arXiv subjects

Ilídio Lopes

Publications and source records attributed to Ilídio Lopes.

At least 55 records · Page 3Linked to original sources

arXiv2020

Constraints on light dark matter particles using white dwarf stars

We report constraints on the nucleon-dark matter particle cross section using the internal luminosity of observed white dwarf stars in the globular cluster Messier 4. Our results cover the parameter space corresponding to relatively light dark matter particles, in the mass range $0.1~GeV-5~GeV$, which is known to be very difficult to be probed by direct dark matter searches. The additional luminosity coming from self-annihilations of dark matter particles captured inside the stars must not exceed the observed luminosity. Imposing that condition, we obtain for the spin independent cross section of light dark matter particles on baryons $σ_{Nχ}$ the upper bound: $σ_{Nχ} < 4 \times 10^{-41}{\rm cm^2}$.

hep-ph↗

arXiv2020

Interior solutions of relativistic stars in the scale-dependent scenario

We study relativistic stars in the scale-dependent scenario, which is one of the approaches to quantum gravity, and where Newton's constant is promoted to a scale-dependent quantity. First, the generalized structure equations are derived here for the first time. Then they are integrated numerically assuming a linear equation-of-state in the simplest MIT bag model for quark matter. We compute the radius, the mass and the compactness of strange quarks stars, and we show that the energy conditions are fulfilled.

gr-qc↗

arXiv2020

White dwarf cooling via gravity portals

We investigate the impact of gravity portal to properties of white dwarfs, such as equation-of-state as well as cooling time. We find that the interaction between dark matter spin-zero bosons and electrons in the mean field approximation softens the equation-of-state, and the object coolers down slower compared to the usual case.

hep-ph↗

arXiv2019

Obtaining Precision Constraints on Modified Gravity with Helioseismology

We propose helioseismology as a new, precision probe of fifth forces at astrophysical scales, and apply it on the most general scalar-tensor theories for dark energy, known as Degenerate Higher-Order Scalar-Tensor theories (DHOST). We explain how the effect of the fifth force on the solar interior leaves an observable imprint on the acoustic oscillations, and under certain assumptions we numerically compute the non-radial pulsation eigenfrequencies within modified gravity. We illustrate its constraining power by showing that helioseismic observations have the potential to improve constraints on the strength of the fifth force by more than $2$ orders of magnitude, as $-1.8 \cdot 10^{-3} \leq Y \leq 1.2 \cdot 10^{-3}$ (at $2σ$). This in turn would suggest constraints of similar order for the theory's free functions around a cosmological background ($α_{\text{H}}, β_{1}$).

astro-ph.CO↗

arXiv2019

Asymmetric Dark Matter Imprint on Low-mass Main-sequence Stars in the Milky Way Nuclear Star Cluster

In this work, we study the impact of asymmetric dark matter (ADM) on low-mass main-sequence stars in the Milky Way's nuclear star cluster, where the dark matter (DM) density is expected to be orders of magnitude above what is found near the Sun (${ρ}_{\mathrm{DM}}\gtrsim {10}^{3}\ \mathrm{GeV}\ {\mathrm{cm}}^{-3}$). Using a modified stellar evolution code and considering a DM particle ($m_χ = 4 \text{ GeV}$) with a spin-dependent interaction cross section close to the limits allowed by direct detection, we found that the interactions of ADM with baryons in the star's core can have two separate effects on the evolution of these stars: a decrease in the hydrogen burning rate, extending the duration of the main-sequence of stars with $M ~ 1M_{\odot}$ by a few Gyr; the suppression of the onset of convection in the core of stars with $M \lesssim 1.5M_{\odot}$ and consequent quench of supply for the nuclear reactions. If we consider $ρ_{\text{DM}} > 10^3 \ \text{GeV cm}^{-3}$ (corresponding to the inner 5 pc of the Milky Way), stars lighter than the Sun will have a main-sequence life span comparable to the current age of the universe. Stars heavier than two solar masses are not sensitive to the DM particles considered here.

astro-ph.SR↗

arXiv2019

Anisotropic strange quark stars with a non-linear equation-of-state

We obtain an exact analytical solution to Einstein's field equations assuming a non-linear equation-of-state and a particular mass function. Our solution describes the interior of anisotropic color flavor locked strange quark stars. All energy conditions are fulfilled, and therefore the solution obtained here is a realistic solution within General Relativity. The mass-to-radius profile is obtained, and the compactness of the star is computed.

