Stellar Structure Research Articles

Physical Implications of Pure Lovelock Geometry on Stellar Structure

AbstractAn exact anisotropic star model with a linear barotropic equation of state and with Finch‐Skea potential is constructed within the framework of pure Lovelock gravity. A comparison with the corresponding Einstein model in a suitable limit is easily deduced. Evidently higher curvature effects induced by the Lovelock contributions generate lower densities, pressures, surface tensions, and anisotropy factors when compared to its Einstein counterpart. The maximum moment of inertia is attained for the Einstein case and hence it may be inferred that Lovelock effects soften the equation of state. The model satisfies various stability tests.

Compact stars in the Einstein dark energy model

We investigate the properties of high density compact objects in a vector type theory, inspired by Einstein's 1919 theory of elementary particles, in which Einstein assumed that elementary particles are held together by gravitational as well as electromagnetic type forces. From a modern perspective, Einstein's theory can be interpreted as a vector type model, with the gravitational action constructed as a linear combination of the Ricci scalar, of the trace of the matter energy-momentum tensor, and of a massive self-interacting vector type field. To obtain the properties of stellar models we consider the field equations for a static, spherically symmetric system, and we investigate numerically their solutions for different equations of state of quark and neutron matter, by assuming that the self-interaction potential of the vector field either vanishes or is quadratic in the vector field potential. We consider quark stars described by the MIT bag model equation of state and in the Color Flavor Locked phase, as well as compact stars consisting of a Bose-Einstein Condensate of neutron matter, with neutrons forming Cooper pairs. Constant density stars, representing a generalization of the Interior Schwarzschild solution of general relativity, are also analyzed. Also, we consider the Douchin-Haensel (SLy) equation of state. The numerical solutions are explicitly obtained in both standard general relativity, and the Einstein dark energy model and an in depth comparison between the astrophysical predictions of these two theories are performed. As a general conclusion of our study, we find that for all the considered equations of state a much larger variety of stellar structures can be obtained in the Einstein dark energy model, including classes of stars that are more massive than their general relativistic counterparts.

Self-consistent modelling of the Milky Way’s nuclear stellar disc

Abstract The nuclear stellar disc (NSD) is a flattened high-density stellar structure that dominates the gravitational field of the Milky Way at Galactocentric radius $30\, {\rm pc}\lesssim R\lesssim 300\, {\rm pc}$. We construct axisymmetric self-consistent equilibrium dynamical models of the NSD in which the distribution function is an analytic function of the action variables. We fit the models to the normalized kinematic distributions (line-of-sight velocities + VIRAC2 proper motions) of stars in the NSD survey of Fritz et al., taking the foreground contamination due to the Galactic Bar explicitly into account using an N-body model. The posterior marginalized probability distributions give a total mass of $M_{\rm NSD} = 10.5^{+1.1}_{-1.0} \times 10^8 \, \, \rm M_\odot$, roughly exponential radial and vertical scale lengths of $R_{\rm disc} = 88.6^{+9.2}_{-6.9} \, {\rm pc}$ and $H_{\rm disc}=28.4^{+5.5}_{-5.5} \, {\rm pc}$, respectively, and a velocity dispersion $\sigma \simeq 70\, {\rm km\, s^{-1}}$ that decreases with radius. We find that the assumption that the NSD is axisymmetric provides a good representation of the data. We quantify contamination from the Galactic Bar in the sample, which is substantial in most observed fields. Our models provide the full 6D (position + velocity) distribution function of the NSD, which can be used to generate predictions for future surveys. We make the models publicly available as part of the software package agama.

A search for stellar structures around nine outer halo globular clusters in the Milky Way

