Exoplanets and Stellar Physics

Ultra-hot Jupiters are tidally locked gas giants with dayside temperatures high enough to dissociate hydrogen and other molecules. Their atmospheres are vastly non-uniform in terms of chemistry, temperature and dynamics, and this makes their high-resolution transmission spectra and cross-correlation signal difficult to interpret. In this work, we use the SPARC/MITgcm global circulation model to simulate the atmosphere of the ultra-hot Jupiter WASP-76b under different conditions, such as atmospheric drag and the absence of TiO and VO. We then employ a 3D Monte-Carlo radiative transfer code, HIRES-MCRT, to self-consistently model high-resolution transmission spectra with iron (Fe I) lines at different phases during the transit. To untangle the structure of the resulting cross-correlation map, we decompose the limb of the planet into four sectors, and we analyse each of their contributions separately. Our experiments demonstrate that the cross-correlation signal of an ultra-hot Jupiter is primarily driven by its temperature structure, rotation and dynamics, while being less sensitive to the precise distribution of iron across the atmosphere. We also show that the previously published iron signal of WASP-76b can be reproduced by a model featuring iron condensation on the leading limb. Alternatively, the signal may be explained by a substantial temperature asymmetry between the trailing and leading limb, where iron condensation is not strictly required to match the data. Finally, we compute the Kp−Vsys maps of the simulated WASP-76b atmospheres, and we show that rotation and dynamics can lead to multiple peaks that are displaced from zero in the planetary rest frame.

We present a velocimetric and spectropolarimetric analysis of 27 observations of the 22-Myr M1 star AU Microscopii (AU Mic) collected with the high-resolution YJHK (0.98-2.35 μm) spectropolarimeter SPIRou from 2019 September 18 to November 14. Our radial velocity (RV) time-series exhibits activity-induced fluctuations of 45 m s-1 rms, ∼3 times smaller than those measured in the optical domain, that we filter using Gaussian Process Regression. We report a 3.9σ detection of the recently discovered 8.46 -d transiting planet AU Mic b, with an estimated mass of 17.1 +4.7−4.5 M⊕ and a bulk density of 1.3 ± 0.4 g cm-3, inducing an RV signature of semi-amplitude K = 8.5 +2.3−2.2 m s-1 in the spectrum of its host star. A consistent detection is independently obtained when we simultaneously image stellar surface inhomogeneities and estimate the planet parameters with Zeeman-Doppler imaging (ZDI). Using ZDI, we invert the time-series of unpolarized and circularly polarized spectra into surface brightness and large-scale magnetic maps. We find a mainly poloidal and axisymmetric field of 475 G, featuring, in particular, a dipole of 450 G tilted at 19° to the rotation axis. Moreover, we detect a strong differential rotation of dΩ = 0.167 ± 0.009 rad d-1 shearing the large-scale field, about twice stronger than that shearing the brightness distribution, suggesting that both observables probe different layers of the convective zone. Even though we caution that more RV measurements are needed to accurately pin down the planet mass, AU Mic b already appears as a prime target for constraining planet formation models, studying the interactions with the surrounding debris disc, and characterizing its atmosphere with upcoming space- and ground-based missions.

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.