Star-planet Magnetic Interaction Research Articles

An Earth-sized Planet on the Verge of Tidal Disruption

TOI-6255 b (GJ 4256) is an Earth-sized planet (1.079 ± 0.065 R ⊕) with an orbital period of only 5.7 hr. With the newly commissioned Keck Planet Finder and CARMENES spectrographs, we determine the planet’s mass to be 1.44 ± 0.14 M ⊕. The planet is just outside the Roche limit, with P orb/P Roche = 1.13 ± 0.10. The strong tidal force likely deforms the planet into a triaxial ellipsoid with a long axis that is ∼10% longer than the short axis. Assuming a reduced stellar tidal quality factor Q⋆′≈107 , we predict that tidal orbital decay will cause TOI-6255 to reach the Roche limit in roughly 400 Myr. Such tidal disruptions may produce the possible signatures of planet engulfment that have been seen on stars with anomalously high refractory elemental abundances compared to their conatal binary companions. TOI-6255 b is also a favorable target for searching for star–planet magnetic interactions, which might cause interior melting and hasten orbital decay. TOI-6255 b is a top target (with an Emission Spectroscopy Metric of about 24) for phase-curve observations with the James Webb Space Telescope.

Planetary perturbers: flaring star–planet interactions in Kepler and TESS

ABSTRACT In many star–planet systems discovered so far, the innermost planet orbits within only a few stellar radii. In these systems, planets could become in situ probes of the extended stellar magnetic field. Because they disturb the field as they move, they are expected to trigger flares in the corona. Potential differences to the energies and morphologies of intrinsic flares are poorly constrained. However, as we expect planet-induced flares to correlate with the planet’s orbital period, we can identify them from a clustering of flares in phase with the planet’s orbit. We used the excellent phase coverage from Kepler and the Transiting Exoplanet Survey Satellite to find flaring star–planet systems, compile a catalogue of all their flares, and measure how much they cluster in orbital phase. In the 1811 searched systems, we found 25 single stars with three or more flares each. We quantified the significance of the clustering in each system, and compared it against the theoretically expected power of magnetic interaction that leads to planet-induced flaring. Most systems do not show any clustering, consistent with low expected power. Those we expect to show clustering fall on two branches. An inactive one, without any signs of clustering, and a tentative active one, where the clustering becomes more pronounced as the expected power of interaction increases. The flares in HIP 67522 are prominently clustered (p < 0.006). This young hot Jupiter system is the most promising candidate for magnetic star–planet interaction in our sample.

ROME. III. The Arecibo Search for Star–Planet Interactions at 5 GHz

After nearly three decades of discovery, many exoplanetary systems have been studied and characterized in detail with one important exception: exoplanet magnetism. Although many surveys sought to detect magnetospheric radio emissions from exoplanets to directly measure their magnetic field strengths, they have yet to reveal an unambiguous detection. However, the indirect detection of exoplanet magnetic fields by measuring their influence on their host stars via magnetic star–planet interactions has recently gained prominence as an alternative method of discovery. This third paper of the Radio Observations of Magnetized Exoplanets series presents the results of a targeted radio survey of eight nearby exoplanet-hosting systems that may engage in star–planet interactions. This survey, conducted with the Arecibo radio telescope at ∼5 GHz, has the greatest frequency coverage of any to date while providing millijansky-level sensitivity over <1 s integration times. No exoplanet-induced stellar radio bursts were detected. The orbital phase coverage of candidate systems for magnetic star–planet interactions is described, and the survey results are examined within the context of the plasma flow–obstacle paradigm and searches for star–planet interactions at other wavelengths.