gr-qc↗

arXiv2019

The partial ionisation zone of heavy elements in F-stars: a study on how it correlates with rotation

We study the relation between the internal structures of 10 benchmark main-sequence F-stars and their rotational properties. Stellar rotation of main-sequence F-type stars can be characterised by two distinct rotational regimes. Early-type F-stars are usually rapid rotators with periods typically below 10 days, whereas later-type F-stars have longer rotation periods. Specifically, and since the two rotational regimes are tightly connected to the effective temperatures of the stars, we investigate in detail the characteristics of the partial ionisation zones in the outer convective envelopes of these stars, which in turn, depend on the internal temperature profiles. Our study shows that the two rotational regimes might be distinguished by the relative locations of the partial ionisation region of heavy elements and the base of the convective zone. Since in all these stars is expected a dynamo-driven magnetic field where the shear layer between convective and radiative zones (tachocline) plays an important role, this result suggests that the magnetic field may be related to the combined properties of convection and ionisation.

astro-ph.SR↗

arXiv2019

Radial oscillations of boson stars made of ultralight repulsive dark matter

We compute the lowest frequency radial oscillation modes of boson stars. It is assumed that the object is made of pseudo-Goldstone bosons subjected to a scalar potential that leads to a repulsive self-interaction force, and which is characterized by two unknown mass scales $m$ (mass of the particle) and $F$ (decay constant). First we integrate the Tolman-Oppenheimer-Volkoff equations for the hydrostatic equilibrium of the star, and then we solve the Sturm-Liouville boundary value problem for the perturbations using the shooting method. The effective potential that enters into the Schr{ö}dinger-like equation as well as several associated eigenfunctions are shown as well. Moreover, we found that the large frequency separation, i.e. the difference between consecutive modes, is proportional to the square root of the mass of the star and the cube of the mass scale defined by $Λ\equiv \sqrt{m F}$.

gr-qc↗

arXiv2019

Acoustic modes of pulsating axion stars: Non-radial oscillations

We compute the lowest frequency non-radial oscillation modes of dilute axion stars. The effective potential that enters into the Schr{ö}dinger-like equation, several associated eigenfunctions, and the large as well as the small frequency separations are shown as well.

gr-qc↗

arXiv2019

Dark matter imprint on $^8$B neutrino spectrum

The next generation of solar neutrino detectors will provide a precision measure of the $^8$B electron-neutrino spectrum in the energy range from 1-15 MeV. Although the neutrino spectrum emitted by $^8$B $β$-decay reactions in the Sun's core is identical to the neutrino spectrum measured in the laboratory, due to vacuum and matter flavor oscillations, this spectrum will be very different from that measured on Earth by the different solar neutrino experiments. We study how the presence of dark matter (DM) in the Sun's core changes the shape of the $^8$B electron-neutrino spectrum. These modifications are caused by local variations of the electronic density and the $^8$B neutrino source, induced by local changes of the temperature, density and chemical composition. Particularly relevant are the shape changes at low and medium energy range $(E_ν\le 10 {\; \rm MeV})$, for which the experimental noise level is expected to be quite small. If such a distortion in the $^8$B$ν_e$ spectrum were to be observed, this would strongly hint in favor of the existence of DM in the Sun's core. The $^8$B electron-neutrino spectrum provides a complementary method to helioseismology and total neutrino fluxes for constraining the DM properties. In particular, we study the impact of light asymmetric DM on solar neutrino spectra. Accurate neutrino spectra measurements could help to determine whether light asymmetric DM exists in the Sun's core, since it has been recently advocated that this type of DM might resolve the solar abundance problem.

hep-ph↗

arXiv2018

The Sterile-Active Neutrino Flavor Model: the Imprint of Dark Matter on the Electron Neutrino Spectra

