Resolving star spots on WASP-85 A using high-resolution transit spectroscopy

TL;DR

By resolving local line profiles during transits of WASP-85 Ab across WASP-85 A with ESPRESSO and HARPS-N, the paper maps active regions and measures magnetic fields in star spots. It reports Zeeman broadening implying local $B$ of 2.7–4.4 kG, and disc-averaged $Bf$ values of $16 \pm 3$ G and $61 \pm 9$ G for opposite hemispheres, indicating magnetic activity stronger than the Sun's at maximum; the 3D obliquity is $\psi \approx 11^\circ$ with an aligned projected spin axis $λ \approx 0^\circ$ when accounting for spot effects. It documents centre-to-limb variations in line shapes and blueshift consistent with 3D MHD predictions, and demonstrates a robust approach to host-star surface characterization that will benefit from future instruments like ANDES on the ELT. The work provides a framework for disentangling stellar activity from planetary signals and informs magnetic environment studies of exoplanet host stars.

Abstract

Stellar surface inhomogeneities such as spots and faculae introduce Doppler variations that challenge exoplanet detection via the radial velocity method. While their impact on disc-integrated spectra is well established, detailed studies of the underlying local line profiles have so far been limited to the Sun. We present an observational campaign targeting the active star WASP-85 A during transits of its hot Jupiter companion. The transits span two stellar rotation periods, allowing us to probe the evolution of active regions. From ground-based photometry we identify seven active regions, six containing dark spots. Using simultaneous ESPRESSO transit spectroscopy, we spatially resolve these regions on the stellar surface by using the planet as a probe. We detect significant bisector shape changes, line broadening, and net redshifts during spot occultations, with velocity shifts of 108-333 m/s (mean uncertainty 50 m/s). The observed broadening is consistent with the Zeeman effect, implying magnetic field strengths (Stokes $I$) $B$ = 2.7-4.4 kG (mean uncertainty 0.6 kG), comparable to solar umbrae. Combined with our photometric spot model, this yields lower limits to the disc-integrated field $Bf = 16 \pm 3$ G and $61 \pm 9$ G for the two hemispheres probed -- at least three times higher than Sun-as-a-star values. We also measure centre-to-limb variations in FWHM, line depth, equivalent width, and convective blueshift, which broadly agree with solar observations and 3D MHD models. This work demonstrates a new way to characterise the surfaces of exoplanet host stars, paving the way for future analyses incorporating synthetic line profiles from 3D MHD simulations.

Figures (16)

• Figure 1: NGTS long-term monitoring of the WASP-85 binary system showing rotational variability in the light curve due to the evolution of spots on one or both stars. Each data point (green) represents a 2h average of 13s exposures. The individual uncertainties of the average points are fixed to 0.5, which we have chosen as a lower limit to the precision due to atmospheric scintillation. The vertical lines (grey) indicate transits observed with various facilities in this work. The solid line (black) is a Gaussian process regression fit to the data to help visualise the variability, composed of the sum of a non-periodic trend and two celeriteRotationTerm kernels with primary periods of 16.3 and 11.5 found from the BGLS periodogram.
• Figure 2:
• Figure 3: Transit observations from Campaign A (left to right column: 2021 February 12, 20, and March 16) using (top to bottom) ESPRESSO, EulerCam, NGTS, and SPECULOOS. The ESPRESSO data show the local (subplanet) velocities based on the reloaded Rossiter-McLaughlin analysis (Section \ref{['sec:reloaded_rm']}). The images in the top row are best-fit spot models of WASP-85 A for each night based on EulerCam data. For the photometric data the coloured points are individual exposures, while the larger circles with white centres are binned to the same exposure time as individual ESPRESSO exposures (400s). The data has been corrected for the dilution and background trend. The median transit models from the MCMC posterior distribution are shown without (solid black) and with (dashed) the spot model. Transit ingress/egress regions are shaded grey, while the highlighted central regions are transit phases between the second and third contact points.
• Figure 4: Transit residuals from subtracting the median transit model (without spots) from the data. The spot model for the in-transit data (dashed) is also shown, with annotations indicating spot occultations of each active region, based on an outlier analysis of the residuals from EulerCam and NGTS. An annotation in black is classified as an outlier in that dataset, while grey annotations are outliers in a different dataset.
• Figure 5: Rossiter-McLaughlin fit to disc-integrated radial velocities (i.e. "classical" Rossiter-McLaughlin analysis) from ESPRESSO and HARPS-N. Coloured points are the radial velocities and black lines are the best-fitting models to the data. The highlighted central regions indicate the transit window, grey areas are out of transit, while the grey sloping line is the Keplerian orbit expected from the orbital solution. Transit dates are indicated in each panel.