ABSTRACT We use deep imaging from the Dark Energy Camera to explore the peripheral regions of nine globular clusters in the outer halo of the Milky Way. Apart from Whiting 1 and NGC 7492, which are projected against the Sagittarius stream, we see no evidence for adjacent stellar populations to indicate any of these clusters is associated with coherent tidal debris from a destroyed host dwarf. We also find no evidence for tidal tails around any of the clusters in our sample; however, both NGC 1904 and 6981 appear to possess outer envelopes. Motivated by a slew of recent Gaia-based discoveries, we compile a sample of clusters with robust detections of extra-tidal structure, and search for correlations with orbital properties. While we observe that clusters with tidal tails are typically on moderately or very eccentric orbits that are highly inclined to the Galactic plane and often retrograde, these are neither necessary nor sufficient conditions for the formation of extra-tidal structure. That many objects with tidal tails appear to be accreted leads us to speculate that this lack of consistency may stem from the inhomogeneous dynamical history of the Milky Way globular cluster system. Finally, we note that clusters with prominent stellar envelopes detected in ground-based imaging (such as NGC 1851 and 7089) are now all known from Gaia to possess long tidal tails – experimental confirmation that the presence of an extended envelope is indicative of tidal erosion.

Nodal precession of WASP-33b for 11 yr by Doppler tomographic and transit photometric observations

ABSTRACTWASP-33b, a hot Jupiter around a hot star, is a rare system in which nodal precession has been discovered. We updated the model for the nodal precession of WASP-33b by adding new observational points. Consequently, we found a motion of the nodal precession spanning 11 yr. We present homogenous Doppler tomographic analyses of eight data sets, including two new data sets from TS23 and HIDES, obtained between 2008 and 2019, to illustrate the variations in the projected spin–orbit obliquity of WASP-33b and its impact parameter. We also present its impact parameters based on photometric transit observations captured by MuSCAT in 2017 and MuSCAT2 in 2018. We derived its real spin–orbit obliquity ψ, stellar spin inclination is, and stellar gravitational quadrupole moment J2 from the time variation models of the two orbital parameters. We obtained $\psi = 108.19^{+0.95}_{-0.97}$ deg, $i_\mathit{ s} = 58.3^{+4.6}_{-4.2}$ deg, and $J_2=(1.36^{+0.15}_{-0.12}) \times 10^{-4}$. Our J2 value was slightly smaller than the theoretically predicted value, which may indicate that its actual stellar internal structure is different from the theoretical one. We derived the nodal precession speed $\dot{\theta }=0.507^{+0.025}_{-0.022}$ deg yr−1, and its period $P_{\mathrm{pre}}=709^{+33}_{-34}$ yr, and found that WASP-33b transits in front of WASP-33 for only ∼ 20 per cent of the entire nodal precession period.

Numerical experiments to help understand cause and effect in massive star evolution

ABSTRACT The evolution of massive stars is affected by a variety of physical processes, including convection, rotation, mass-loss, and binary interaction. Because these processes modify the internal chemical abundance profiles in multiple ways simultaneously, it can be challenging to determine which properties of the stellar interior are primarily driving the overall evolution. Building on previous work, we develop a new modelling approach called snapshot that allows us to isolate the key features of the internal abundance profile that drive the evolution of massive stars. Using our approach, we compute numerical stellar structure models in thermal equilibrium covering key phases of stellar evolution. For the main sequence, we demonstrate that models with the same mass and very similar surface properties can have different internal distributions of hydrogen and convective core masses. We discuss why massive stars expand after the main sequence and the fundamental reasons for why they become red, blue or yellow supergiants. For the post-main sequence, we demonstrate that small changes in the abundance profile can cause very large effects on the surface properties. We also discuss the effects that produce blue supergiants and the cause of blue loops. Our models show that massive stars with lower metallicity tend to be more compact due to the combined effect of lower CNO abundances in the burning regions and lower opacity in the envelope.

The Smallest Scale of Hierarchy Survey (SSH) – II. Extended star formation and bar-like features in the dwarf galaxy NGC 3741: recent merger or ongoing gas accretion?

