Radius Of Star Research Articles

Stellar mass and radius estimation using artificial intelligence

Context. Estimating stellar masses and radii for most stars is a challenge, but it is critical to know them for many different astrophysical fields, such as exoplanet characterization or stellar structure and evolution. One of the most extended techniques for estimating these variables is the so-called empirical relations. Aims. We propose a group of frontier artificial intelligence (AI) regression models, with the aim of studying their proficiency in estimating stellar masses and radii. We select the model that provides the best accuracy with the least possible bias. Some of these AI techniques do not treat uncertainties properly, but in the current context, in which statistical analyses of massive databases in different fields are conducted, the most accurate estimate possible of stellar masses and radii can provide valuable information. We publicly release the database, the AI models, and an online tool for stellar mass and radius estimation to the community. Methods. We used a sample of 726 MS stars from the literature with accurate M, R, Teff, L, log ɡ, and [Fe/H]. We split our data sample into training and testing sets and then analyzed the different AI techniques with them. In particular, we experimentally evaluated the accuracy of the following models: linear regression, Bayesian regression, regression trees, random forest, support-vector regression (SVR), neural networks, K-nearest neighbour, and stacking. We propose a series of experiments designed to evaluate the accuracy of the estimates, and also the generalization capability of AI models. We also analyzed the impact of reducing the number of input parameters and compared our results with those from current empirical relations in the literature. Results. We have found that stacking several regression models is the most suitable technique for estimating masses and radii. In the case of the mass, neural networks also provide precise results, and for the radius, SVR and neural networks work as well. Compared with other currently used empirical relation-based models, our stacking improves the accuracy by a factor of two for both variables. In addition, bias is reduced to one order of magnitude in the case of stellar mass. Finally, we found that using our stacking and only Teff and L as input features, the accuracies obtained are slightly higher than 5%, with a bias of ≈1.5%. In the case of the mass, including [Fe/H] significantly improves the results. For the radius, including log ɡ yields better results. Finally, the proposed AI models exhibit an interesting generalization capability: they are able to perform estimations for masses and radii that were never observed during the training step.

Dark matter admixed neutron star properties in light of gravitational wave observations: A two fluid approach

We consider the effect of density dependent dark matter on the neutron star mass, radius, and tidal deformability. Nuclear matter (normal matter) as well as the fermionic dark matter sector is considered in a mean field model. We adopt the two fluid formalism to investigate the effect of dark matter on the neutron star properties. In the two fluid picture, there is no direct interaction between the dark matter and the nuclear matter. Rather these two sectors interact only through gravitational interaction. The nuclear matter sector is described by the $\ensuremath{\sigma}\ensuremath{-}\ensuremath{\omega}\ensuremath{-}\ensuremath{\rho}$ meson interaction in the ``FSU2R'' parametrization. In the dark matter sector, we use the Bayesian parameter optimization technique to fix the unknown parameters in the dark matter equation of state. In the two fluid picture, we solve the coupled Tolman-Oppenheimer-Volkoff (TOV) equations to obtain the mass and radius of dark matter admixed neutron stars (DANSs). We also estimate the effect of the density dependent dark matter sector on the tidal deformability of dark matter admixed neutron stars (DANSs).

Constraining scalar-tensor theories using neutron star mass and radius measurements

We use neutron star mass and radius measurements to constrain the spontaneous scalarization phenomenon in scalar-tensor theories using Bayesian analysis. Neutron star structures in this scenario can be significantly different from the case of general relativity, which can be used to constrain the theory parameters. We utilize this idea to obtain lower bounds on the coupling parameter $\beta$ for the case of massless scalars. These constraints are currently weaker than the ones coming from binary observations, and they have relatively low precision due to the approximations in our method. Nevertheless, our results clearly demonstrate the power of the mass-radius data in testing gravity, and can be further improved with future observations. The picture is different for massive scalars, for which the same data is considerably less effective in constraining the theory parameters in an unexpected manner. We identify the main reason for this to be a large high-likelihood region in the parameter space where deviations from general relativity are relatively small. We hope this initial study to be an invitation to use neutron star structure measurements more commonly to test alternative theories in general.

Low-Energy Supernovae Severely Constrain Radiative Particle Decays.

