arXiv subjects

Ilidio Lopes

Publications and source records attributed to Ilidio Lopes.

36 records · Page 2Linked to original sources

arXiv2017

Neutron stars: A novel equation of state with induced surface tension

A novel equation of state with the surface tension induced by particles' interactions was generalized to describe the properties of the neutron stars (NSs). In this equation the interaction between particles occurs via the hard core repulsion by taking into account the proper volumes of particles. Recently, this model was successfully applied to the description of the properties of nuclear and hadron matter created in collisions of nucleons. The new approach is free of causality problems and is fully thermodynamically consistent, which enables us to use it for the investigation of the strongly interacting matter phase-diagram properties in a wide range of temperatures and baryon densities, including NSs. Here, we have calculated the mass-radius relations for a compact star using the Tolmann-Oppenheimer-Volkov (TOV) equation for two sets of parameters that satisfy the existing constraints. Accordingly, we found parameter values that are in good agreement with the same ones obtained from the nuclear-nuclear collision data analysis. The astrophysical constraints suggest that the hard core radius of baryons can vary between 0.425 fm and 0.476 fm.

astro-ph.HE↗

arXiv2017

Radial oscillations of strange quark stars admixed with condensed dark matter

We compute the 20 lowest frequency radial oscillation modes of strange stars admixed with condensed dark matter. We assume a self-interacting bosonic dark matter, and we model dark matter inside the star as a Bose-Einstein condensate. In this case the equation of state is a polytropic one with index $1+1/n=2$ and a constant $K$ that is computed in terms of the mass of the dark matter particle and the scattering length. Assuming a mass and a scattering length compatible with current observational bounds for self-interacting dark matter, we have integrated numerically first the Tolman-Oppenheimer-Volkoff equations for the hydrostatic equilibrium, and then the equations for the perturbations $ξ=Δr/r$ and $η=ΔP/P$. For a compact object with certain mass and radius we have considered here three cases, namely no dark matter at all and two different dark matter scenarios. Our results show that i) the separation between consecutive modes increases with the amount of dark matter, and ii) the effect is more pronounced for higher order modes. These effects are relevant even for a strange star made of 5\% dark matter.

gr-qc↗

arXiv2017

The dark matter effect on realistic equation of state in neutron stars

In this work we apply relativistic mean-field theory in neutron stars assuming that fermionic dark matter is trapped inside the star and interacts directly with neutrons by exchanging Standard Model Higgs bosons. For realistic values of the parameters of the model we compute numerically the equation of state, and we compare it to the standard one. Furthermore, the mass-to-radius relation for both equations of state (pure neutron matter as well as admixed DM-neutron star) is shown, and the highest star mass for both cases is reported.

hep-ph↗

arXiv2017

Constraining the parameter space of branon dark matter using white dwarf stars

In the present work we study the branon dark matter particles impact on compact objects, and we provide the first constraints of the parameter space using white dwarf stars. The branon dark matter model is characterized by two free parameters, namely the branon mass particle M and the brane tension factor $f$. The latter determines the strength of the interaction of branon dark matter particles with baryons. By considering a typical white dwarf star we were able to obtain constraints on branon dark matter and compare with current limits obtained by direct detection searches and dark matter abundance. In particular our results show that i) for heavy branons with a mass $M > 10 GeV$ white dwarfs fail to provide us with bounds better than current limits from DM direct detection searches, and ii) for light branons in the mass range $2 keV < M < 1 GeV$, which cannot be probed neither with current dark matter experiments nor with the next generation of detectors, the dark matter abundance constrain determines $f$ as a function of $M$ in the range $0.1 GeV < M < 1 GeV$ for the branon mass and $1 GeV < f < 5 GeV$ for the brane tension factor. Furthermore, our findings indicate that the limits from white dwarfs are not stronger than the dark matter abundance constrain.

hep-ph↗

arXiv2017

Gravitational effects of condensed dark matter on strange stars

In the present work we study the gravitational effects of condensed dark matter on strange stars. We consider self-interacting dark matter particles with properties consistent with current observational constraints, and dark matter inside the star is modelled as a Bose-Einstein condensate. We integrate numerically the Tolman-Oppenheimer-Volkoff equations in the two-fluid formalism assuming that strange stars are made of up to 4 per cent of dark matter. It is shown that for a mass of the dark matter particles in the range $50 MeV-160 MeV$ strange stars are characterized by a maximum mass and radius similar to the ones found for neutron stars.