Hunting for exoplanets via magnetic star–planet interactions: geometrical considerations for radio emission

ABSTRACT Recent low-frequency radio observations suggest that some nearby M dwarfs could be interacting magnetically with undetected close-in planets, powering the emission via the electron cyclotron maser (ECM) instability. Confirmation of such a scenario could reveal the presence of close-in planets around M dwarfs, which are typically difficult to detect via other methods. ECM emission is beamed, and is generally only visible for brief windows depending on the underlying system geometry. Due to this, detection may be favoured at certain orbital phases, or from systems with specific geometric configurations. In this work, we develop a geometric model to explore these two ideas. Our model produces the visibility of the induced emission as a function of time, based on a set of key parameters that characterize magnetic star–planet interactions. Utilizing our model, we find that the orbital phases where emission appears are highly dependent on the underlying parameters, and does not generally appear at the quadrature points in the orbit as is seen for the Jupiter–Io interaction. Then using non-informative priors on the system geometry, we show that untargeted radio surveys are biased towards detecting emission from systems with planets in near face-on orbits. While transiting exoplanets are still likely to be detectable, they are less likely to be seen than those in near face-on orbits. Our forward model serves to be a powerful tool for both interpreting and appropriately scheduling radio observations of exoplanetary systems, as well as inverting the system geometry from observations.

Coherent radio bursts from known M-dwarf planet-host YZ Ceti

Observing magnetic star–planet interactions (SPIs) offers promise for determining the magnetic fields of exoplanets. Models of sub-Alfvénic SPIs predict that terrestrial planets in close-in orbits around M dwarfs can induce detectable stellar radio emission, manifesting as bursts of strongly polarized coherent radiation observable at specific planet orbital positions. Here we present 2–4 GHz detections of coherent radio bursts on the slowly rotating M dwarf YZ Ceti, which hosts a compact system of terrestrial planets, the innermost of which orbits with a two-day period. Two coherent bursts occur at similar orbital phases of YZ Ceti b, suggestive of an enhanced probability of bursts near that orbital phase. We model the system’s magnetospheric environment in the context of sub-Alfvénic SPIs and determine that YZ Ceti b can plausibly power the observed flux densities of the radio detections. However, we cannot rule out stellar magnetic activity without a well-characterized rate of non-planet-induced coherent radio bursts on slow rotators. YZ Ceti is therefore a candidate radio SPI system, with unique promise as a target for long-term monitoring.

Magnetic activity variability from Hα line intensive monitoring of two F-type stars with a hot Jupiter, τ Bootis A and υ Andromedae A

Abstract We report the results of intensive monitoring of the variability in the Hα line for two F-type stars, τ Boo and υ And, over the last four years 2019–2022, in order to investigate their stellar magnetic activity. The 4 yr Hα line intensity data taken with the 1.88 m reflector at the Okayama Branch Office, Subaru Telescope, shows the existence of a possible ∼123 d magnetic activity cycle for τ Boo. The result of the Hα variability as another tracer of the magnetic activity on the chromosphere is consistent with previous studies of the Ca ii H&amp;K line and suggests that the magnetic activity cycle persists in τ Boo. For υ And, we suggest a quadratic long-term trend in the Hα variability. Meanwhile, the short-term monitoring shows no significant period corresponding to specific variations likely induced by their hot Jupiter in both cases (≈3.31 and 4.62 d, respectively). In this Hα observation, we could not find any signature of the star–planet magnetic interaction. It is speculated that the detected magnetic activity variability of the two F-type stars is related to the stellar intrinsic dynamo.

Flares, Rotation, Activity Cycles, and a Magnetic Star–Planet Interaction Hypothesis for the Far-ultraviolet Emission of GJ 436