The hot and dense core formed in the collapse of a massive star is a powerful source of hypothetical feebly interacting particles such as sterile neutrinos, dark photons, axionlike particles (ALPs), and others. Radiative decays such as a→2γ deposit this energy in the surrounding material if the mean free path is less than the radius of the progenitor star. For the first time, we use a supernova (SN) population with particularly low explosion energies as the most sensitive calorimeters to constrain this possibility. These SNe are observationally identified as low-luminosity events with low ejecta velocities and low masses of ejected ^{56}Ni. Their low energies limit the energy deposition from particle decays to less than about 0.1B, where 1 B(bethe)=10^{51} erg. For 1-500MeV-mass ALPs, this generic argument excludes ALP-photon couplings G_{aγγ} in the 10^{-10}-10^{-8} GeV^{-1} range.

X-ray eclipse mapping constrains the binary inclination and mass ratio of Swift J1858.6−0814

ABSTRACT X-ray eclipse mapping is a promising modelling technique, capable of constraining the mass and/or radius of neutron stars (NSs) or black holes (BHs) in eclipsing binaries and probing any structure surrounding the companion star. In eclipsing systems, the binary inclination, i, and mass ratio, q relate via the duration of totality, te. The degeneracy between i and q can then be broken through detailed modelling of the eclipse profile. Here, we model the eclipses of the NS low-mass X-ray binary Swift J1858.6−0814 utilizing archival NICER observations taken while the source was in outburst. Analogous to EXO 0748−676, we find evidence for irradiation driven ablation of the companion’s surface by requiring a layer of stellar material to surround the companion star in our modelling. This material layer extends ∼7000–14 000 km from the companion’s surface and is likely the cause of the extended, energy-dependent and asymmetric ingress and egress that we observe. Our fits return an inclination of i ∼ 81○ and a mass ratio q ∼ 0.14. Using Kepler’s law to relate the mass and radius of the companion star via the orbital period (∼21.3 h), we subsequently determine the companion to have a low mass in the range 0.183 M⊙ ≤ Mcs ≤ 0.372 M⊙ and a large radius in the range 1.02 R⊙ ≤ Rcs ≤ 1.29 R⊙. Our results, combined with future radial velocity amplitudes measured from stellar absorption/emission lines, can place precise constraints on the component masses in this system.

The Stability Analysis of Coupled Differential Equations in Stellar Structure

Astrophysical and Mathematical Models usually describes the evolutionary processes of life cycle of stellar structure. The process of star formation undergoes various stages during its life cycle. In human time scale it is not possible to observe whole of the evolution of stellar structure. From the birth of star to the death of star, different complex cosmological phenomenon occurs in the galactic region. In general theory of astrophysics stars are assumed to be static and spherical in nature. This nature results from hydrostatic and thermostatic equilibrium. Self-gravitating forces makes the star to collapse gradually and intermittently. At the same time thermal nature of chemical composition inside the star develops the pressure and hinders the sudden collapse. As a result, there is change in physical quantities like temperature, density, mass, radius, luminosity opacity and many more structural behaviours of star. These quantities are taken as variable in differential equation which are evolved in due process of star evolution. During the evolution of star, hydrostatic equilibrium and thermal equilibrium remains the major areas of study, therefore emphasis is laid on the stability of stellar structure with main focus on the hydrostatic and thermal equilibrium.

Crust Cooling of Soft X-Ray Transients—the Uncertainties of Shallow Heating

Crust cooling of soft X-ray transients has been observed after outbursts, but an additional shallow heating during accretion in outburst is needed to explain the quiescent light curve. However, shallow heating is significantly different between sources and even within one source between different outbursts, and the source of shallow heat is as yet unknown. Using the open source code “dStar” which solves the fully general relativistic heat diffusion equation for the crust, we investigate the effect of magnitude and depth of shallow heating on crust cooling and find that some exceptional sources (Swift J174805.3-244637, MAXI J0556-332 during outburst II and GRO J1750-27) in which shallow heating may be inactive could be explained by a deeper shallow heating mechanism. We compare our results with those from previous works and find that the shallow heating is model dependent. In addition, the effects of mass and radius of a neutron star on shallow heating are studied, and it is shown that the more compact the star, the less shallow heating will be needed to fit the crust cooling light curves.

Decoding the density dependence of the nuclear symmetry energy

The large imbalance in the neutron and proton densities in very neutron rich systems increases the nuclear symmetry energy so that it governs many aspects of neutron stars and their mergers. Extracting the density dependence of the symmetry energy therefore constitutes an important scientific objective. Many analyses have been limited to extracting values for the symmetry energy, S0, and its “derivative”, L, at saturation density ρ0≈2.6×1014g/cm3≈0.16nucleons/fm3, resulting in constraints that appear contradictory. We show that most experimental observables actually probe the symmetry energy at densities far from ρ0, making the extracted values of S0 or L imprecise. By focusing on the densities these observables actually probe, we obtain a detailed picture of the density dependence of the symmetry energy from 0.25ρ0 to 2.0ρ0. From this experimentally derived density functional, we extract L01=54±6 MeV corresponding to the symmetry pressure of P01=1.8±0.2 MeV/fm3 at ρ≈0.10fm−3, a neutron skin thickness for P208b of Rnp=0.23±0.03 fm, and symmetry pressure at saturation density of P0=4.4±1.3 MeV/fm3. The extrapolated symmetry pressure at high density is consistent with results from recent measurements of neutron star radii from NICER and deformability from LIGO.

