee, Kevin McBride, Walker Mettling, Jessica Rosner, Suzanne Scanlan, Nell Painter, Allison Pappas, Pamela A. Parmal, Shiyanthi Thavapalan, and Nick White.https://digitalcommons.risd.edu/risdmuseum_journals/1038/thumbnail.jp

Quantitative imaging of concentrated suspensions under flow

We review recent advances in imaging the flow of concentrated suspensions, focussing on the use of confocal microscopy to obtain time-resolved information on the single-particle level in these systems. After motivating the need for quantitative (confocal) imaging in suspension rheology, we briefly describe the particles, sample environments, microscopy tools and analysis algorithms needed to perform this kind of experiments. The second part of the review focusses on microscopic aspects of the flow of concentrated model hard-sphere-like suspensions, and the relation to non-linear rheological phenomena such as yielding, shear localization, wall slip and shear-induced ordering. Both Brownian and non-Brownian systems will be described. We show how quantitative imaging can improve our understanding of the connection between microscopic dynamics and bulk flow.Comment: Review on imaging hard-sphere suspensions, incl summary of methodology. Submitted for special volume 'High Solid Dispersions' ed. M. Cloitre, Vol. xx of 'Advances and Polymer Science' (Springer, Berlin, 2009); 22 pages, 16 fig

The Evolution of Compact Binary Star Systems

We review the formation and evolution of compact binary stars consisting of white dwarfs (WDs), neutron stars (NSs), and black holes (BHs). Binary NSs and BHs are thought to be the primary astrophysical sources of gravitational waves (GWs) within the frequency band of ground-based detectors, while compact binaries of WDs are important sources of GWs at lower frequencies to be covered by space interferometers (LISA). Major uncertainties in the current understanding of properties of NSs and BHs most relevant to the GW studies are discussed, including the treatment of the natal kicks which compact stellar remnants acquire during the core collapse of massive stars and the common envelope phase of binary evolution. We discuss the coalescence rates of binary NSs and BHs and prospects for their detections, the formation and evolution of binary WDs and their observational manifestations. Special attention is given to AM CVn-stars -- compact binaries in which the Roche lobe is filled by another WD or a low-mass partially degenerate helium-star, as these stars are thought to be the best LISA verification binary GW sources.Comment: 105 pages, 18 figure

Evolutionary and pulsational properties of white dwarf stars

Abridged. White dwarf stars are the final evolutionary stage of the vast majority of stars, including our Sun. The study of white dwarfs has potential applications to different fields of astrophysics. In particular, they can be used as independent reliable cosmic clocks, and can also provide valuable information about the fundamental parameters of a wide variety of stellar populations, like our Galaxy and open and globular clusters. In addition, the high densities and temperatures characterizing white dwarfs allow to use these stars as cosmic laboratories for studying physical processes under extreme conditions that cannot be achieved in terrestrial laboratories. They can be used to constrain fundamental properties of elementary particles such as axions and neutrinos, and to study problems related to the variation of fundamental constants. In this work, we review the essentials of the physics of white dwarf stars. Special emphasis is placed on the physical processes that lead to the formation of white dwarfs as well as on the different energy sources and processes responsible for chemical abundance changes that occur along their evolution. Moreover, in the course of their lives, white dwarfs cross different pulsational instability strips. The existence of these instability strips provides astronomers with an unique opportunity to peer into their internal structure that would otherwise remain hidden from observers. We will show that this allows to measure with unprecedented precision the stellar masses and to infer their envelope thicknesses, to probe the core chemical stratification, and to detect rotation rates and magnetic fields. Consequently, in this work, we also review the pulsational properties of white dwarfs and the most recent applications of white dwarf asteroseismology.Comment: 85 pages, 28 figures. To be published in The Astronomy and Astrophysics Revie

Interleukin-10 enhances the intestinal epithelial barrier in the presence of corticosteroids through p38 MAPK activity in Caco-2 monolayers : a possible mechanism for steroid responsiveness in ulcerative colitis