Variability in the far-ultraviolet (FUV) emission produced by stellar activity affects photochemistry and heating in orbiting planetary atmospheres. We present a comprehensive analysis of the FUV variability of GJ 436, a field-age M2.5V star (P rot ≈ 44 days) that is orbited by a warm Neptune-sized planet (M ≈ 25 M ⊕, R ≈ 4.1 M ⊕, P orb ≈ 2.6 days). Observations at three epochs from 2012 to 2018 span nearly a full activity cycle, sample two rotations of the star and two orbital periods of the planet, and reveal a multitude of brief flares. From 2012 to 2018, the star’s 7.75 ± 0.10 yr activity cycle produced the largest observed variations, 38% ± 3% in the summed flux of the major FUV emission lines. In 2018, the variability due to rotation was 8% ± 2%. An additional 11% ± 1% scatter at a cadence of 10 minutes, which is treated as white noise in the fits, likely has both instrumental and astrophysical origins. Flares increased time-averaged emission by 15% over the 0.88 days of cumulative exposure, peaking as high as 25× quiescence. We interpret these flare values as lower limits given that flares too weak or too infrequent to have been observed likely exist. GJ 436’s flare frequency distribution at FUV wavelengths is unusual compared to other field-age M dwarfs, exhibiting a statistically significant dearth of high-energy (>4 × 1028 erg) events, which we hypothesize to be the result of a magnetic star–planet interaction (SPI) triggering premature flares. If an SPI is present, GJ 436 b’s magnetic field strength must be ≲100 G to explain the statistically insignificant increase in the orbit-phased FUV emission.

Weak evidence for variable occultation depth of 55 Cnc e with TESS

Context.55 Cnc e is in a 0.73 day orbit transiting a Sun-like star. It has been observed that the occultation depth of this super-Earth, with a mass of 8M⊕and radius of 2R⊕, changes significantly over time at mid-infrared wavelengths. Observations withSpitzermeasured a change in its day-side brightness temperature of 1200 K, possibly driven by volcanic activity, magnetic star-planet interaction, or the presence of a circumstellar torus of dust.Aims.Previous evidence for the variability in occultation was in the infrared range. Here we aim to explore if the variability also exists in the optical range.Methods.The Transiting Exoplanet Survey Satellite (TESS) observed 55 Cnc during sectors 21, 44, and 46. We carefully detrended the data and fitted a transit and occultation model for each sector in a Markov chain Monte Carlo (MCMC) routine. In a later stage, we used the leave-one-out (LOO) cross-validation statistic to compare with a model of constant occultation for the complete set and a model with no occultation.Results.We report an occultation depth of 8 ± 2.5 ppm for the complete set of TESS observations. In particular, we measured a depth of 15 ± 4 ppm for sector 21, while for sector 44 we detected no occultation. In sector 46 we measured a weak occultation of 8 ± 5 ppm. The occultation depth varies from one sector to the next between 1.6 and 3.4σsignificance. We derived the possible contribution on reflected light and thermal emission and set an upper limit on the geometric albedo. Based on our model comparison, the presence of an occultation is favoured considerably over no occultation, and the model with varying occultation across sectors takes most of the statistical weight.Conclusions.Our analysis confirms a detection of the occultation in TESS. Moreover, our results weakly lean towards a varying occultation depth between each sector, while the transit depth is constant across visits.

Searching for flaring star–planet interactions in AU Mic TESS observations

ABSTRACT Planets that closely orbit magnetically active stars are thought to be able to interact with their magnetic fields in a way that modulates stellar activity. This modulation in phase with the planetary orbit, such as enhanced X-ray activity, chromospheric spots, radio emission, or flares, is considered the clearest sign of magnetic star–planet interaction (SPI). However, the magnitude of this interaction is poorly constrained, and the intermittent nature of the interaction is a challenge for observers. AU Mic is an early M dwarf, and the most actively flaring planet host detected to date. Its innermost companion, AU Mic b, is a promising target for magnetic SPI observations. We used optical light curves of AU Mic obtained by the Transiting Exoplanet Survey Satellite to search for signs of flaring SPI with AU Mic b using a customized Anderson–Darling test. In the about 50 d of observations, the flare distributions with orbital, rotational, and synodic periods were generally consistent with intrinsic stellar flaring. We found the strongest deviation (p = 0.07, n = 71) from intrinsic flaring with the orbital period of AU Mic b, in the high-energy half of our sample (ED &amp;gt; 1 s). If it reflects the true SPI signal from AU Mic b, extending the observing time by a factor of 2–3 will yield a &amp;gt;3σ detection. Continued monitoring of AU Mic may therefore reveal flaring SPI with orbital phase, while rotational modulation will smear out due to the star’s strong differential rotation.