ABSTRACTUsing Large Binocular Telescope deep imaging data from the Smallest Scale of Hierarchy Survey (SSH) and archival Hubble Space Telescope data, we reveal the presence of two elongated stellar features contiguous to a bar-like stellar structure in the inner regions of the dwarf irregular galaxy NGC 3741. These structures are dominated by stars younger than a few hundred Myr and collectively are about twice as extended as the old stellar component. These properties are very unusual for dwarf galaxies in the nearby Universe and difficult to explain by hydro-dynamical simulations. From the analysis of archival 21-cm observations, we find that the young stellar ‘bar’ coincides with an H i high-density region proposed by previous studies to be a purely gaseous bar; we furthermore confirm radial motions of a few km s−1, compatible with an inflow/outflow, and derive a steeply rising rotation curve and high H i surface density at the centre, indicating a very concentrated mass distribution. We propose that the peculiar properties of the stellar and gaseous components of NGC 3741 may be explained by a recent merger or ongoing gas accretion from the intergalactic medium, which caused gas inflows towards the galaxy centre and triggered star formation a few hundred Myr ago. This event may explain the young and extended stellar features, the bar-like structure, the very extended H i disc and the central H i spiral arms. The high central H i density and the steeply rising rotation curve suggest that NGC 3741 may be the progenitor or the descendant of a starburst dwarf.

The Contribution of M-dwarf Flares to the Thermal Escape of Potentially Habitable Planet Atmospheres

Abstract The habitability of planets around M dwarfs (≲0.5 M ⊙) can be affected by the X-rays + extreme UV (XUV) emission of these stars, with flares occasionally increasing the XUV flux by more than 2 orders of magnitude above quiescent levels. This wavelength range can warm and ionize terrestrial planets’ upper atmospheres, which expands the planetary radius and promotes atmospheric loss. In this work, we study the contribution of the XUV flux due to flares on the atmospheric escape of Earth-like planets orbiting M dwarfs through numerical simulations. We considered the first Gyr of planets with initial surface water abundances between 1 and 10 terrestrial oceans (TO), a small primordial hydrogen envelope (≤10−3 M ⊕), and with host-star masses between 0.2 and 0.6 M ⊙. In this parameter range, we find that flares can remove up to two TO more than nonflaring stars, which, in some cases, translates to a doubling of the total water loss. We also find that flaring can increase atmospheric oxygen partial pressures by hundreds of bars in some cases. These results were obtained by adding a new module for flares to the VPLanet software package and upgrading its atmospheric escape module to account for Roche lobe overflow and radiation/recombination-limited escape.

The VMC survey – XLVII. Turbulence-controlled hierarchical star formation in the Large Magellanic Cloud

ABSTRACT We perform a statistical clustering analysis of upper main-sequence stars in the Large Magellanic Cloud (LMC) using data from the Visible and Infrared Survey Telescope for Astronomy survey of the Magellanic Clouds. We map over 2500 young stellar structures at 15 significance levels across ∼120 square degrees centred on the LMC. The structures have sizes ranging from a few parsecs to over 1 kpc. We find that the young structures follow power-law size and mass distributions. From the perimeter–area relation, we derive a perimeter–area dimension of 1.44 ± 0.20. From the mass–size relation and the size distribution, we derive two-dimensional fractal dimensions of 1.50 ± 0.10 and 1.61 ± 0.20, respectively. We find that the surface density distribution is well represented by a lognormal distribution. We apply the Larson relation to estimate the velocity dispersions and crossing times of these structures. Our results indicate that the fractal nature of the young stellar structures has been inherited from the gas clouds from which they form and that this architecture is generated by supersonic turbulence. Our results also suggest that star formation in the LMC is scale-free from 10 to 700 pc.

Anisotropic spheres via embedding approach in f(R) gravity

This study explores the anisotropic stellar structures in the background of f(R) modified gravity, where R is a Ricci scalar. To complete this analysis, we use the spherically symmetric space-time. Further, an anisotropic source of matter is used to investigate the stability of stellar structures. The embedded class-I approach is used to demonstrate the behavior of stellar structures. Schwarzschild space-time is taken as exterior space-time to calculate the values of involved parameters. The observational data of three different stars, like LMC X-4, Cen X-3, and EXO 1785-248, are taken for the current analysis. For this study, we used Starobinsky-like model f(R) = R + βR2 for f(R) modified gravity. The stability analysis is discussed via the equation of state (EoS) parameters, Tolman–Oppenheimer–Volkoff (TOV) equation, anisotropy, etc. Graphical and analytical results are provided as viable results in f(R) gravity.

Manifold Alignment Aware Ants: A Markovian Process for Manifold Extraction.

