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We study the conformally reduced R+R2 theory of gravity and we show that the theory is asymptotically safe with an ultraviolet critical manifold of dimension three. In particular, we discuss the universality properties of the fixed point and its stability under the use of different regulators with the help of the proper-time flow equation. We find three relevant directions, corresponding to the g, gR, and gR2 operators, whose critical properties are very similar to the ones shared by the full theory. Our result shows that the basic mechanism at the core of the asymptotic safety program is still well described by the conformal sector also beyond the Einstein-Hilbert truncation. Possible consequences for the asymptotic safety program are discussed.

It is known that for a star like the Sun, rotation and magnetic activity evolve with age. Indeed, in 1972, Skumanich showed that on the main sequence, a solar-like star spins down and becomes less magnetically active with age. Such evolution opened the possibility to use rotation and magnetic activity as stellar clocks, known as gyrochronology for the former and magneto chronology for the latter. While these relations have been based on spectroscopic indexes such as CaHK and spectropolarimetric observations from surveys, the sample used is still small with up to a few thousands of stars. The recent catalog of rotation periods and photometric magnetic activity proxies for more than 55,000 stars observed by the Kepler mission is a goldmine to study the evolution of magnetic activity of solar-like stars. Using stellar models that include angular momentum evolution, we computed ages for the full sample of stars. We investigated relations between Sph, a photometric magnetic activity index, stellar ages as well as other stellar fundamental parameters (such as luminosity, metallicity, rotation periods). Such analysis is based on two different methods: a Bayesian fit of analytical relations and a Random Forest (RF) regressor.While our results depend on the stellar models used, the methodologies can be performed with other stellar models. In this talk, I will show the promising results found to predict ages using these methods with up to 5-6% median differences between the predictions and the stellar models. In particular, we show that adding the magnetic activity proxy, Sph, as input improves the age prediction. Our results are now being implemented in the PLATO stellar analysis system.

We study the conformally reduced $R+R^2$ theory of gravity and we show that the theory is asymptotically safe with an ultraviolet critical manifold of dimension three. In particular, we discuss the universality properties of the fixed point and its stability under the use of different regulators with the help of the proper-time flow equation. We find three relevant directions, corresponding to the $\sqrt{g}$, $\sqrt{g} R$ and $\sqrt{g} R^2$ operators, whose critical properties are very similar to the ones shared by the full theory. Our result shows that the basic mechanism at the core of the Asymptotic Safety program is still well described by the conformal sector also beyond the Einstein-Hilbert truncation. Possible consequences for the asymptotic safety program are discussed., 14 pages, 4 figures

The ages of solar-like stars have been at the center of many studies such as exoplanet characterization or Galactic-archaeology. While ages are usually computed from stellar evolution models, relations linking ages to other stellar properties, such as rotation and magnetic activity, have been investigated. With the large catalog of 55,232 rotation periods, $P_{\rm rot}$, and photometric magnetic activity index, $S_{\rm ph}$ from Kepler data, we have the opportunity to look for such magneto-gyro-chronology relations. Stellar ages are obtained with two stellar evolution codes that include treatment of angular momentum evolution, hence using $P_{\rm rot}$ as input in addition to classical atmospheric parameters. We explore two different ways of predicting stellar ages on three subsamples with spectroscopic observations: solar analogs, late-F and G dwarfs, and K dwarfs. We first perform a Bayesian analysis to derive relations between $S_{\rm ph}$ and ages between 1 and 5 Gyr, and other stellar properties. For late-F and G dwarfs, and K dwarfs, the multivariate regression favors the model with $P_{\rm rot}$ and $S_{\rm ph}$ with median differences of 0.1%.and 0.2% respectively. We also apply Machine Learning techniques with a Random Forest algorithm to predict ages up to 14 Gyr with the same set of input parameters. For late-F, G and K dwarfs together, predicted ages are on average within 5.3% of the model ages and improve to 3.1% when including $P_{\rm rot}$. These are very promising results for a quick age estimation for solar-like stars with photometric observations, especially with current and future space missions., Comment: 29 pages, 13 figures, including 8 pages of Appendix. Accepted for publication in ApJ