gr-qc↗

arXiv2017

Constraining the scalar singlet and inert dark matter models using neutron stars

In the present work we study the scalar singlet as well as the Two-Higgs Doublet model inert dark matter particles impact on compact objects, and we provide the first constraints of the parameter space using neutron stars. The models discussed here are characterized by two free parameters, namely the mass $M_χ$ of the scalar particle that plays the role of the dark matter in the Universe, and a dimensionless coupling constant $λ_χ$ that determines the strength of the interaction of the dark matter particles with the Standard Model Higgs boson. By considering a typical neutron star we were able to obtain constraints on scalar dark matter depending on the DM annihilation cross section and self-interaction coupling constant. Our findings show that i) for heavy DM particles neutron stars can provide us with bounds better that the current limits from direct detection searches only when the self-annihilations of DM particles are negligible and the DM self-interaction coupling constant is very small, while ii) for light DM particles the bounds obtained here are comparable to limits from Higgs invisible decays unless the DM particles are extremely light.

hep-ph↗

arXiv2015

Looking for granulation and periodicity imprints in the sunspot time series

The sunspot activity is the end result of the cyclic destruction and regeneration of magnetic fields by the dynamo action. We propose a new method to analyze the daily sunspot areas data recorded since 1874. By computing the power spectral density of daily data series using the Mexican hat wavelet, we found a power spectrum with a well-defined shape, characterized by three features. The first term is the 22 yr solar magnetic cycle, estimated in our work to be of 18.43 yr. The second term is related to the daily volatility of sunspots. This term is most likely produced by the turbulent motions linked to the solar granulation. The last term corresponds to a periodic source associated with the solar magnetic activity, for which the maximum of power spectral density occurs at 22.67 days. This value is part of the 22-27 day periodicity region that shows an above-average intensity in the power spectra. The origin of this 22.67 day periodic process is not clearly identified, and there is a possibility that it can be produced by convective flows inside the star. The study clearly shows a north-south asymmetry. The 18.43 yr periodical source is correlated between the two hemispheres, but the 22.67 day one is not correlated. It is shown that towards the large timescales an excess occurs in the northern hemisphere, especially near the previous two periodic sources. To further investigate the 22.67 day periodicity we made a Lomb-Scargle spectral analysis. The study suggests that this periodicity is distinct from others found nearby.

astro-ph.SR↗

arXiv2014

A particle dark matter footprint on the first generation of stars

Dark matter particles with properties identical to dark matter candidates that are hinted at by several international collaborations dedicated to experimental detection of dark matter (DAMA, COGENT, CRESST and CDMS-II, although not, most notably, by LUX), and which also have a dark matter asymmetry identical to the observed baryon asymmetry (Planck and Wilkinson Microwave Anisotropy Probe), may produce a significant impact on the evolution of the first generation of low-metallicity stars. The lifetimes of these stars in different phases of stellar evolution are significantly extended, namely, in the pre-main sequence, main sequence, and red giant phases. In particular, intermediate-mass stars in the red giant phase experience significant changes in their luminosity and chemical composition. The annihilations of dark matter particles affect the interior of the star in such a way that the $3α-$reaction becomes less efficient in the production of carbon and oxygen. This dark matter effect contradicts the excess of carbon and other metals observed today in stars of low mass and low metallicity. Hence, we can impose an upper limit on the dark matter halo density, and therefore on the redshift, at which the first generation of low-metallicity stars formed.

astro-ph.CO↗

arXiv2014

Local perturbations of the upper layers of a Sun-like star: the impact on the acoustic oscillation spectrum

In the last decade the quality and the amount of observational asteroseismic data that has been made available by space based missions had a tremendous upgrowth. The determination of asteroseismic parameters to estimate the fundamental physical processes occurring in stars' interiors, can be done today in a way that has never been possible before. In this work we propose to compute the seismic observable $β$, which is a proxy of the phase shift of the acoustic modes propagating in the envelope of the Sun-like stars. This seismic parameter $β$ can be used to identify rapid variation regions usually known as glitches. We show that a small variation in the structure, smaller than 1% in the sound speed, produces a glitch in the acoustic potential that could explain the oscillatory character of $β$. This method allows us to determine the location and the thickness of the glitch with precision. We applied this idea to the Sun-like star $α$ Centauri A and found a glitch located at approximately $0.94\,R$ (1400 s) and with a thickness of 0.2% of the stars' radius. This is fully consistent with the data and also validates other seismic tests.

astro-ph.SR↗

arXiv2013

Search for global f-modes and p-modes in the 8B neutrino flux

The impact of global acoustic modes on the 8B neutrino flux time series is computed for the first time. It is shown that the time fluctuations of the 8B neutrino flux depend on the amplitude of acoustic eigenfunctions in the region where the 8B neutrino flux is produced: modes with low n (or order) that have eigenfunctions with a relatively large amplitude in the Sun's core, strongly affect the neutrino flux; conversely, modes with high n that have eigenfunctions with a minimal amplitude in the Sun's core have a very small impact on the neutrino flux. It was found that the global modes with a larger impact on the 8B neutrino flux have a frequency of oscillation in the interval 250 μHz to 500 μHz (or a period in the interval 30 minutes to 70 minutes), such as the f-modes (n=0) for the low degrees, radial modes of order n smaller or equal to 3, and the dipole mode of order n=1. Their corresponding neutrino eigenfunctions are very sensitive to the solar inner core and are unaffected by the variability of the external layers of the solar surface. If time variability of neutrinos is observed for these modes, it will lead to new ways of improving the sound speed profile inversion in the central region of the Sun.