MOVES – V. Modelling star–planet magnetic interactions of HD 189733

ABSTRACT Magnetic interactions between stars and close-in planets may lead to a detectable signal on the stellar disc. HD 189733 is one of the key exosystems thought to harbour magnetic interactions, which may have been detected in 2013 August. We present a set of 12 wind models at that period, covering the possible coronal states and coronal topologies of HD 189733 at that time. We assess the power available for the magnetic interaction and predict its temporal modulation. By comparing the predicted signal with the observed signal, we find that some models could be compatible with an interpretation based on star–planet magnetic interactions. We also find that the observed signal can be explained only with a stretch-and-break interaction mechanism, while that the Alfvén wings scenario cannot deliver enough power. We finally demonstrate that the past observational cadence of HD 189733 leads to a detection rate of only between 12 and 23 per cent, which could explain why star–planet interactions have been hard to detect in past campaigns. We conclude that the firm confirmation of their detection will require dedicated spectroscopic observations covering densely the orbital and rotation period, combined with scarcer spectropolarimetric observations to assess the concomitant large-scale magnetic topology of the star.

A model for spin–orbit commensurability and synchronous starspot activity in stars with close-by planets

Context. The rotation period of some planet-hosting stars appears to be in close commensurability with the orbital period of their close-by planets. In some cases, starspots rotating with a commensurable period have been detected, while the star displays latitudinal differential rotation. Aims. A model is proposed to interpret such a phenomenon based on the excitation of resonant oscillations in the interior magnetic field of the star by a component of the tidal potential with a very low frequency in the reference frame rotating with the star. Methods. A magnetic flux tube located in the overshoot layer of the star is assumed in order to study the excitation of the resonant oscillations in the magnetostrophic regime. The model considers a planet on a circular oblique orbit, and the growth timescale of the oscillations is estimated. To keep the system in resonance with the exciting potential despite the variations in the magnetic field or tidal frequency, a self-regulating mechanism is proposed. Results. The model is applied to ten systems and proves capable of accounting for the observed close commensurability in eight of them by assuming a magnetic field between 102 and 104 G. Systems with distant low-mass planets, such as AU Mic and HAT-P-11, cannot be interpreted by the proposed model. Conclusions. Consequences for the spin–orbit evolution of the systems, including the dynamical tides and gyrochronology of planet-hosting stars, are discussed together with the effects on the chromospheric features produced by star–planet magnetic interactions.

The TESS View of LOFAR Radio-emitting Stars

Abstract The recent detection of the M dwarf GJ 1151 at 144 MHz low radio frequencies using LOFAR has been interpreted as evidence of an exoplanet magnetically interacting with its host star. This would be the first exoplanet detected around a main-sequence star by a radio telescope. Radial velocity confirmation of such a planet has proven inconclusive and it remains possible that the radio emission could be generated by a stellar coronal process. Using data from TESS, we shed light on this question by probing the stellar activity and flares of GJ 1151 as well as 14 other M dwarfs detected by LOFAR. GJ 1151 and three other star–planet interaction candidates are found to be inactive, with no rotational modulation and few, if any, flares. The remainder of the LOFAR-detected M dwarfs flare frequently. We consider it unlikely that stellar activity is responsible for the bright, circularly polarized emission from GJ 1151 and its three analogs and support the star–planet magnetic interaction interpretation.

