On stellar limb darkening and exoplanetary transits

TL;DR

This paper explores the challenges of comparing stellar limb-darkening coefficients from models and observations, especially in exoplanet transits, highlighting discrepancies and proposing synthetic photometry methods for better comparison.

Contribution

It introduces a synthetic photometry approach to compare model-derived and observed limb-darkening coefficients in exoplanet transits, addressing inconsistencies in previous methods.

Findings

Good agreement for some stars between models and observations.

Discrepancies observed even among stars with similar parameters.

Synthetic photometry can improve comparison accuracy.

Abstract

This paper examines how to compare stellar limb-darkening coefficients evaluated from model atmospheres with those derived from photometry. Different characterizations of a given model atmosphere can give quite different numerical results (even for a given limb-darkening `law'), while light-curve analyses yield limb-darkening coefficients that are dependent on system geometry, and that are not directly comparable to any model-atmosphere representation. These issues are examined in the context of exoplanetary transits, which offer significant advantages over traditional binary-star eclipsing systems in the study of stellar limb darkening. `Like for like' comparisons between light-curve analyses and new model-atmosphere results, mediated by synthetic photometry, are conducted for a small sample of stars. Agreement between the resulting synthetic-photometry/atmosphere-model (SPAM) limb-darkening coefficients and empirical values ranges from very good to quite poor, even though the targets investigated show only a small dispersion in fundamental stellar parameters.