The knowledge of the evolution of rotation and magnetic activity of solar-like stars is key to understand our Sun. This was indeed realized several decades ago with the Mount Wilson spectroscopic survey where the goal was to characterize the magnetism of stars other than the Sun in order to put it into context and in time. By investigating the magnetic activity of other stars in a different parameter space, we can provide additional constraints to dynamo models as a function of stellar age and spectral type.While several surveys followed the Mount Wilson one, the number of stars that can be followed from the ground is quite limited. The recent catalog of rotation periods and photometric magnetic activity proxies for more than 55,000 stars observed by the Kepler mission opens the possibility to study the evolution of surface magnetic activity of a much larger sample of solar-like stars. We will present a sample of main-sequence stars where we compare the Sun to Sun-like stars. We also find that the magnetic activity of the Sun is comparable to the one of stars with properties very similar to the Sun. We also compute ages based on models that take into account the weakened magnetic braking theory observed in the Kepler asteroseismic sample. This allows us to study the evolution of magnetic activity as a function of age, providing a more complete picture to understand the dynamical changes on the main sequence.

A convincing explanation for the nature of the dark energy and dark matter is still missing. In recent works a RG-improved swiss-cheese cosmology with an evolving cosmological constant dependent on the \sch radius has been proven to be a promising model to explain the observed cosmic acceleration. In this work we extend this model to consider the combined scaling of the Newton constant $G$ and the cosmological constant $\Lambda$ according to the IR-fixed point hypothesis. We shall show that our model easily generates the observed recent passage from deceleration to acceleration without need of extra energy scales, exotic fields or fine tuning. In order to check the generality of the concept, two different scaling relations have been analysed and we proved that both are in very good agreement with $\Lambda$CDM cosmology. We also show that our model satisfies the observational local constraints on $\dot{G}/G$., Comment: 15 pages, 3 figures

A convincing explanation for the nature of the dark energy and dark matter is still missing. In recent works a RG-improved swiss-cheese cosmology with an evolving cosmological constant dependent on the \sch radius has been proven to be a promising model to explain the observed cosmic acceleration. In this work we extend this model to consider the combined scaling of the Newton constant $G$ and the cosmological constant $Λ$ according to the IR-fixed point hypothesis. We shall show that our model easily generates the observed recent passage from deceleration to acceleration without need of extra energy scales, exotic fields or fine tuning. In order to check the generality of the concept, two different scaling relations have been analysed and we proved that both are in very good agreement with $Λ$CDM cosmology. We also show that our model satisfies the observational local constraints on $\dot{G}/G$., 15 pages, 3 figures

Black holes encountered in general relativity are characterized by spacetime singularities hidden within an event horizon. These singularities provide a key motivation to go beyond general relativity and look for regular black holes where the spacetime curvature remains bounded everywhere. A prominent mechanism achieving this replaces the singularity by a regular patch of de Sitter space. The resulting regular geometries exhibit two horizons: the outer event horizon is supplemented by an inner Cauchy horizon. The latter could render the geometry unstable against perturbations through the so-called mass-inflation effect, i.e., an exponential growth of the mass function. This chapter reviews the mass-inflation effect for spherically symmetric black hole spacetimes contrasting the dynamics of the mass function for Reissner-Nordst\"om and regular black holes. We also cover recent developments related to the late-time attractors induced by Hawking radiation which exorcise the exponential growth of the spacetime curvature encountered in the standard mass-inflation scenario. In order to make the exposition self-contained, we also briefly discuss basic properties of regular black holes including their thermodynamics., Comment: 34 pages, Invited chapter for the edited book "Regular Black Holes: Towards a New Paradigm of the Gravitational Collapse" (Ed. C. Bambi, Springer Singapore, expected in 2023)