The presence of manifolds is a common assumption in many applications, including astronomy and computer vision. For instance, in astronomy, low-dimensional stellar structures, such as streams, shells, and globular clusters, can be found in the neighborhood of big galaxies such as the Milky Way. Since these structures are often buried in very large data sets, an algorithm, which can not only recover the manifold but also remove the background noise (or outliers), is highly desirable. While other works try to recover manifolds either by pushing all points toward manifolds or by downsampling from dense regions, aiming to solve one of the problems, they generally fail to suppress the noise on manifolds and remove background noise simultaneously. Inspired by the collective behavior of biological ants in food-seeking process, we propose a new algorithm that employs several random walkers equipped with a local alignment measure to detect and denoise manifolds. During the walking process, the agents release pheromone on data points, which reinforces future movements. Over time the pheromone concentrates on the manifolds, while it fades in the background noise due to an evaporation procedure. We use the Markov chain (MC) framework to provide a theoretical analysis of the convergence of the algorithm and its performance. Moreover, an empirical analysis, based on synthetic and real-world data sets, is provided to demonstrate its applicability in different areas, such as improving the performance of t-distributed stochastic neighbor embedding (t-SNE) and spectral clustering using the underlying MC formulas, recovering astronomical low-dimensional structures, and improving the performance of the fast Parzen window density estimator.

Study of anisotropic and non-uniform Gravastars by gravitational decoupling in [formula omitted] gravity

Adopting gravitational decoupling through minimal geometric deformation (MGD) procedure, we develop an analytical version of gravastar model with non-uniform and anisotropic features, in the framework of f(R,T) gravity. This new non-uniform model describes an ultracompact stellar structure of radius RS=2M, whose interior solution smoothly joins a conformally deformed Schwarzschild exterior solution, and also it is matched to the standard Schwarzschild exterior solution under some restrictions on f(R,T) coupling constant λ. The constructed solution presents a family of stellar models satisfying some of the fundamental properties of a stable configuration, including a positive energy density everywhere with monotonically decreasing behavior from the center to surface. Besides, a non-uniform pressure is observed with monotonic behavior. All the energy conditions except the strong one are satisfied inside the ultracompact stellar configuration for all the values of f(R,T) coupling constant λ, which are compatible with regularity condition.

Study of anisotropic polytropes in f (, T) Theory

This paper examines the general formalism and applications of isotropic as well as anisotropic polytropic stars in curvature-matter coupled gravity. For this purpose, we consider static spherical and Schwarzschild spacetimes in the interior and exterior regions, respectively. We use two polytropic equations of state to obtain physically viable solutions of the field equations. The hydrostatic equilibrium and Lane-Emden equations are developed for both isotropic as well as anisotropic cases. We study the effects of anisotropic pressure on the stellar structure. Moreover, we graphically inspect the physical behavior of isotropic as well as anisotropic polytropes through energy conditions and stability criterion. Finally, we discuss Tolman mass to explore some characteristics of the models. It is concluded that more viable and stable polytropes are found in this theory as compared to general relativity.

Class I stars in the background of [formula omitted] cosmological model

The present paper is designed to study the structure of some compact objects such as PSRJ1614−2230,LMXB4U1608−52,CenX−3,EXO1785−248 and SMCX−1 in the framework of f(T,τ) gravity, where τ and T represent the trace of energy–momentum tensor and torsion scalar, respectively. For this work, we use anisotropic fluid distribution filled in spherical symmetric geometry providing the interior of star model and utilize the off-diagonal tetrad components for deriving the corresponding set of field equations. We construct stellar structure by using a well-known model of f(T,τ) gravity, given by f(T,τ)=αT(r)2+βτ(r)+ϕ, where α, β and ϕ are arbitrary constants. This model has interesting cosmological properties, in addition to providing an effective dark energy sector explanation (Harko et al., 2014). For checking viability of the constructed model, we set α<0, β<0 and discuss physical and stability features like anisotropy, energy conditions, EoS parameters, TOV equation, redshift, compactness and mass function of strange stars. It is found that these features comprehensively match with the realistic nature of compact stars in the realm of f(T,τ) gravity.

