Exploring the surface of HD 189733 via Doppler shadow analysis of planetary transits

Abstract

Transmission spectroscopy has greatly advanced the study of exoplanet atmospheres, but stellar surface heterogeneities can contaminate transit spectra. Characterising how stellar spectra vary across the stellar disc is therefore essential to disentangle stellar and planetary contributions. Transit observations can probe the local stellar spectra along the planet's transit chord. We study centre-to-limb variations of line profiles across the surface of HD 189733 using the ESPRESSO spectrograph. Building on previous work, we assess the feasibility of applying the Doppler shadow technique with ESPRESSO and compare the results with solar observations and numerical simulations. We analyse spectra obtained during two transits of HD 189733 b. Each spectrum was cross-correlated with two masks of selected Fe I lines, producing four sets of cross-correlation functions (CCFs). Using a Doppler shadow methodology, we retrieved local stellar profiles along the transit chord. These were compared with previous studies, with disc-resolved solar spectra from IAG ATLAS and with transit simulations generated using SOAPv4 and synthetic spectra from Turbospectrum based on MARCS stellar atmosphere models under LTE and NLTE conditions. For three Fe I CCF sets we detect a statistically significant increase in line depth from stellar limb to centre, consistent with Turbospectrum predictions, although solar data show a weaker gradient. For one CCF set we also find that line widths decrease from limb to centre, consistent with solar observations but not reproduced by the simulations. These results demonstrate the capability of ESPRESSO to measure centre-to-limb variations of spectral line profiles on other stars. While the local CCF profiles of HD 189733 agree with solar data, discrepancies in line widths suggest that additional physical processes are required to reproduce the observed profiles.

Figures (17)

• Figure 1: Airmass variation and S/N of an ESPRESSO spectral order covering wavelengths from 567.76 nm to 576.42 nm, chosen as a midpoint in the selected FeI spectral lines, as a function of the orbital phase of HD 189733 b. Blue points correspond to observations from 11 August 2021, and red points to those from 31 August 2021. The darker shaded region marks the phases where the planet is fully in transit, with lighter shading indicating the ingress and egress phases.
• Figure 2: Comparison of the same section of the spectra (normalised to continuum) before (top) and after telluric correction with Molecfit (bottom). The colour bar represents the orbital phases of the corresponding spectra. The telluric lines can be seen varying in depth due to changes in airmass before the correction.
• Figure 3: Spectral line positions of FeI lines taken from Dravins_2018 with one example spectrum (uncorrected for tellurics). Top: Example spectrum shown in grey. Middle: Wavelength positions of the weak FeI lines shown as red vertical lines. Bottom: Wavelength positions of the strong FeI lines, shown as green vertical lines.
• Figure 4: Schematic of the Doppler shadow method for the system HD 189733 b.
• Figure 5: Radial velocities of the strong FeI CCFs for the 11 of August 2021. The shaded area represents the time of transit, with the lighter sections representing the ingress and egress of the transit. The dots in red represent the in-transit observations, while blue represent out-of-transit observations. The blue line represents the linear fit to the motion of the star around its system's barycentre.