Phase Behaviors of ABA Star Polymer and Nanoparticles Confined in a Sphere with Soft Inner Surface.

The phase behaviors of an ABA star polymer and nanoparticles confined in a sphere with soft inner surface, which is grafted with homopolymer brushes have been studied by the self-consistent field theory (SCFT). The morphologies of mixture in the center slice of sphere were focused. Two cases are considered: one is that the nanoparticles interact with the B blocks and the other is that the nanoparticles preferentially wet the B blocks. Under the two conditions, through changing the block ratio of the ABA star polymer, the concentration and radius of the nanoparticles, the phase behaviors of the mixtures confined the soft sphere are studied systematically. With increasing the concentration of nanoparticles, the entropy and the steric repulsive interaction of nanoparticles, and the nanoparticle density distributions along the perpendicular line through the center of sphere are plotted. The phase diagram is also constructed to analyze the effects of the nanoparticle volume fraction and radius on morphologies of ABA star polymers, and to study the effect of confinement on the phase behaviors. The results in this work provide a useful reference for controlling the ordered structures in experiment, which is an effective way to fabricate the newly multifunctional materials.

Uncertainty limits on neutron star radius measurements with gravitational waves

Upcoming observing campaigns with improved detectors will yield numerous detections of gravitational waves from neutron star binary inspirals. Rare loud signals together with numerous signals of moderate strength promise stringent constraints on the properties of neutron star matter, with a projected radius statistical uncertainty of $50-200$m with ${\cal{O}}(2000)$ sources. Given this precision we revisit all analysis assumptions and identify sources of systematic errors, quantify their impact on radius extraction, and discuss their relative importance and ways to mitigate them.

Six new compact triply eclipsing triples found withTESS

ABSTRACTIn this work, we report the discovery and analysis of six new compact triply eclipsing triple star systems found with the TESS mission: TICs 37743815, 42565581, 54060695, 178010808, 242132789, and 456194776. All of these exhibit distinct third-body eclipses where the inner eclipsing binary (EB) occults the third (‘tertiary’) star, or vice versa. We utilized the TESS photometry, archival photometric data, and available archival spectral energy distribution curves (SED) to solve for the properties of all three stars, as well as many of the orbital elements. We describe in detail our SED fits, search of the archival data for the outer orbital period, and the final global photodynamical analyses. From these analyses, we find that all six systems are coplanar to within 0°−5°, and are viewed nearly edge on (i.e. within a couple of degrees). The outer orbital periods and eccentricities of the six systems are {Pout (days), e}: {68.7, 0.36}, {123, 0.16}, {60.7, 0.01}, {69.0, 0.29}, {41.5, 0.01}, {93.9, 0.29}, respectively, in the order the sources are listed above. The masses of all 12 EB stars were in the range of 0.7–1.8 M⊙ and were situated near the main sequence. By contrast, the masses and radii of the tertiary stars ranged from 1.5 to 2.3 M⊙ and 2.9 to 12 R⊙, respectively. We use this information to estimate the occurrence rate of compact flat triple systems..

Asymmetric Nuclear Matter in Relativistic Mean-field Models with Isoscalar- and Isovector-meson Mixing

Using the relativistic mean-field model with nonlinear couplings between the isoscalar and isovector mesons, we study the properties of isospin-asymmetric nuclear matter. Not only the vector mixing, ω μ ω μ ρ ν ρ ν , but also the quartic interaction due to the scalar mesons, σ 2 δ 2, is taken into account to investigate the density dependence of nuclear symmetry energy, E sym, and the neutron star properties. It is found that the δ meson increases E sym at high densities, whereas the σ–δ mixing makes E sym soft above the saturation density. Furthermore, the δ meson and its mixing have a large influence on the radius and tidal deformability of a neutron star. In particular, the σ–δ mixing reduces the neutron star radius; thus, the present calculation can simultaneously reproduce the dimensionless tidal deformabilities of a canonical 1.4 M ⊙ neutron star observed from the binary neutron star merger GW170817 and the compact binary coalescence GW190814.