astro-ph.SR↗

arXiv2013

Solar neutrino physics: Sensitivity to light dark matter particles

Neutrinos are produced in several neutrino nuclear reactions of the proton-proton chain and carbon-nitrogen-oxygen cycle that take place at different radius of the Sun's core. Hence, measurements of solar neutrino fluxes provide a precise determination of the local temperature. The accumulation of non-annihilating light dark matter particles (with masses between 5 GeV and 16 GeV in the Sun produces a change in the local solar structure, namely, a decrease in the central temperature of a few percent. This variation depends on the properties of the dark matter particles, such as the mass of the particle and its spin-independent scattering cross-section on baryon-nuclei, specifically, the scattering with helium, oxygen, and nitrogen among other heavy elements. This temperature effect can be measured in almost all solar neutrino fluxes. In particular, by comparing the neutrino fluxes generated by stellar models with current observations, namely 8B neutrino fluxes, we find that non-annihilating dark matter particles with a mass smaller than 10 GeV and a spin-independent scattering cross-section with heavy baryon-nuclei larger than 3 x 10^{-37} cm^-2 produce a variation in the 8B neutrino fluxes that would be in conflict with current measurements.

astro-ph.SR↗

arXiv2013

Planetary influence on the young Sun's evolution: the solar neutrino probe

Recent observations of solar twin stars with planetary systems like the Sun, have uncovered that these present a peculiar surface chemical composition. This is believed to be related to the formation of earth-like planets. This suggests that twin stars have a radiative interior that is richer in heavy elements than their envelopes. Moreover, the current standard solar model does not fully agree with the helioseismology data and solar neutrino flux measurements. In this work, we find that this agreement can improve if the Sun has mass loss during the pre-main sequence, as was previously shown by other groups. Despite this better agreement, the internal composition of the Sun is still uncertain, especially for elements heavier than helium. With the goal of inferring the chemical abundance of the solar interior, we tested several chemical compositions. We found that heavy element abundances influence the sound speed and solar neutrinos equally. Nevertheless, the carbon-nitrogen-oxygen (CNO;13N, 15O and 17F) neutrino fluxes are the most affected; this is due to the fact that contrarily to proton-proton (pp, pep, 8B and 7Be) neutrino fluxes, the CNO neutrino fluxes are less dependent on the total luminosity of the star. Furthermore, if the central solar metallicity increases by 30%, as hinted by the solar twin stars observations, this new solar model predicts that 13N, 15O and 17F neutrino fluxes increase by 25%-80% relative to the standard solar model. Finally, we highlight that the next generation of solar neutrino experiments will not only put constraints on the abundances of carbon, oxygen and nitrogen, but will also give some information about their radial distribution.

astro-ph.SR↗

arXiv2012

Solar constraints on asymmetric dark matter

The dark matter content of the Universe is likely to be a mixture of matter and antimatter, perhaps comparable to the measured asymmetric mixture of baryons and antibaryons. During the early stages of the Universe, the dark matter particles are produced in a process similar to baryogenesis, and dark matter freeze-out depends on the dark matter asymmetry and the annihilation cross section (s-wave and p-wave annihilation channels). In these η-parametrised asymmetric dark matter models (η-ADM), the dark matter particles have an annihilation cross section close to the weak interaction cross section, and a value of η-dark matter asymmetry close to the baryon asymmetry η_B. Furthermore, we assume that dark matter scattering of baryons, namely, the spin-independent scattering cross section, is of the same order as the range of values suggested by several theoretical particle physics models used to explain the current unexplained events reported in the DAMA/LIBRA, CoGeNT and CRESST experiments. Here, we constrain η-ADM models by investigating the impact of such a type of dark matter on the evolution of the Sun, namely, the flux of solar neutrinos and helioseismology. We find that dark matter particles with a mass smaller than 15 GeV, a spin-independent scattering cross section on baryons of the order of a picobarn, and an η-dark matter asymmetry with a value in the interval 10^{-12}-10^{-10}, would induce a change in solar neutrino fluxes in disagreement with current neutrino flux measurements. A natural consequence of this model is suppressed annihilation, thereby reducing the tension between indirect and direct dark matter detection experiments, but the model also allows a greatly enhanced annihilation cross section. All the cosmological η-asymmetric dark matter scenarios that we discuss are in agreement with the current WMAP measured values.