An internal heating mechanism operating in ultra-short-period planets orbiting magnetically active stars

Context. Rocky planets with orbital periods shorter than ~1 day have been discovered by the method of transits and their study can provide information on Earth-like planets not available from bodies on longer period orbits. Aims. A new mechanism for the internal heating of such ultra-short-period planets is proposed based on the gravitational perturbation produced by a non-axisymmetric quadrupole moment of their host stars. Such a quadrupole is due to the magnetic flux tubes in the stellar convection zone, unevenly distributed in longitude and persisting for many stellar rotations as observed in young late-type stars. Methods. The rotation period of the host star evolves from its shortest value on the zero-age main sequence (ZAMS) to longer periods due to the loss of angular momentum through a magnetized wind. If the stellar rotation period comes close to twice the orbital period of the planet, the quadrupole leads to a spin-orbit resonance that excites oscillations of the star-planet separation. As a consequence, a strong tidal dissipation is produced inside the planet that converts the energy of the oscillations into internal heat. The total heat released inside the planet scales as a−8, where a is the orbit semimajor axis, and it is largely independent of the details of the planetary internal dissipation or the lifetime of the stellar magnetic flux tubes. Results. We illustrate the operation of the mechanism by modeling the evolution of the stellar rotation and of the innermost planetary orbit under the action of the stellar wind and the tides in the cases of CoRoT-7, Kepler-78, and K2-141 whose present orbital periods range between 0.28 and 0.85 days. If the spin-orbit resonance occurs, the maximum power dissipated inside the planets ranges between 1018 and 1019 W, while the total dissipated energy is on the order of 1030−1032 J over a time interval as short as (1−4.5) × 104 yr. Conclusions. Our illustrative models suggest that, if their host stars started their evolution on the ZAMS as fast rotators with periods between 0.5 and 1.0 days, the resonance occurred after about 40 Myr since the host stars settled on the ZAMS in all the three cases. This huge heating over such a short time interval produces a complete melting of the planetary interiors and may shut off their hydromagnetic dynamos. These may initiate a successive phase of intense internal heating owing to unipolar magnetic star-planet interactions and affect the composition and the escape of their atmospheres, producing effects that could be observable during the entire lifetime of the planets.

Magnetic and tidal migration of close-in planets

Context.Over the last two decades, a large population of close-in planets has been detected around a wide variety of host stars. Such exoplanets are likely to undergo planetary migration through magnetic and tidal interactions.Aims.We aim to follow the orbital evolution of a planet along the structural and rotational evolution of its host star, simultaneously taking into account tidal and magnetic torques, in order to explain some properties of the distribution of observed close-in planets.Methods.We rely on a numerical model of a coplanar circular star–planet system called ESPEM, which takes into account stellar structural changes, wind braking, and star–planet interactions. We browse the parameter space of the star–planet system configurations and assess the relative influence of magnetic and tidal torques on its secular evolution. We then synthesize star–planet populations and compare their distribution in orbital and stellar rotation periods toKeplersatellite data.Results.Magnetic and tidal interactions act together on planetary migration and stellar rotation. Furthermore, both interactions can dominate secular evolution depending on the initial configuration of the system and the evolutionary phase considered. Indeed, tidal effects tend to dominate for high stellar and planetary masses as well as low semi-major axis; they also govern the evolution of planets orbiting fast rotators while slower rotators evolve essentially through magnetic interactions. Moreover, three populations of star–planet systems emerge from the combined action of both kinds of interactions. First, systems undergoing negligible migration define an area of influence of star–planet interactions. For sufficiently large planetary magnetic fields, the magnetic torque determines the extension of this region. Next, planets close to fast rotators migrate efficiently during the pre-main sequence, which engenders a depleted region at low rotation and orbital periods. Then, the migration of planets close to slower rotators, which happens during the main sequence, may lead to a break in gyrochronology for high stellar and planetary masses. This also creates a region at high rotation periods and low orbital periods not populated by star–planet systems. We also find that star–planet interactions significantly impact the global distribution in orbital periods by depleting more planets for higher planetary masses and planetary magnetic fields. However, the global distribution in stellar rotation periods is marginally affected, as around 0.5% of G-type stars and 0.1% of K-type stars may spin up because of planetary engulfment. More precisely, star–planet magnetic interactions significantly affect the distribution of super-Earths around stars with a rotation period higher than around 5 days, which improves the agreement between synthetic populations and observations at orbital periods of less than 1 day. Tidal effects for their part shape the distribution of giant planets.

