White dwarfs with a surface electrical charge distribution: Equilibrium and stability

The equilibrium configuration and the radial stability of white dwarfs composed of charged perfect fluid are investigated. These cases are analyzed through the results obtained from the solution of the hydrostatic equilibrium equation. We regard that the fluid pressure and the fluid energy density follow the relation of a fully degenerate electron gas. For the electric charge distribution in the object, we consider that it is centralized only close to the white dwarfs' surfaces. We obtain larger and more massive white dwarfs when the total electric charge is increased. To appreciate the effects of the electric charge in the structure of the star, we found that it must be in the order of $10^{20}\,[{\rm C}]$ with which the electric field is about $10^{16}\,[{\rm V/cm}]$. For white dwarfs with electric fields close to the Schwinger limit, we obtain masses around $2\,M_{\odot}$. We also found that in a system constituted by charged static equilibrium configurations, the maximum mass point found on it marks the onset of the instability. This indicates that the necessary and sufficient conditions to recognize regions constituted by stable and unstable equilibrium configurations against small radial perturbations are respectively $dM/d\rho_c>0$ and $dM/d\rho_c<0$.Comment: This is a preprint. The original paper will be published in EPJ

ARE DRY-LAND STRENGTH METRICS AND FORCES EXERTED IN-WATER RELATED WITH HIGH SWIMMING VELOCITY IN YOUNG ATHLETES?

This study aimed to assess strength metrics in 3 dry-land exercises, forces exerted inwater in 3 tethering conditions, and to analyze possible relationships between those variables with high swimming velocity. Mean power, mean forces and 50 m maximum swimming velocity, were recorded and calculated for ten male young swimmers. High correlations were noticed between the dry-land exercises, with the lat pull down presenting the higher correlation with swimming velocity (r = 0.695, p = 0.026). The higher correlation of swimming velocity with forces exerted in-water was observed through the only arms condition (r = 0.762, p = 0.010). Results suggest that for high swimming velocity forces exerted in-water by the arms are a major criteria for success, and that lat pull down may be an appropriate dry-land exercise for its development

Stellar equilibrium configurations of white dwarfs in the f(R,T)f(R,T) gravity

In this work we investigate the equilibrium configurations of white dwarfs in a modified gravity theory, na\-mely, $f(R,T)$ gravity, for which $R$ and $T$ stand for the Ricci scalar and trace of the energy-momentum tensor, respectively. Considering the functional form $f(R,T)=R+2\lambda T$, with $\lambda$ being a constant, we obtain the hydrostatic equilibrium equation for the theory. Some physical properties of white dwarfs, such as: mass, radius, pressure and energy density, as well as their dependence on the parameter $\lambda$ are derived. More massive and larger white dwarfs are found for negative values of $\lambda$ when it decreases. The equilibrium configurations predict a maximum mass limit for white dwarfs slightly above the Chandrasekhar limit, with larger radii and lower central densities when compared to standard gravity outcomes. The most important effect of $f(R,T)$ theory for massive white dwarfs is the increase of the radius in comparison with GR and also $f(R)$ results. By comparing our results with some observational data of massive white dwarfs we also find a lower limit for $\lambda$, namely, $\lambda >- 3\times 10^{-4}$.Comment: To be published in EPJ

Perspectives on environment and human health: an editorial

Human health and wellbeing are intimately linked to the state of the environment [...]This editorial work received from the European Union through the European Regional Development Fund based on COMPETE 2020 (Programa Operacional da Competitividade e Internacionalização) and projects ICT UIDB/04683/2020 and UIDP/04683/2020

ETHERED SWIMMING AS AN USEFUL TOOL TO MEASURE UNBALANCE BETWEEN ARMS AND FORCE PRODUCTION DECREASE

Our aim of present study was to investigate the differences in force production between arms during front crawl tethered swimming (TS). Firstly, 14 young male swimmers (14.2 ± 1.09 yrs; 168.3 ± 2.22 cm; 59.9 ± 4.77 kg) undertook a 30 s maximum front crawl TS test. It was observed that preferred arm (P_Fmax) produces a maximum force higher than non-preferred arm (NP_Fmax). Additionally, was verified that the decrease in maximum force was higher for P_Fmax than NP_Fmax. In the second part of the study, 6 elite male swimmers (19.8 ± 2.23 yrs; 183.6 ± 3.64 cm; 77.3 ± 3.64 kg) replicated the methodology, being the individual curves assessed trough polynomial curves, which allowed identifying the unbalance between arms. This methodology may detect a limiting factor of performance being a useful tool for coaches training prescription