Altres ajuts: 2012 Spanish Gastroenterological Association i CIBER G0034Glucocorticosteroids are the first line therapy for moderate-severe flare-ups of ulcerative colitis. Despite that, up to 60% of patients do not respond adequately to steroid treatment. Previously, we reported that low IL-10 mRNA levels in intestine are associated with a poor response to glucocorticoids in active Crohn's disease. Here, we test whether IL-10 can favour the response to glucocorticoids by improving the TNFα-induced intestinal barrier damage (assessed by transepithelial electrical resistance) in Caco-2 monolayers, and their possible implications on glucocorticoid responsiveness in active ulcerative colitis. We show that the association of IL-10 and glucocorticoids improves the integrity of TNFα-treated Caco-2 cells and that p38 MAPK plays a key role. In vitro, IL-10 facilitates the nuclear translocation of p38 MAPK-phosphorylated thereby modulating glucocorticoids-receptor-α, IL-10-receptor-α and desmoglein-2 expression. In glucocorticoids-refractory patients, p38 MAPK phosphorylation and membrane desmoglein-2 expression are reduced in colonic epithelial cells. These results suggest that p38 MAPK-mediated synergism between IL-10 and glucocorticoids improves desmosome straightness contributing to the recovery of intestinal epithelium and reducing luminal antigens contact with lamina propria in ulcerative colitis. This study highlights the link between the intestinal epithelium in glucocorticoids-response in ulcerative colitis

Effects of macromolecular crowding on intracellular diffusion from a single particle perspective

Compared to biochemical reactions taking place in relatively well-defined aqueous solutions in vitro, the corresponding reactions happening in vivo occur in extremely complex environments containing only 60–70% water by volume, with the remainder consisting of an undefined array of bio-molecules. In a biological setting, such extremely complex and volume-occupied solution environments are termed ‘crowded’. Through a range of intermolecular forces and pseudo-forces, this complex background environment may cause biochemical reactions to behave differently to their in vitro counterparts. In this review, we seek to highlight how the complex background environment of the cell can affect the diffusion of substances within it. Engaging the subject from the perspective of a single particle’s motion, we place the focus of our review on two areas: (1) experimental procedures for conducting single particle tracking experiments within cells along with methods for extracting information from these experiments; (2) theoretical factors affecting the translational diffusion of single molecules within crowded two-dimensional membrane and three-dimensional solution environments. We conclude by discussing a number of recent publications relating to intracellular diffusion in light of the reviewed material

Nuclear astrophysics: the unfinished quest for the origin of the elements

Half a century has passed since the foundation of nuclear astrophysics. Since then, this discipline has reached its maturity. Today, nuclear astrophysics constitutes a multidisciplinary crucible of knowledge that combines the achievements in theoretical astrophysics, observational astronomy, cosmochemistry and nuclear physics. New tools and developments have revolutionized our understanding of the origin of the elements: supercomputers have provided astrophysicists with the required computational capabilities to study the evolution of stars in a multidimensional framework; the emergence of high-energy astrophysics with space-borne observatories has opened new windows to observe the Universe, from a novel panchromatic perspective; cosmochemists have isolated tiny pieces of stardust embedded in primitive meteorites, giving clues on the processes operating in stars as well as on the way matter condenses to form solids; and nuclear physicists have measured reactions near stellar energies, through the combined efforts using stable and radioactive ion beam facilities. This review provides comprehensive insight into the nuclear history of the Universe and related topics: starting from the Big Bang, when the ashes from the primordial explosion were transformed to hydrogen, helium, and few trace elements, to the rich variety of nucleosynthesis mechanisms and sites in the Universe. Particular attention is paid to the hydrostatic processes governing the evolution of low-mass stars, red giants and asymptotic giant-branch stars, as well as to the explosive nucleosynthesis occurring in core-collapse and thermonuclear supernovae, gamma-ray bursts, classical novae, X-ray bursts, superbursts, and stellar mergers.Comment: Invited Review. Accepted for publication in "Reports on Progress in Physics" (version with low-resolution figures