The large-scale magnetic field of Proxima Centauri near activity maximum

ABSTRACT We report the detection of a large-scale magnetic field at the surface of the slowly rotating fully convective (FC) M dwarf Proxima Centauri. 10 circular polarization spectra, collected from 2017 April to July with the HARPS-Pol spectropolarimeter, exhibit rotationally modulated Zeeman signatures suggesting a stellar rotation period of 89.8 ± 4.0 d. Using Zeeman–Doppler Imaging, we invert the circular polarization spectra into a surface distribution of the large-scale magnetic field. We find that Proxima Cen hosts a large-scale magnetic field of typical strength 200 G, whose topology is mainly poloidal, and moderately axisymmetric, featuring, in particular, a dipole component of 135 G tilted at 51° to the rotation axis. The large-scale magnetic flux is roughly 3× smaller than the flux measured from the Zeeman broadening of unpolarized lines, which suggests that the underlying dynamo is efficient at generating a magnetic field at the largest spatial scales. Our observations occur ∼1 yr after the maximum of the reported 7 yr-activity cycle of Proxima Cen, which opens the door for the first long-term study of how the large-scale field evolves with the magnetic cycle in an FC very low mass star. Finally, we find that Proxima Cen’s habitable zone planet, Proxima-b, is likely orbiting outside the Alfvèn surface, where no direct magnetic star–planet interactions occur.

Chasing Star–Planet Magnetic Interactions: The Case of Kepler-78

Abstract Observational evidence of star–planet magnetic interactions (SPMIs) in compact exosystems have been looked for in the past decades. Indeed, planets in close-in orbit can be magnetically connected to their host star and can channel Alfvén waves carrying large amounts of energy toward the central star. The strength and temporal modulation of SPMIs are primarily set by the magnetic topology of the host star and the orbital characteristics of the planet. As a result, SPMI signals can be modulated over the rotational period of the star, the orbital period of the planet, or a complex combination of the two. The detection of SPMIs thus has to rely on multiple-epoch and multiple-wavelength observational campaigns. We present a new method to characterize SPMIs and apply it to Kepler-78, a late G star with a super-Earth on an 8.5 hr orbit. We model the corona of Kepler-78 using the large-scale magnetic topology of the star observed with Zeeman–Doppler imaging. We show that the closeness of Kepler-78b allows the interaction with channel energy flux densities up to a few kW m−2 toward the central star. We show that this flux is large enough to be detectable in classical activity tracers such as Hα. It is nonetheless too weak to explain the modulation observed by Moutou et al. We furthermore demonstrate how to predict the temporal modulation of SPMI signals in observed systems such as Kepler-78. The methodology presented here thus paves the way toward denser, more specific observational campaigns that would allow proper identification of SPMIs in compact star–planet systems.

Magnetic field strengths of hot Jupiters from signals of star–planet interactions

Evidence of star-planet interactions in the form of planet-modulated chromospheric emission has been noted for a number of hot Jupiters. Magnetic star-planet interactions involve the release of energy stored in the stellar and planetary magnetic fields. These signals thus offer indirect detections of exoplanetary magnetic fields. Here we report the derivation of the magnetic field strengths of four hot Jupiter systems using the power observed in Ca II K emission modulated by magnetic star-planet interactions. By approximating the fractional energy released in the Ca II K line we find that the surface magnetic field values for the hot Jupiters in our sample range from 20 G to 120 G, ~10-100 times larger than the values predicted by dynamo scaling laws for planets with rotation periods of ~2 - 4 days. On the other hand, these value are in agreement with scaling laws relating the magnetic field strength to the internal heat flux in giant planets. Large planetary magnetic field strengths may produce observable electron-cyclotron maser radio emission by preventing the maser from being quenched by the planet's ionosphere. Intensive radio monitoring of hot Jupiter systems will help confirm these field values and inform on the generation mechanism of magnetic fields in this important class of exoplanets.