Stellar dynamos are affected by boundary conditions imposed by stellar coronae. Under some approximations it is possible to find analytical solutions. Interior dynamo models often consider a current-free coronae without taking into account the constraints imposed by the presence of currents in the corona. We aim to analytically evaluate the effect of coronal currents and of an outer boundary condition on the efficiency of an $\alpha^2$ dynamo. We intend to estimate the change in geometry and dinamo excitation numbers with respect to the current-free case. We analytically solve the turbulent dynamo induction equation for a homogeneous, non-mirror symmetric turbulence, in a spherical domain surrounded by a linear force-free corona with magnetic field B satisfying curl B = \beta B. The main result is that the dynamo number is a decreasing function of \beta. Moreover, if the current is parallel to the field (\beta > 0) the dynamo number is smaller than in the force-free case. On the contrary (\beta < 0) the dynamo number is greater than the in force-free case. We conclude that the presence of currents in the corona needs to be taken into account because it affects the condition for excitation of a dynamo., Comment: Accepted for publication in A&A

Helioseismology can provide strong constraints on the evolution of Newton's constant over cosmic time. We make use of the best possible estimate of 8640 days of low-$\ell$ BISON data, corrected for the solar cycle variation, to obtain a new constraint on an evolving gravitational constant. In particular, by means of a Bayesian analysis we conclude that ${{\dot{G}/G}_{\rm today}}=(1.25\pm 0.30) \times 10^{-13} \; \mathrm{yr}^{-1}$. Our result, a 4-$\sigma$ effect, is more than one order of magnitude stronger than previous constraints obtained with helioseismology. We also take into account possible systematic effects by considering the theoretical uncertainties on the efficiency of the proton-proton $(pp)$ fusion cross-section. We show that models with variable $G$ significantly outclass models with no secular variation of $G$, viz by a Bayes factor exceeding 30., Comment: 3 pages, 1 figure

Stellar dynamos are affected by boundary conditions imposed by stellar coronae. Under some approximations it is possible to find analytical solutions. Interior dynamo models often consider a current-free coronae without taking into account the constraints imposed by the presence of currents in the corona. We aim to analytically evaluate the effect of coronal currents and of an outer boundary condition on the efficiency of an $��^2$ dynamo. We intend to estimate the change in geometry and dinamo excitation numbers with respect to the current-free case. We analytically solve the turbulent dynamo induction equation for a homogeneous, non-mirror symmetric turbulence, in a spherical domain surrounded by a linear force-free corona with magnetic field B satisfying curl B = ��B. The main result is that the dynamo number is a decreasing function of ��. Moreover, if the current is parallel to the field (��> 0) the dynamo number is smaller than in the force-free case. On the contrary (��< 0) the dynamo number is greater than the in force-free case. We conclude that the presence of currents in the corona needs to be taken into account because it affects the condition for excitation of a dynamo., Accepted for publication in A&A

The Asymptotically Safe Gravity provides a framework for the description of gravity from the trans-Planckian regime to cosmological scales. According to this scenario, the cosmological constant and Newton's coupling are functions of the energy scale whose evolution is dictated by the renormalization group equations. The formulation of the renormalization group equations on foliated spacetimes, based on the Arnowitt-Deser-Misner (ADM) formalism, furnishes a natural way to construct the RG energy scale from the spectrum of the laplacian operator on the spatial slices. Combining this idea with a Renormalization Group improvement procedure, in this work we study quantum gravitational corrections to the Einstein-Hilbert action on Friedmann-Lema\^{i}tre-Robertson-Walker (FLRW) backgrounds. The resulting quantum-corrected Friedmann equations can give rise to both bouncing cosmologies and emergent universe solutions. Our bouncing models do not require the presence of exotic matter and emergent universe solutions can be constructed for any allowed topology of the spatial slices., Comment: 8 pages, 1 figure. V2: references added

