Equilibrium, radial stability and non-adiabatic gravitational collapse of anisotropic neutron stars

In this work we construct families of anisotropic neutron stars for an equation of state compatible with the constraints of the gravitational-wave event GW170817 and for four anisotropy ansatze. Such stars are subjected to a radial perturbation in order to study their stability against radial oscillations and we develop a dynamical model to describe the non-adiabatic gravitational collapse of the unstable anisotropic configurations whose ultimate fate is the formation of a black hole. We find that the standard criterion for radial stability $dM/d\rho_c >0$ is not always compatible with the calculation of the oscillation frequencies for some anisotropy ansatze, and each anisotropy parameter is constrained taking into account the recent restriction of maximum mass of neutron stars. We further generalize the TOV equations within a non-adiabatic context and we investigate the dynamical behaviour of the equation of state, heat flux, anisotropy factor and mass function as an unstable anisotropic star collapses. After obtaining the evolution equations we recover, as a static limit, the background equations.Comment: 17 pages, 7 figure

Constraint on Einstein-Gauss-Bonnet Gravity from Neutron Stars

Within the framework of Einstein-Gauss-Bonnet theory in five-dimensional spacetime ($5D$ EGB), we derive the hydrostatic equilibrium equations and solve them numerically to obtain the neutron stars for both isotropic and anisotropic distribution of matter. The mass-radius relations are obtained for SLy equation of state, which describes both the solid crust and the liquid core of neutron stars, and for a wide range of the Gauss-Bonnet coupling parameter $\alpha$. More specifically, we find that the contribution of the Gauss-Bonnet term leads to substantial deviations from the Einstein gravity. We also discuss that after a certain value of $\alpha$, the theory admits higher maximum masses compared with general relativity, however, the causality condition is violated in the high-mass region. Finally, our results are compared with the recent observations data on mass-radius diagram.Comment: 9 pages, 5 figure

Relativistic structure of charged quark stars in energy-momentum squared gravity

Within the context of energy-momentum squared gravity (EMSG), where non-linear matter contributions appear in the gravitational action, we derive the modified TOV equations describing the hydrostatic equilibrium of charged compact stars. We adopt two different choices for the matter Lagrangian density ($\mathcal{L}_m= p$ versus $\mathcal{L}_m= -\rho$) and investigate the impact of each one on stellar structure. Furthermore, considering a charge profile where the electric charge density $\rho_{\rm ch}$ is proportional to the standard energy density $\rho$, we solve numerically the stellar structure equations in order to obtain the mass-radius diagrams for the MIT bag model equation of state (EoS). For $\mathcal{L}_m= p$ and given a specific value of $\beta$ (including the uncharged case when $\beta= 0$), the maximum-mass values increase (decrease) substantially as the gravity model parameter $\alpha$ becomes more negative (positive). However, for uncharged configurations and considering $\mathcal{L}_m= -\rho$, our numerical results reveal that when we increase $\alpha$ (from a negative value) the maximum mass first increases and after reaching a maximum value it starts to decrease. Remarkably, this makes it a less trivial behavior than that caused by the first choice when we take into account the presence of electric charge ($\beta \neq 0$).Comment: 15 pages, 6 figures, 2 tables. Version accepted in Annals of Physic

E-Commerce, social media and Social Inclusion: A Typology of Users Over 60 Years of Age in Spain

The growth of e-commerce and the ageing of the world population converges in a situation which represents significant challenges for the economy: a large population of over 65year-olds with major limitations for online shopping. The digital division, although reduced in certain sectors, in others, such as e-commerce persists. This research involves the study of 405 subjects, Spanish seniors aged between 60 and 79, to discover and analyse the barriers, motivators and behaviour with regards to shopping online. Using a telephone survey and a random sample, a typology of users was established as per their behaviour and perception of e-commerce. The main finding is the segmentation in eleven typologies of users, identified through cluster analysis. The major conclusions confirm the barriers related to data security or levels of confidence. Furthermore, with basic digital skills, seniors consider themselves capable of completing purchases on the Internet. Online shoppers feel empowered being digitally self-sufficient, while non-shoppers desiring to be independent, require the implementation of measures to aid the process on commercial websites.El crecimiento del comercio electrónico y el envejecimiento de la población mundial convergen en una situación que representa importantes retos para la economía: una gran población de mayores de 65 años con grandes limitaciones para realizar compras en línea. La brecha digital, aunque se ha reducido en algunos sectores, en otros, como el del comercio electrónico, persiste. Esta investigación consiste en un estudio de 405 sujetos, españoles de entre 60 y 79 años, para conocer y analizar las barreras, las motivaciones y el comportamiento ante las compras online. Mediante una encuesta telefónica y con una muestra aleatoria, se estableció una tipología de usuarios según su comportamiento y percepción del comercio electrónico. El principal hallazgo es la segmentación en once tipologías de usuarios, identificadas a través del análisis de clúster. Las principales conclusiones confirman las barreras relacionadas con la seguridad de los datos o los niveles de confianza. Por otro lado, con unas habilidades digitales básicas, los mayores se consideran capaces de realizar compras en Internet. Los compradores online se sienten capacitados siendo autosuficientes digitalmente, mientras que los no compradores que desean ser independientes, requieren la implementación de medidas de ayuda al proceso en los sitios web comerciales

Lesions of pemphigus vulgaris on irradiated skin

Summary Pemphigus vulgaris (PV) is an autoimmune blistering disease produced by IgG autoantibodies against desmoglein (Dsg)3. Lesions on the skin and mucosa can, in rare cases, be induced by radiotherapy. We report a patient with a history of microprolactinoma and PV, who had only oral lesions from the beginning of her illness but 2 months after treatment with radiotherapy for a breast neoplasia, developed skin lesions limited to the irradiated area. Over the following few months, she also developed autoantibodies against Dsg