Classification of magnetized star--planet interactions: bow shocks, tails, and inspiraling flows

Close-in exoplanets interact with their host stars gravitationally as well as via their magnetized plasma outflows. The rich dynamics that arises may result in distinct observable features. Our objective is to study and classify the morphology of the different types of interaction that can take place between a giant close-in planet (a Hot Jupiter) and its host star, based on the physical parameters that characterize the system. We perform 3D magnetohydrodynamic numerical simulations to model the star--planet interaction, incorporating a star, a Hot Jupiter, and realistic stellar and planetary outflows. We explore a wide range of parameters and analyze the flow structures and magnetic topologies that develop. Our study suggests the classification of star--planet interactions into four general types, based on the relative magnitudes of three characteristic length scales that quantify the effects of the planetary magnetic field, the planetary outflow, and the stellar gravitational field in the interaction region. We describe the dynamics of these interactions and the flow structures that they give rise to, which include bow shocks, cometary-type tails, and inspiraling accretion streams. We point out the distinguishing features of each of the classified cases and discuss some of their observationally relevant properties. The magnetized interactions of star--planet systems can be categorized, and their general morphologies predicted, based on a set of basic stellar, planetary, and orbital parameters.Comment: Accepted for publication in A&

Covariant formulation of refracted gravity

We propose a covariant formulation of refracted gravity (RG), a classical theory of gravity based on the introduction of gravitational permittivity (GP), a monotonic function of the local mass density, in the standard Poisson equation. GP mimics dark matter (DM) phenomenology. The covariant formulation of RG (CRG) that we propose belongs to the class of scalar-tensor theories, where the scalar field $\varphi$ has a self-interaction potential $V(\varphi)=-\Xi\varphi$, with $\Xi$ a normalization constant. We show that $\varphi$ is twice the GP in the weak-field limit. Far from a spherical source of density $\rho_s(r)$, the transition between the Newtonian and the RG regime appears below the acceleration scale $a_\Xi=(2\Xi-8\pi G\rho/\varphi)^{1/2}$, with $\rho=\rho_s+\rho_{bg}$, $\rho_{bg}$ being an isotropic and homogeneous background. In the limit $2\Xi\gg 8\pi G\rho/\varphi$, we obtain $a_\Xi\sim 10^{-10}$~m~s$^{-2}$. This is comparable to the acceleration $a_0$ originally introduced in MOND. From CRG, we also derived the modified Friedmann equations for an expanding, homogeneous, and isotropic universe. We find that the same scalar field that mimics DM also drives the accelerated expansion of the Universe. From the stress-energy tensor of $\varphi$, we derived the equation of state of a redshift-dependent effective dark energy (DE) $w_{DE}=p_{DE}/\rho_{DE}$. Current observational constraints on $w_{DE}$ and distance modulus data of SNIa suggest that $\Xi$ has a comparable value to the cosmological constant $\Lambda$ in the standard model. CRG, therefore, suggests a natural explanation of the known relation $a_0\sim \Lambda^{1/2}$ and appears to describe both the dynamics of cosmic structure and the expanding Universe with a single scalar field, highlighting a possible deep connection between phenomena currently attributed to DM and DE separately.Comment: 16 pages, 6 appendices, 3 figures, matches the accepted version in A&

Dynamics of DiskMass Survey galaxies in refracted gravity

We test if Refracted Gravity (RG) can describe the dynamics of disk galaxies without resorting to dark matter. RG is a classical theory of gravity where the standard Poisson equation is modified by the gravitational permittivity, $\epsilon$, a universal monotonic function of the local mass density. We use the rotation curves and the vertical velocity dispersions of 30 galaxies in the DiskMass Survey (DMS) to determine $\epsilon$. RG describes the kinematic profiles with mass-to-light ratios consistent with SPS models, and disk thicknesses in agreement with observations, once observational biases are considered. Our results rely on setting the three free parameters of $\epsilon$ for each galaxy. However, we show that the differences of these parameters from galaxy to galaxy could be ascribed to statistical fluctuations. We adopt an approximate method to find a single set of parameters that may properly describe the kinematics of the entire sample and suggest the universality of $\epsilon$. Finally, we show that the RG models of the individual rotation curves can only partly describe the radial acceleration relation (RAR). Evidently, the RG models underestimate the observed accelerations of 0.1-0.3 dex at low Newtonian accelerations. Another problem is the strong correlation, at much more than 5$\sigma$, between the residuals of the RAR models and three radially-dependent properties of galaxies, whereas the DMS data show a considerably less significant correlation, at more than 4$\sigma$, for only two of them. These correlations might originate the non-null intrinsic scatter of the RG models, at odds with the observed intrinsic scatter of galaxy samples different from DMS, which is consistent with 0. Further studies are required to assess if these discrepancies in the RAR originate from the DMS sample, which might not be ideal for deriving the RAR, or if they are genuine failures of RG.Comment: 36 pages, 22 figures, 7 tables, published in Astronomy & Astrophysics, Section 2. Astrophysical processes of Astronomy and Astrophysics; v2: minor corrections due to editorial process, notes added below the tables, arXiv references updated, italics removed from the titl

Covariant formulation of refracted gravity

We propose a covariant formulation of refracted gravity (RG), which is a classical theory of gravity based on the introduction of gravitational permittivity – a monotonic function of the local mass density – in the standard Poisson equation. Gravitational permittivity mimics dark matter phenomenology. The covariant formulation of RG (CRG) that we propose belongs to the class of scalar-tensor theories, where the scalar field φ has a self-interaction potential (φ) = − Ξφ, with Ξ being a normalization constant. We show that the scalar field is twice the gravitational permittivity in the weak-field limit. Far from a spherical source of density ρs(r), the transition between the Newtonian and the RG regime appears below the acceleration scale aΞ = (2Ξ − 8πGρ/φ)1/2, with ρ = ρs + ρbg and ρbg being an isotropic and homogeneous background. In the limit 2Ξ ≫ 8πGρ/φ, we obtain aΞ ∼ 10−10 m s−2. This acceleration is comparable to the acceleration a0 originally introduced in MOdified Newtonian Dynamics (MOND). From CRG, we also derived the modified Friedmann equations for an expanding, homogeneous, and isotropic universe. We find that the same scalar field φ that mimics dark matter also drives the accelerated expansion of the Universe. From the stress-energy tensor of φ, we derived the equation of state of a redshift-dependent effective dark energy wDE = pDE/ρDE. Current observational constraints on wDE and distance modulus data of type Ia supernovae suggest that Ξ has a comparable value to the cosmological constant Λ in the standard model. Since Ξ also plays the same role of Λ, CRG suggests a natural explanation of the known relation a0 ∼ Λ1/2. CRG thus appears to describe both the dynamics of cosmic structure and the expanding Universe with a single scalar field, and it falls within the family of models that unify the two dark sectors, highlighting a possible deep connection between phenomena currently attributed to dark matter and dark energy separately