Recent numerical results of current-driven instabilities at low magnetic Prandtl number and high Hartmann number support the possibility of a saturation state characterized by helicity oscillations. We investigate the underlying mechanism by analyzing this possibility using an higher-order Landau-Ginzburg effective Lagrangian for the weakly non-linear amplitude dynamics, where the magnetic and velocity perturbations are linearly dependent. We find that, if the mirror symmetry between left- and right-handed modes is spontaneously broken, it is impossible to achieve an oscillating helical state. We argue that the result is likely to hold also adding higher-order terms and in the presence of an explicit symmetry breaking. We conclude that an oscillating saturating state for the Tayler instability is unlikely to depend on the interaction of chiral modes., Comment: 10 pages, 6 figures

We consider combined rotational, magnetic, and thermal evolution of the neutron star during the accretion phase in a binary system. A rapid accretion-driven decay of the magnetic field decreases substantially the efficiency of angular momentum transfer. As a result, the neutron star cannot spin up to periods shorter than some limiting value even if accretion is very long and accretion rate is high. The proposed mechanism can explain a discrepancy between the shortest detected period and minimal possible spin period of neutron stars., 6 pages, 2 figures, MNRAS in press, figure 2 label typo corrected

We studied solar-like oscillations in 115 red giants in the three open clusters NGC 6791, NGC 6811, and NGC 6819, based on photometric data covering more than 19 months with NASA's Kepler space telescope. We present the asteroseismic diagrams of the asymptotic parameters \delta\nu_02, \delta\nu_01 and \epsilon, which show clear correlation with fundamental stellar parameters such as mass and radius. When the stellar populations from the clusters are compared, we see evidence for a difference in mass of the red giant branch stars, and possibly a difference in structure of the red clump stars, from our measurements of the small separations \delta\nu_02 and \delta\nu_01. Ensemble \'{e}chelle diagrams and upper limits to the linewidths of l = 0 modes as a function of \Delta\nu of the clusters NGC 6791 and NGC 6819 are also shown, together with the correlation between the l = 0 ridge width and the T_eff of the stars. Lastly, we distinguish between red giant branch and red clump stars through the measurement of the period spacing of mixed dipole modes in 53 stars among all the three clusters to verify the stellar classification from the color-magnitude diagram. These seismic results also allow us to identify a number of special cases, including evolved blue stragglers and binaries, as well as stars in late He-core burning phases, which can be potentially interesting targets for detailed theoretical modeling., Comment: 30 pages, 15 figures, 1 table, accepted to ApJ

International audience

We briefly review the turbulent mean-field dynamo action in protoneutron stars that are subject to convective and neutron finger instabilities during the early evolutionary phase. By solving the mean-field induction equation with the simplest model of $��$-quenching we estimate the strength of the generated magnetic field. If the initial period of the protoneutron star is short, then the generated large-scale field is very strong ($> 3 \times 10^{13}$G) and exceeds the small-scale field at the neutron star surface, while if the rotation is moderate, then the pulsars are formed with more or less standard dipole fields ($< 3 \times 10^{13}$G) but with surface small-scale magnetic fields stronger than the dipole field. If rotation is very slow, then the mean-field dynamo does not operate, and the neutron star has no global field., 4 pages, 2 figures, Exotic States of Nuclear Matter. Proceedings of the International Symposium EXOCT07. Held 11-15 June 2007 in Catania University, Italy. Edited by Umberto Lombardo, Marcello Baldo, Fiorella Burgio & Hans-Josef Schulze. Published by World Scientific Publishing

A class of exact solutions of the field equations with higher derivative terms is presented when the matter field is a pressureless null fluid plus a Maxwellian static electric component. It is found that the stable solutions are black holes in anti de Sitter background. The issue of the stability of the Cauchy horizon is discussed., REVTex, 13 pages, to appear on the proceedings of the workshop on " The Internal Structure of Black Holes and Spacetime Singularities", held at Technion -Israel Institute of Technology-, Haifa,Israel,June 29-July 3, 1997