A look into the mass–radius relation of the neutron star in modified Chaplygin gas background

Motivated by the work of Astashenok et al. [Causal limit of neutron star maximum mass in f(R) gravity in view of GW190814, Phys. Lett. B 816 (2021) 136222], the present work aims at studying the behavior of f in f(R) gravity and attempts to study the causal mass–radius relation of a massive neutron star. To initiate the study, the background evolution is taken as of modified Chaplygin gas. We observed the positive behavior of f. The Tolman–Oppenheimer–Volkoff equation has been taken into consideration which states the spherical stellar structure of non-rotating neutron stars. We also observed that the mass–radius relationship is close to the Tolman–Oppenheimer–Volkoff limit. The change of density of neutron stars with respect to the radius clearly manifests the actual variation of density from the core to the surface of a neutron star in the background evolution of modified Chaplygin gas, which can unify the early inflation with the late time acceleration of the universe.

Electrically charged quark stars in 4D Einstein\u2013Gauss\u2013Bonnet gravity

In this work we study the properties of compact spheres made of a charged perfect fluid with a MIT bag model EoS for quark matter. Considering static spherically symmetric spacetime we derive the hydrostatic equilibrium equations in the recently formulated four dimensional Einstein–Gauss–Bonnet (4D EGB) gravity theory. In this setting, the modified TOV equations are solved numerically with the aim to investigate the impact of electric charge on the stellar structure. A nice feature of 4D EGB theory is that the Gauss–Bonnet term has a non-vanishing contribution to the gravitational dynamics in 4D spacetime. We therefore analyse the effects of Gauss–Bonnet coupling constant alpha and the charge fraction beta on the mass–radius (M{-}R) diagram and also the mass–central density (M{-}rho _c) relation of quark stars. Finally, we conclude that depending on the choice of coupling constant one could have larger mass and radius compared with GR and can also be relevant for more massive compact objects due to the effect of the repulsive Coulomb force.

Dynamical parallax, physical parameters, and evolutionary status of the components of the bright eclipsing binary α Draconis

Aims. Both components of the bright eclipsing binary α Dra have been resolved using long baseline interferometry and the secondary component has been shown to contribute approximately 15% of the total flux; however, a spectroscopic detection of the companion star has so far been unsuccessful. We aim for a firm spectroscopic detection of the secondary component of α Dra using state-of-the-art spectroscopic analysis methods for very high-quality spectroscopic observations. This will allow the determination of fundamental and atmospheric properties of the components in the system with high precision and accuracy. Methods. To achieve our goals, we use a combined data set from interferometry with the Navy Precision Optical Interferometer (NPOI), photometry with the TESS space observatory, and high-resolution spectroscopy with the HERMES fibre-fed spectrograph at the La Palma observatory. We use the method of spectral disentangling to search for the contribution of a companion star in the observed composite HERMES spectra, to separate the spectral contributions of both components, and to determine orbital elements of the α Dra system. TESS light curves are analysed in an iterative fashion with spectroscopic inference of stellar atmospheric parameters to determine fundamental stellar properties and their uncertainties. Finally, NPOI interferometric measurements are used for determination of the orbital parameters of the system and angular diameters of both binary components. Results. We report the first firm spectroscopic detection of the secondary component in α Dra and deliver disentangled spectra of both binary components. The components’ masses and radii are inferred with high precision and accuracy, and are MA = 3.186 ± 0.044 M⊙, RA = 4.932 ± 0.036 R⊙, and MB = 2.431 ± 0.019 M⊙, RB = 2.326 ± 0.052 R⊙ for the primary and secondary components, respectively. Combined astrometric and spectroscopic analysis yields the semi-major axis of the system, which is ultimately used to derive the dynamical parallax of π = 11.48 ± 0.13 mas, and the distance d = 87.07 ± 1.03 pc to the α Dra system. Evolutionary analysis of both binary components with MESA stellar structure and evolution models suggests the primary is an evolved post-TAMS A-type star, while the companion is a main-sequence A-type star with a convective core mass of Mcc = 0.337 ± 0.011 M⊙. Positions of both binary components in the Kiel- and HR-diagrams suggest a value of the convective core overshooting parameter fov well below 0.010 Hp, and we infer the age of the system to be 310 ± 25 Myr. Conclusions. The inferred near-core mixing properties of both components do not support a dependence of the convective core overshooting on the stellar mass. At the same time, the α Dra system provides extra support to hypothesise that the mass discrepancy in eclipsing spectroscopic double-lined binaries is associated with inferior atmospheric modelling of intermediate- and high-mass stars, and less so with the predictive powerof stellar structure and evolution models as to the amount of near-core mixing and mass of the convective core.