Observational features of hot spots on neutron stars with a photon sphere

Abstract We describe and illustrate light deflection near neutron stars possessing a photon sphere motivated by modern confirmations of general relativity and its significance for relativistic astrophysics. In this paper, we model the spacetime outside the star with the Schwarzschild metric and summarize the properties of the photon orbits. More especially we illustrate the relativistic appearance of the hot spots on the neutron star surfaces over the celestial screen of an asymptotic observer. We visualize the relativistic images of the neutron star surface for various ratios of neutron star radius to Schwarzschild radius. We establish a noticeable increscent of the visual size of ultra-compact neutron stars with a photon sphere, which could distinguish stars with extremely strong surface gravity with respect to less compact neutron stars. We found an appearance of additional relativistic images of the hot spots leading to a higher magnification of the neutron star luminosity emission regions.

Compact stellar model in Tolman spacetime in the presence of pressure anisotropy

In this paper, we develop a new relativistic compact stellar model for a spherically symmetric anisotropic matter distribution. The model has been obtained through generating a new class of solutions by invoking the Tolman ansatz for one of the metric potentials [Formula: see text] and a physically reasonable selective profile of radial pressure. We have matched our obtained interior solution to the Schwarzschild exterior spacetime over the bounding surface of the compact star. These matching conditions together with the condition of vanishing the radial pressure across the boundary of the star have been utilized to determine the model parameters. We have shown that the central pressure of the star depends on the parameter [Formula: see text]. We have estimated the range of [Formula: see text] by using the recent data of compact stars 4U 1608-52 and Vela X-1. The effect of [Formula: see text] on different physical parameters, e.g. pressure anisotropy, the subliminal velocity of sound, relativistic adiabatic index, etc., has also been discussed. The developed model of the compact star is elaborately discussed both analytically and graphically to justify that it satisfies all the criteria demanded by a realistic star. From our analysis, we have shown that the effect of anisotropy becomes small for higher values of [Formula: see text]. The mass–radius (MR) relationship which indicates the maximum mass admissible for observed pulsars for a given surface density has also been investigated in our model. Moreover, the variation of radius and mass with central density has been shown which allows us to estimate central density for a given radius (or mass) of a compact star. We have explored the tidal deformabilities by calculating the Love numbers and showed the variation of tidal Love numbers with the central pressure of a star.

Efficient method for estimating the time evolution of the proto-neutron star mass and radius from a supernova neutrino signal

ABSTRACTIn this paper, we present a novel method to estimate the time evolution of the proto-neutron star (PNS) structure from the neutrino signal in a core-collapse supernova (CCSN). Employing recent results from multidimensional CCSN simulations, we delve into a relation between the total emitted neutrino energy (TONE) and PNS mass/radius, and we find that they are strongly correlated with each other. We fit the relation by simple polynomial functions connecting the TONE to the mass and radius of the PNS as a function of time. By combining another fitting function representing the correlation between the TONE and the cumulative number of events at each neutrino observatory, the PNS mass and radius can be retrieved from purely observed neutrino data. We demonstrate retrievals of PNS mass and radius from mock data of the neutrino signal, and we assess the capability of our proposed method. While underlining the limitations of the method, we also discuss the importance of the joint analysis with the gravitational wave signal. This would reduce uncertainties of parameter estimations in our method, and may narrow down the possible neutrino oscillation model. The proposed method is a very easy and inexpensive computation, which will be useful in real data analysis of the CCSN neutrino signal.

Large and massive neutron stars: Implications for the sound speed within QCD of dense matter

The NASA telescope NICER has recently measured x-ray emissions from the heaviest of the precisely known two-solar mass neutron stars, PSR J0740+6620. Analysis of the data [Miller et al., Astrophys. J. Lett. 918, L28 (2021); Riley et al., Astrophys. J. Lett. 918, L27 (2021)] suggests that PSR J0740+6620 has a radius in the range of $R_{2.0} \approx (11.4-16.1)$ km at the $68\%$ credibility level. In this article, we study the implications of this analysis for the sound speed in the high-density inner cores by using recent chiral effective field theory ($\chi$EFT) calculations of the equation of state at next-to-next-to-next-to-leading order to describe outer regions of the star at modest density. We find that the lower bound on the maximum speed of sound in the inner core, $\textbf{min}\{c^2_{s, {\rm max}}\}$, increases rapidly with the radius of massive neutron stars. If $\chi$EFT remains an efficient expansion for nuclear interactions up to about twice the nuclear saturation density, $R_{2.0}\geqslant 13$ km requires $\textbf{min}\{c^2_{s, {\rm max}}\} \geqslant 0.562$ and $0.442$ at the $68\%$ and $95\%$ credibility level, respectively.

