The center-to-limb variation across the Fraunhofer lines of HD 189733; Sampling the stellar spectrum using a transiting planet

TL;DR

This study uses high-resolution transit spectra of HD 189733 to analyze the center-to-limb variation in Fraunhofer lines, revealing limb brightening effects that are crucial for accurate exoplanet atmosphere characterization.

Contribution

It provides the first detailed observational analysis of CLV effects in specific Fraunhofer lines of an active star using a transiting planet as a probe.

Findings

Fraunhofer line wings are limb brightened as predicted by models.

CLV effects can mimic or influence exoplanet atmospheric signals.

Careful modeling of stellar CLV is essential for accurate transit spectroscopy.

Abstract

The center-to-limb variation (CLV) describes the brightness of the stellar disk as a function of the limb angle. Across strong absorption lines, the CLV can vary quite significantly. We obtained a densely sampled time series of high-resolution transit spectra of the active planet host star HD 189733 with UVES. Using the passing planetary disk of the hot Jupiter HD 189733 b as a probe, we study the CLV in the wings of the Ca II H and K and Na I D1 and D2 Fraunhofer lines, which are not strongly affected by activity-induced variability. In agreement with model predictions, our analysis shows that the wings of the studied Fraunhofer lines are limb brightened with respect to the (quasi-)continuum. The strength of the CLV-induced effect can be on the same order as signals found for hot Jupiter atmospheres. Therefore, a careful treatment of the wavelength dependence of the stellar CLV in strong absorption lines is highly relevant in the interpretation of planetary transit spectroscopy.