Daniele Viganò

Aims. This study focuses on the low-mass binary 2MASS J05082729-2101444 (2M0508-21), one of the few known radio-loud members of the complex periodic variable sample. Our aim is to use very long baseline interferometry to constrain the orbit. Methods. We observed the system with the Very Long Baseline Array (VLBA) in three epochs at a frequency of 4.85 GHz, which provides an angular resolution of about 3 mas. We combined the three radio astrometric observations, 119 RVs (60 VIS and 59 NIR) obtained with the CARMENES high-resolution spectrograph over a period of 8.1 years, and a relative astrometric measurement of an archival H -band Keck NIRC adaptive optics image to fit the orbital motion of the binary system. Results. The VLBA observations resolved the binary system and show emission from both stellar components, with similar fluxdensity levels (0.34-0.67 mJy), and showing slight temporal flux variations. The emission appears quiescent, with no significant circular polarization, and with no flare events. We obtained an orbital motion fit of the binary system, which shows an eccentric orbit ( e = 0.71), an orbital period of 2.19 yr, and a semimajor axis of 26.964 mas (1.3 au). Conclusions. The VLBA observations made it possible to resolve the binary system and identify both stars as radio-loud sources. The combined fit shows that 2M0508-21 is an M-dwarf binary with a total dynamical mass of 0.459 ± 0.007 M _⊙ , assuming Gaia ’s parallax. This mass is slightly higher than those estimated from the luminosity and theoretical evolutionary models. The upper limit of the circular polarization at 4.85 GHz (Gaia ’s parallax of the system. A complete characterization of the system will help improve evolutionary models for young objects at the substellar boundary.

Hot Jupiters (HJs) are commonly thought to host the strongest dynamo-generated magnetic fields among exoplanets, potentially exceeding Jupiter’s by an order of magnitude. Yet, despite extensive radio and spectroscopic searches for star–planet interactions (SPI), no unambiguous detections have been made. Here, we present new magnetic field estimates for individual HJs together with general magnetic field scaling laws. Assuming their dynamos operate in the fast-rotating regime (Ro < 0.1), we combine one-dimensional evolutionary models with integral magnetic field scaling laws under two heating scenarios: deep and shallow energy deposition within the convective zone. Using interpolators built from the simulation data, we derive optimistic (deep heating) and pessimistic (shallow heating) magnetic field strengths, and construct scaling relations based on observables, i.e., planetary mass, radius, and incident stellar flux. The deep-heating scenario maximizes convective heat flux in the dynamo and predicts strong magnetic fields of up to ~100 G, whereas shallow heating suppresses convection and yields much weaker fields of order ~1 G. These results challenge the standard assumption that inflated HJs necessarily host exceptionally strong magnetic fields because of their large radii and vigorous interiors. Instead, many HJs may generate fields too weak to produce detectable coherent radio emission or strong SPI signatures, naturally explaining the persistent non-detections in radio surveys. Future facilities may distinguish between these interior-heating scenarios through improved magnetic field constraints.

Theory predicts that a planet with a sufficiently strong magnetic field orbiting close to its host star could induce star-planet magnetic interactions. This is potentially observable as an optical or radio stellar activity signal synchronised with the planet’s orbital period. We analyze 18 years of high-resolution optical spectroscopy of GJ 436, a low mass star orbited by a Neptune-sized exoplanet on a polar eccentric orbit. Stellar activity indicators show enhancements at a period corresponding to the exoplanet orbit, modulated by stellar rotation, and the star’s 8-year magnetic cycle. We interpret this as a signal of star-planet magnetic interaction. Using a geometric model, we reproduce these periods if GJ 436 b has a magnetic field strength of 6 to 110 Gauss.

The strong magnetic fields of neutron stars are closely linked to their observed thermal, spectral, and timing properties, such as the distribution of spin periods and their derivatives. To understand the evolution of astrophysical observables over time, it is essential to develop robust theoretical frameworks and numerical models that solve the coupled thermal and magnetic field evolution equations, incorporating detailed microphysics such as thermal and electrical conductivities and neutrino emission rates. These efforts are key to uncovering how the strength and geometry of magnetic fields change with age, ultimately shedding light on the diverse phenomenology of neutron stars. In this review, we outline the fundamental theory underlying magneto-thermal evolution models, with an emphasis on numerical methods and a comprehensive set of benchmark tests intended to guide current and future code development. We revisit established results from axisymmetric simulations, highlight recent progress in fully three-dimensional models, and offer a perspective on the anticipated developments in this rapidly evolving field.