astro-ph.SR↗

arXiv2010

Low-mass stars within dense dark matter halos

We studied the formation and evolution of low-mass stars within halos with high concentration of dark matter (DM) particles, using a highly sophisticated expression to calculate the rate at which DM particles are captured inside the star. For very high DM densities in the host halo (ρ_χ>10^10 GeV cm^-3 for a 1 M_{\odot} star), we found that young stars stop sooner their gravitational collapse in the pre-Main Sequence phase, reaching states of equilibrium in which DM annihilation is their only source of energy. The lower effective temperature of these stars, which depends on the properties of the DM particles and DM halo, may be used as an alternative method to investigate the nature of DM.

astro-ph.CO↗

arXiv2010

The formation and evolution of young low-mass stars within halos with high concentration of dark matter particles

The formation and evolution of low-mass stars within dense halos of dark matter (DM) leads to evolution scenarios quite different from the classical stellar evolution. As a result of our detailed numerical work, we describe these new scenarios for a range of DM densities on the host halo, a range of scattering cross sections of the DM particles considered, and for stellar masses from 0.7 to 3 M_{\odot}. For the first time, we also computed the evolution of young low-mass stars in their Hayashi track in the pre-main sequence phase and found that, for high DM densities, these stars stop their gravitational collapse before reaching the main sequence, in agreement with similar studies on first stars. Such stars remain indefinitely in an equilibrium state with lower effective temperatures (|ΔT_eff| > 10^3 K for a star of one solar mass), the annihilation of captured DM particles in their core being the only source of energy. In the case of lower DM densities, these proto-stars continue their collapse and progress through the main sequence burning hydrogen at a lower rate. A star of 1 M_{\odot} will spend a time greater than the current age of the universe consuming all the hydrogen in its core if it evolves in a halo with DM density ρ_χ=10^9 GeV cm^-3. We also show the strong dependence of the effective temperature and luminosity of these stars on the characteristics of the DM particles and how this can be used as an alternative method for DM research.

astro-ph.CO↗

arXiv2002

The implications for helioseismology of experimental uncertainties in Newton's constant

The experimental uncertainties in G between different experiments have important implications for helioseismology. We show that these uncertainties for the standard solar model lead to a range in the value of the square of the sound speed in the nuclear region that is as much as 0.15% higher than the inverted helioseismic sound speed. While a lower value of G is preferred for the standard model, any definite prediction is masked by the uncertainties in the solar models available in the literature. However future refinements of helioseismology with an accuracy of the order of 1E-3 to 1E-4 in the square of the sound speed, especially in combination with precision measurements of the 8B solar neutrino flux should be capable of independently testing these experimental values of G.

astro-ph↗

arXiv2002

Solar Seismic Model as a New Constraint on Supersymmetric Dark Matter

If the Milky Way is populated by WIMPs as predicted by cosmological models of the large-scale structure of the universe and as motivated by SUSY, the capture of high-mass WIMPs by the Sun would affect the temperature, density and chemical composition of the solar core. We use the sound speed and the density profiles inferred from the helioseismic instruments on the Solar and Heliospheric Observatory (SOHO) to discuss the effect of WIMP accretion and annihilation on the evolution of the Sun. The WIMP transport of energy inside the Sun is not negligible for WIMPs with a mass smaller than 60 GeV and annihilating WIMPs with ~ 10^{-27}cm^3/sec. WIMP-accreting models with WIMP masses smaller than 30 GeV are in conflict with the most recent seismic data. We combine our new constraints with the analysis of predicted neutrino fluxes from annihilating WIMPs in the solar core. Working in the framework of the Minimal Supersymmetric Standard Model and considering the neutralino as the best dark matter particle candidate, we find that supersymmetric models, consistent with solar seismic data and with recent measurements of dark matter relic density, lead to a measured muon flux on Earth in the range of 1 to 10^4 km^{-2} yr^{-1}, for neutralino masses between 30 and 400 GeV.

astro-ph↗

arXiv2001

Solar Neutrinos: Probing the Quasi-Isothermal Solar Core Produced by SUSY Dark Matter Particles

SNO measurements strongly constrain the central temperature of the Sun within a precision of much less than 1%. This result can be used to constrain the parameter space of SUSY dark matter particle candidates. In this first analysis we find a lower limit for the WIMP mass of 60 GeV, well above the WIMP evaporation limit of 10 GeV. Furthermore, in the event that WIMPs create a quasi-isothermal core within the Sun, they will produce a peculiar distribution of the solar neutrino fluxes measured on Earth. Typically, a WIMP with a mass of 100 GeV and annihilation cross-section of 1E-34 cm^3/sec, will decrease the standard solar model neutrino predictions, by up to 4% for the Cl, by 3% for the heavy water, and by 1% for the Ga detectors.

astro-ph↗