Contact interactions between sterile neutrinos and dark matter particles in a hidden sector have been suggested as a good solution to simultaneously resolve the dark matter problem and anomalies in neutrino experiments. In this non-standard particle physics model, sterile and active neutrinos change their through vacuum oscillations and matter (or Mikheyev--Smirnov--Wolfenstein) oscillations, in which the latter mechanism of flavor oscillation depends strongly on the concentration of dark matter in the Sun's core. We found that a large concentration of dark matter in the Sun's interior changes substantially the shape of ${\rm ^8B}$ and $\rm hep$ electron neutrino spectra, but has an insignificant impact on the other neutrino spectra (i.e., $\rm pp$, $\rm pep$, $\rm ^7Be$ and $\rm ^{15}O$, $\rm ^{13}N$ and $\rm ^{17}F$). The strength of the interaction of the dark matter particles with neutrinos depends on an effective coupling constant, $G_χ$, which is an analog of the Fermi constant for the hidden sector. By using the latest $\rm ^8B$ solar neutrino flux, we found that $G_χ$ must be smaller than $\rm 0.5\times 10^9$ $G_{\rm F}$ for this particle physics model to be in agreement with the data.

astro-ph.CO↗

arXiv2018

The spectroscopy of solar sterile neutrinos

We predict the sterile neutrino spectrum of some of the key solar nuclear reactions and discuss the possibility of these being observed by the next generation of solar neutrino experiments. By using an up-to-date standard solar model with good agreement with current helioseismology and solar neutrino flux data sets, we found that from solar neutrino fluxes arriving on Earth only 3\%-4\% correspond to the sterile neutrino. The most intense solar sources of sterile neutrinos are the $pp$ and $^7Be$ nuclear reactions with a total flux of $2.2\times 10^{9}\;{\rm cm^2 s^{-1}}$ and $1.8\times 10^{8}\;{\rm cm^2 s^{-1}}$, followed by the $^{13}N$ and $^{15}O$ nuclear reactions with a total flux of $1.9\times 10^{7}\;{\rm cm^2 s^{-1}}$ and $1.7\times 10^{7}\;{\rm cm^2 s^{-1}}$. Moreover, we compute the sterile neutrino spectra of the nuclear proton-proton nuclear reactions -- $pp$, $hep$ and $^8B$ and the carbon-nitrogen-oxygen -- $^{13}N$, $^{15}O$ and $^{17}F$ and the spectral lines of $^7Be$.

hep-ph↗

arXiv2018

Millisecond pulsars modelled as strange quark stars admixed with condensed dark matter

We study for the first time how a new class of stars could impact an ensemble of pulsars with known masses and spin-periods. These new compact objects are strange stars admixed with condensed dark matter. In this exploratory theoretical work, our goal is to determine how the basic parameters of pulsars are modified for such a new class of compact objects. In particular we consider three different scenarios that correspond to a dark matter mass fraction of $~5\%$, $~11\%$ and $~25\%$. Within each scenario with fixed parameters we predict theoretically other properties of the pulsars, such as the radius, the compactness, the moment of inertia, as well as the angular momentum. Our numerical results are summarized in tables and also shown graphically for better visualization, where a comparison between the different scenarios can be made.

gr-qc↗

arXiv2018

From cool to hot F-stars: the influence of two ionization regions in the acoustic oscillations

The high-precision data available from the Kepler satellite allows us to venture in the study of the complex outer convective envelopes of solar-type stars. We use a seismic diagnostic, specialized for investigating the outer layers of solar-type stars, to infer the impact of the ionization processes on the oscillation spectrum, for a sample of Kepler stars. These stars, of spectral type F, cover all of the observational seismic domain of the acoustic oscillation spectrum in solar-type stars. They also cover the range between a cool F-dwarf (\sim 6000 K) and a hotter F-star (\sim 6400 K). Our study reveals the existence of two relevant ionization regions. One of these regions, which is located closer to the surface of the star, is commonly associated with the second ionization of helium, although other chemical species also contribute to ionization. The second region, located deeper in the envelope, is linked with the ionization of heavy elements. Specifically, in this study, we analyze the elements carbon, nitrogen, oxygen, neon, and iron. Both regions can be related to the K electronic shell. We show that, while for cooler stars like the Sun, the influence of this second region on the oscillation frequencies is small; in hotter stars, its influence becomes comparable to the influence of the region of the second ionization of helium. This can guide us in the study of the outer layers of F-stars, specifically with the understanding of phenomena related to rotation and magnetic activity in these stars.

astro-ph.SR↗

arXiv2017

Gravitational Waves from Stellar Black Hole Binaries and the Impact on Nearby Sun-like Stars