The inflated radii observed in hundreds of Hot Jupiters (HJs) represents a long-standing open issue, with Ohmic dissipation derived from atmospheric magnetic induction being one of the most promising mechanisms for a quantitative explanation. Using the evolutionary code MESA, we simulate the evolution of irradiated giant planets, spanning the observed range of masses and equilibrium temperatures. We incorporate Ohmic dissipation, accounting for atmospheric induction and realistic profiles of electrical conductivity, and, for the first time, we study how it couples with the dynamo-generated internal field, which is assumed to scale as the internal heat flux as in fully convective stars and Solar planets. We find that, contrarily to the widespread expectations of large magnetic fields in HJs, Ohmic dissipation can partially suppress convection and keep the dynamo-generated magnetic fields at Jovian-like values maximum (few gauss). This has consequence in terms of measurability of atmospheric wind velocities, which depend on the magnetic drag. This talk is based on Viganò et al. 2025, A&A.

Context. The magneto-rotational instability (MRI) plays a crucial role in accretion disk modelling, driving magnetohydrodynamic turbulence and facilitating enhanced angular momentum transport. Notably, MRI is believed to be pivotal in the development of large-scale poloidal magnetic fields during binary neutron star mergers. However, the few numerical simulations that start from weak seed magnetic fields and capture its growth until saturation show the effects of small-scale turbulence and winding but lack convincing evidence of MRI activity. Aims. We investigated how the axisymmetric MRI is impacted by magnetic fields with the realistic, complex topologies of the post-merger phase, where field gradients cannot be neglected. This analysis aims to improve our understanding of the conditions under which MRI develops in more realistic astrophysical scenarios. Methods. We performed a linear analysis of axisymmetric MRI under these extended conditions and studied the resulting generalized dispersion relation. After deriving the extended MRI criteria, we first applied them to simple analytical disk models. Finally, we analysed the results obtained from a numerical relativity simulation of a long-lived neutron star merger remnant. Results. We find that radial magnetic field gradients can significantly impact the instability, slowing it and suppressing it entirely if sufficiently large. Specifically, we derived modified expressions for the growth rate and wavelength of the fastest-growing mode. Evaluating them in the context of binary neutron star merger remnants, we find that conditions that potentially allow the instability to be active and grow on short enough timescales might be reached only in small portions of the post-merger remnant, and only at late times, t ≳ 100 ms after the merger. Conclusions. Our results indicate that the role of the axisymmetric MRI in amplifying the poloidal magnetic field in the post-merger environment during the first 𝒪(100) ms is likely limited.

Context. The observational efficiency of pulsars, defined as the ratio of the observationally derived isotropic-equivalent luminosity, 4 πd _obs ² F _obs , where F _obs is the average pulsed energy flux of a pulsar and d _obs is its estimated distance, to its energy budget, shows a wide range of values. This dispersion is believed to be a combination of beaming effects, different geometries, and case-by-case variability of the emission mechanism efficiency, but it is not clear in what proportion. Aims. In this work we focused on the γ -ray range and analysed the four main ingredients that likely contribute to this dispersion: the geometrical term arising from the anisotropic emission (beaming), viewing and inclination angles, the uncertainty on the pulsar distance, the uncertainty on the moment of inertia, and the intrinsic efficiency of the mechanism producing the γ -ray emission. Methods. Estimating the expected ranges of the moment of inertia and the distance errors, and considering a geometrical and spectral model that we have recently used to fit the light curves and spectra of the entire γ -ray pulsar population, we estimate the a priori distribution of the first three ingredients in order to obtain the a posteriori distribution of the intrinsic efficiency of the mechanism. Results. We found the latter to peak at ∼5 − 15% (depending on the trial distribution) and to have a dispersion of around one order of magnitude. That is, we found the intrinsic efficiency of the mechanism to be the leading factor in the observed dispersion. In addition, we found little sensitivity of these results on different distributions of the estimated pulsar distance errors, and saw that the weak, alleged correlation with the spin-down power can only explain part of the observed dispersion. This methodology can be easily applied to other geometrical models of the emission, to test the sensitivity of these results on the beaming distribution.

