| Resumo : |
In this work, we analyze the structure and stability of white dwarfs at finite temperature, making comparisons with observational data and analyzing the insertion of a an electric charge due to a polarization of the matter on their envelopes. Regarding the stellar fluid, we define it as being a composition of electrons and nucleons in a Wigner-Seitz cell, as well as free photons. Because a hot degenerate lump with conductive transport forms these white dwarfs, the temperature is approximately constant in this region. In the non-degenerate envelope, however, there is energy transport due to convection and radiation, which creates a temperature distribution. As we only considered massive white dwarfs in all analyses, the determination of this envelope does not change the structure of the star (although it is necessary to guarantee hydrostatic balance). Using the Tolman-Oppenheimer-Volkoff (TOV) equations, we numerically obtain the structure of hot white dwarfs. Through some observational stars present in the Sloan Digital Sky Survey and Extreme Ultraviolet Sky Survey catalogs, we observed that some of them had masses and radii compatible with very high core temperature curves. We then fit the observational surface gravity and effective temperature data to find the core mass, radius and temperature of these stars. We find that due to general relativity, these stars with very high surface gravity have smaller masses and radii than those previously reported. In order to improve the next estimates of the mass and radius of these stars, we obtain an equation for these quantities as a function of the effective temperature and surface gravity. We also analyzed the stability of very massive hot white dwarfs by pycnonuclear reactions, $\beta$-inverse and radial oscillations. We obtain that white dwarfs with core temperature from $10^8\rm [K]$ have their stability analyzed according to radial oscillations. Additionally, we also study a possible electric charge due to a polarized matter in the envelope of hot white dwarfs. We investigate this polarization by introducing conditions to the charge location, which ensures a global charge neutralization. By analyzing extreme cases, we consider that the core temperature would be in the order of $10^8\rm[K]$, in which lattice interaction effects can be neglected. We solve the Maxwell-Einstein equations obtaining bigger and more massive stars (masses of approximately $2.4M_\odot$ for polarized charge of $Q=1.5\times10^{19}\rm ~[C]$). When analyzing these stars stability, we found that pycnonuclear reactions may be use to settle a limit for the star central density. Considering effects due to the star rotation we found maximum magnetic fields of the order of $10^8 \rm [G]$. For the first time we provide white dwarf structures with masses beyond the Chandrasekhar limit, with zero external electric field. |