Inferring astrophysical parameters of core-collapse supernovae from their gravitational-wave emission

Nearby core-collapse supernovae (CCSNe) are powerful multimessenger sources for gravitational-wave, neutrino, and electromagnetic telescopes as they emit gravitational waves in the ideal frequency band for ground based detectors. Once a CCSN gravitational-wave signal is detected, we will need to determine the parameters of the signal and understand how those parameters relate to the source's explosion, progenitor, and remnant properties. This is a challenge due to the stochastic nature of CCSN explosions, which is imprinted on their time series gravitational waveforms. In this paper, we perform Bayesian parameter estimation of CCSN signals using an asymmetric chirplet signal model to represent the dominant high-frequency mode observed in spectrograms of CCSN gravitational-wave signals. We use design sensitivity Advanced LIGO noise and CCSN waveforms from four different hydrodynamical supernova simulations with a range of different progenitor stars. We determine how well our model can reconstruct time-frequency images of the emission modes and show how well we can determine parameters of the signal such as the frequency, amplitude, and duration. We show how the parameters of our signal model may allow us to place constraints on the protoneutron star mass and radius, the turbulent kinetic energy onto the protoneutron star, and the time of shock revival.

Physical viability of anisotropic compact stars solutions under Karmarkar condition in f(ℛ,𝒯) theory of gravity

In this paper, we investigate the generalized symmetric, static compact objects under anisotropic fluid in the background of Karmarkar embedding class-1 condition. To find the exact solutions of the modified field equations, we consider the gravitational Lagrangian as a linear function of the Ricci scalar and the trace of the stress–energy tensor, i.e. [Formula: see text], where [Formula: see text] is a constant. By employing the concept of Karmarkar condition, we calculate a [Formula: see text] metric component, while for a specific form of [Formula: see text], the gravitational potential is chosen. To attain the closed-form solutions of the modified field equations, we match the interior spacetime metric with the exterior Schwarzschild metric at the boundary and using the numerical values of mass and radius of the compact stars like PSR J1903+327, 4U 1820-30, LMC X-4, and EXO 1785-248, we constraint some unknown parameters. We have discussed physical stability of the stellar configuration by adopting the energy bounds, Tolman–Oppenheimer–Volkov (TOV) equation, adiabatic index, central density and pressure as well. It is concluded that our resulting anisotropic solutions did well behave and a reasonable degree of precision with recent observations.

Neutron stars in scalar–tensor gravity with quartic order scalar potential

In this work we investigate the effects of a non-minimally coupled quartic order scalar model on static neutron stars, with the non-minimal coupling in the Jordan frame being of the form f(ϕ)=1+ξϕ2. Particularly we derive the Einstein frame Tolman–Oppenheimer–Volkoff equations, and by numerically integrating them for both the interior and the exterior of the neutron star, using a double shooting python 3 based numerical code, we extract the masses and radii of the neutron stars evaluated finally in the Jordan frame, along with several other related physical quantities of interest. With regard to the equation of state for the neutron star, we use a piecewise polytropic equation of state with the central part being Skyrme-Lyon (SLy), Akmal–Pandharipande–Ravenhall (APR) or the Wiringa–Fiks–Fabrocini (WFF1) equations of state. The resulting M–R graphs are compatible with the observational bounds imposed by the GW170817 event which require the radius of a static M∼1.6M⊙ neutron star to be larger than R=10.68−0.04+15km and the radius of a static neutron star corresponding to the maximum mass of the star to be larger than R=9.6−0.03+0.14km. Moreover, the WFF1 EoS, which was excluded for static neutron stars in the context of general relativity, for the quartic order scalar model neutron star model provides realistic results compatible with the GW170817 event.

Neutron star mass formula with nuclear saturation parameters

We derive the empirical formulas for the neutron star mass and gravitational redshift as a function of the central density and specific combination of the nuclear saturation parameters, which are applicable to the stellar models constructed with the central density up to threefold nuclear saturation density. Combining the both empirical formulas, one also estimates the neutron star radius. In practice, we find that the neutron star mass (radius) can be estimated within $\sim 10\%$ (a few percent) accuracy by comparing the mass and radius evaluated with our empirical formulas to those determined with the specific equation of state. Since our empirical formulas directly connect the neutron star mass and radius to the nuclear saturation parameters, one can discuss the neutron star properties with the specific values of nuclear saturation parameters constrained via nuclear experiments.