Could star-planet magnetic interactions lead to planet migration and influence stellar rotation?

AbstractThe distribution of hot Jupiters, for which star-planet interactions can be significant, questions the evolution of exosystems. We aim to follow the orbital evolution of a planet along the rotational and structural evolution of the host star by taking into account the coupled effects of tidal and magnetic torques from ab initio prescriptions. It allows us to better understand the evolution of star-planet systems and to explain some properties of the distribution of observed close-in planets. To this end we use a numerical model of a coplanar circular star-planet system taking into account stellar structural changes, wind braking and star-planet interactions, called ESPEM (Benbakoura et al. (2019)). We find that depending on the initial configuration of the system, magnetic effects can dominate tidal effects during the various phases of the evolution, leading to an important migration of the planet and to significant changes on the rotational evolution of the star. Both kinds of interactions thus have to be taken into account to predict the evolution of compact star-planet systems.

Evidence of Magnetic Star–Planet Interactions in the HD 189733 System from Orbitally Phased Ca ii K Variations

Abstract Magnetic star–planet interactions (SPI) provide a detection method and insight into exoplanet magnetic fields and, in turn, exoplanet interiors and atmospheric environments. These signatures can be sporadic and difficult to confirm for single-epoch observations of a system due to inhomogeneous stellar magnetospheres and periodic variability in stellar magnetism. Thus, an ideal SPI search consists of multiple epochs containing observations on consecutive nights spanning at least one complete planetary orbit. Such data sets are rare but do exist for some of the most intensely studied hot Jupiter systems. One such system is HD 189733 for which six suitable SPI data sets exist, the result of spectroscopic monitoring to perform some of the first SPI searches and also to study the star’s magnetic field. Here we perform a uniform analysis of six archival Ca ii K data sets for HD 189733, spanning 2006 June through 2015 July, in order to search for magnetic SPI signatures in the chromospheric line variations. We find significant evidence for modulations of Ca ii K with a 2.29 ± 0.04 day period in the 2013 August data, which is consistent with the planet’s orbital period. The peak in the orbital variations occurs at ϕ orb ≈ 0.9, which corresponds to the SPI emission leading the planet with a phase difference of Δϕ ≈ 40° from the sub-planetary point. This is consistent with the phase-lead predictions of nonlinear force-free magnetic field SPI models. The stellar magnetic field strength at the planet’s orbit was greatest in 2013 August, which, due to the energy released in magnetic SPI scaling with B *, lends strength to the SPI interpretation.

Detection of α Centauri at radio wavelengths: chromospheric emission and search for star–planet interaction

At radio wavelengths, solar-type stars emit thermal free-free and gyroresonance, gyrosynchrotron, and impulsive coherent emission. Thermal free-free emission originates at layers where the optical depth is close to unit, while high brightness temperature, variable emission, can be due to flares via gyrosynchrotron emission. We observed the alpha Cen system with the Australian Telescope Compact Array at 2 GHz for three days and 17 GHz for one day. Both stars have been detected at 17 GHz, while only an upper limit has been obtained at low frequency despite the longer integration time. The brightness temperatures are consistent with the temperature of the upper chromosphere of the Sun. Inverting the formulae of the free-free emission, the average electron density of the plasma has been inferred. The same procedure was applied to the data in the millimetre recently acquired with ALMA. A comparison with the atmospheric solar models reveals a higher level of activity in alpha Cen B rather than in alpha Cen A, even if still at quiescent level. The non detection at 2 GHz allows us to put a lower limit in the filling factor of active regions. The claimed detection of an Earth size planet in close orbit to alpha Cen B, although doubtful, opens the opportunity to check the existence of Star-Planet Magnetic Interaction (SPMI). We constructed dynamic spectra in the 1.3 - 2.9 GHz of the 2 stars to search for time-variable coherent emission but obtained a null result.