We investigate the impact of resonant gravitational waves on quadrupole acoustic modes of Sun-like stars located nearby stellar black hole binary systems (such as GW150914 and GW151226). We find that the stimulation of the low-overtone modes by gravitational radiation can lead to sizeable photometric amplitude variations, much larger than the predictions for amplitudes driven by turbulent convection, which in turn are consistent with the photometric amplitudes observed in most Sun-like stars. For accurate stellar evolution models, using up-to-date stellar physics, we predict photometric amplitude variations of $1$ -- $10^3$ ppm for a solar mass star located at a distance between 1 au and 10 au from the black hole binary, and belonging to the same multi-star system. The observation of such a phenomenon will be within the reach of the Plato mission because telescope will observe several portions of the Milky Way, many of which are regions of high stellar density with a substantial mixed population of Sun-like stars and black hole binaries.

astro-ph.GA↗

arXiv2017

Quadrupole stellar oscillations: The impact of gravitational waves from the Galactic Center

Acoustic quadrupole modes of sunlike stars vibrate when perturbed by a passing gravitational wave generated somewhere in the Universe. Here, we compute the imprint of the gravitational waves on the acoustic spectrum of these stars for gravitational events occurring near the supermassive black hole located at the center of the Milky Way. We found that in most cases the impact of gravitational waves in low-order quadrupole modes is not above the current observational threshold of detectability, although this should be in the reach of the next generation of near infrared observatories and asteroseismology satellite missions. Equally, we found that it is possible to follow the end phase of the coalescence of binaries with large chirp masses, as these phenomena have a unique imprint in the spectra of sunlike stars affecting sequentially several low-order quadrupole modes. Moreover, we discuss the different imprints on the acoustic spectra of the different types of binary systems constituted either by two white dwarfs, two neutron stars, two black holes or a compact star and a massive black hole.

astro-ph.SR↗

arXiv2017

A Theoretical Study of the Outer Layers of Eight Kepler F-stars: The Relevance of Ionization Processes

We have analyzed the theoretical model envelopes of eight Kepler F-stars by computing the phase shift of the acoustic waves, $α(ω)$, and its related function, $β(ω)$. The latter is shown to be a powerful probe of the external stellar layers since it is particularly sensitive to the partial ionization zones located in these upper layers. We found that these theoretical envelopes can be organized into two groups, each of which is characterized by a distinct $β(ω)$ shape that we show to reflect the differences related to the magnitudes of ionization processes. Since $β(ω)$ can also be determined from the experimental frequencies, we compared our theoretical results with the observable $β(ω)$. Using the function $β(ω)$, and with the purpose of quantifying the magnitude of the ionization processes occurring in the outer layers of these stars, we define two indexes, $Δβ_1$ and $Δβ_2$. These indexes allow us to connect the microphysics of the interior of the star with macroscopic observable characteristics. Motivated by the distinct magnetic activity behaviors of F-stars, we studied the relation between the star's rotation period and these indexes. We found a trend, in the form of a power-law dependence, that favors the idea that ionization is acting as an underlying mechanism, which is crucial for understanding the relation between rotation and magnetism and even observational features such as the Kraft break.

astro-ph.SR↗

arXiv2017

New neutrino physics and the altered shapes of solar neutrino spectra

Neutrinos coming from the Sun's core are now measured with a high precision, and fundamental neutrino oscillations parameters are determined with a good accuracy. In this work, we estimate the impact that a new neutrino physics model, the so-called generalized Mikheyev-Smirnov-Wolfenstein (MSW) oscillation mechanism, has on the shape of some of leading solar neutrino spectra, some of which will be partially tested by the next generation of solar neutrino experiments. In these calculations, we use a high-precision standard solar model in good agreement with helioseismology data. We found that the neutrino spectra of the different solar nuclear reactions of the proton-proton chains and carbon-nitrogen-oxygen cycle have quite distinct sensitivities to the new neutrino physics. The $HeP$ and $^8B$ neutrino spectra are the ones for which their shapes are more affected when neutrinos interact with quarks in addition to electrons. The shape of the $^{15}O$ and $^{17}F$ neutrino spectra are also modified, although in these cases the impact is much smaller. Finally, the impact in the shape of the $PP$ and $^{13}N$ neutrino spectra is practically negligible.

astro-ph.SR↗