The extended atmosphere and circumstellar environment of the cool evolved star VX Sagittarii as seen by MATISSE

Context. VX Sgr is a cool, evolved, and luminous red star whose stellar parameters are difficult to determine, which affects its classification. Aims. We aim to spatially resolve the photospheric extent as well as the circumstellar environment. Methods. We used interferometric observations obtained with the MATISSE instrument in the L (3–4 μm), M (4.5–5 μm), and N (8–13 μm) bands. We reconstructed monochromatic images using the MIRA software. We used 3D radiation-hydrodynamics simulations carried out with CO5BOLD and a uniform disc model to estimate the apparent diameter and interpret the stellar surface structures. Moreover, we employed the radiative transfer codes OPTIM3D and RADMC3D to compute the spectral energy distribution for the L, M, and N bands, respectively. Results. MATISSE observations unveil, for the first time, the morphology of VX Sgr across the L, M, and N bands. The reconstructed images show a complex morphology with brighter areas whose characteristics depend on the wavelength probed. We measured the angular diameter as a function of the wavelength and showed that the photospheric extent in the L and M bands depends on the opacity through the atmosphere. In addition to this, we also concluded that the observed photospheric inhomogeneities can be interpreted as convection-related surface structures. The comparison in the N band yielded a qualitative agreement between the N-band spectrum and simple dust radiative transfer simulations. However, it is not possible to firmly conclude on the interpretation of the current data because of the difficulty in constraing the model parameters using the limited accuracy of our absolute flux calibration. Conclusions. MATISSE observations and the derived reconstructed images unveil the appearance of VX Sgr’s stellar surface and circumstellar environment across a very large spectral domain for the first time.

Relationships between Stellar Velocity Dispersion and the Atmospheres of Early-type Galaxies

Abstract The Voit et al. black hole feedback valve model predicts relationships between stellar velocity dispersion and atmospheric structure among massive early-type galaxies. In this work, we test that model using the Chandra archival sample of 49 early-type galaxies from Lakhchaura et al. We consider relationships between stellar velocity dispersion and entropy profile slope, multiphase gas extent, and the ratio of cooling time to freefall time. We also define subsamples based on data quality and entropy profile properties that clarify those relationships and enable more specific tests of the model predictions. We find that the atmospheric properties of early-type galaxies generally align with the predictions of the Voit et al. model, in that galaxies with a greater stellar velocity dispersion tend to have radial profiles of pressure, gas density, and entropy with steeper slopes and less extended multiphase gas. Quantitative agreement with the model predictions improves when the sample is restricted to have low central entropy and a stellar velocity dispersion of between 220 and 300 km s−1.

Predictions for Gravity-mode Periods and Surface Abundances in Intermediate-mass Dwarfs from Shear Mixing and Radiative Levitation

Abstract The treatment of chemical mixing in the radiative envelopes of intermediate-mass stars has hardly been calibrated so far. Recent asteroseismic studies demonstrated that a constant diffusion coefficient in the radiative envelope is not able to explain the periods of trapped gravity modes in the oscillation spectra of γ Doradus pulsators. We present a new generation of MESA stellar models with two major improvements. First, we present a new implementation for computing radiative accelerations and Rosseland mean opacities that requires significantly less CPU time. Second, the inclusion of shear mixing based on rotation profiles computed with the 2D stellar structure code ESTER is considered. We show predictions for the mode periods of these models covering stellar masses from 1.4 to 3.0 M ⊙ across the main sequence, computed for different metallicities. The morphology of the chemical mixing profile resulting from shear mixing in combination with atomic diffusion and radiative levitation does allow for mode trapping, while the diffusion coefficient in the outer envelope is large (&gt;106 cm2 s−1). Furthermore, we make predictions for the evolution of surface abundances for which radiative accelerations can be computed. We find that the N/C and C/O abundance ratios correlate with stellar age. We predict that these correlations are observable with precisions ≲ 0.1 dex on these ratios, given that a precise age estimate can be made.