Magnetar outbursts are among the most noteworthy manifestations of magnetism in neutron stars. They are episodes in which the X-ray luminosity of a strongly magnetised neutron star swiftly rises by several orders of magnitude to then decay over the course of several months. In this work, we present simulations of outbursts as a consequence of localised heat deposition in a magnetised neutron star crust, and the subsequent surface cooling. In particular, we employed a magnetothermal evolution code adapted to the study of short-term phenomena; that is, one including in its integration domain the outer layers of the star, where heat diffusion is faster. This choice entailed the development and use of heat blanketing envelope models that are thinner than those found in the literature as the surface boundary condition. We find that such envelopes can support a higher surface temperature than the thicker ones (albeit for less time), which can account for the typical luminosities observed in outbursts even when coming from small hotspots (few km in radius). We study several parameters related to the energetics and geometry of the heating region, concluding that the cooling of a crustal hotspot found in the outer part of the crust can account for the luminosity evolution observed in outbursts both in terms of peak luminosity and timescales. Finally, we discuss the key observables that must be studied in future observations to better constrain the nature of the underlying mechanism.

DG CVn is an eruptive variable star and represents the closest member of the known sample of complex periodic variables, or scallop-shell stars. Over the years, this M dwarf binary system has shown significant flaring activity at a wide range of frequencies. Here, we present a detailed analysis of ∼14 hours of radio observations of this stellar system, taken with the Karl G. Jansky Very Large Array at band L, centered at 1.5 GHz. In both 7-hour-long observations we have found a quiescent, weakly polarized component that could be ascribable to the incoherent, gyro-synchrotron emission coming from the magnetosphere surrounding one or both stars, along with multiple ∼90% right-circularly polarized bursts, some of which last for a few minutes and others of which last longer, ≳30 minutes Some of these bursts show a drift in frequency and time, possibly caused due to beaming effects or the motion of the plasma responsible for the emission. We assess the possible modulation of burst frequency with the primary and secondary periods, and discuss the properties of these bursts, favoring electron cyclotron maser over plasma emission as the likely underlying mechanism. We compare DG CVn’s dynamic spectrum to other young M dwarfs and find many similarities. A proper, dedicated, simultaneous radio/optical follow-up is needed to monitor the long-term variability and increase the statistics of bursts, in order to test whether the corotating absorbers detected in the optical can drive the observed radio emission, and whether the occurrence of radio bursts correlates with the rotational phase of either star.

Aims. The inflated radii observed in hundreds of hot Jupiters (HJ) represent a long-standing open issue. In this study, we quantitatively investigate this phenomenon within the framework of Ohmic dissipation arising from magnetic induction in the atmosphere, one of the most promising mechanisms for explaining the radius anomaly. Methods. Using MESA, we simulated the evolution of irradiated giant planets spanning the observed range of masses and equilibrium temperatures, incorporating an internal source of Ohmic dissipation that extends to deep layers of the envelope. We considered the heat-flux-dependent evolution of the deep-seated dynamo field on which the induced field depends. We adopted a state-of-the-art electrical conductivity, considering the thermal ionisation of alkali metals in the outer layers and the pressure-ionisation in the interior and the corresponding solutions for the induced currents across the planet. Results. We inferred that, in order to reproduce the range of observed radii, the atmospheric wind intensities averaged in the region p ≲ 10 bar have to be in the range O0.01-1 km/s and to decrease roughly linearly with planetary mass and much more steeply with equilibrium temperature. This is consistent with the expected effects of magnetic drag from the induced field, which is higher for more intense irradiation, via conductivity, and for larger masses, which have higher dynamo fields. Due to the evolution of the dynamo field and the proportionality of the induced currents on it, the Ohmic efficiency typically decreases by at least one order of magnitude from 0.1 to 10 Gyr, which is in contrast with the common assumption of a constant-in-time value. Notably, the extent of the main convective region and the associated heat flux supporting the dynamo is reduced in the presence of strong Ohmic dissipation, which in turn depends on the dynamo field strength, generating a non-trivial coupling of the latter with the atmospheric induction and potentially leading to the oscillatory behaviour of the field strength. These findings remain generally valid even when accounting for a long-term increase in the main-sequence host star luminosity, although this case can more readily lead to HJ re-inflation, consistent with previous studies.