The Information Content in Analytic Spot Models of Broadband Precision Lightcurves

TL;DR

This paper investigates the limitations and degeneracies in modeling starspot lightcurves, highlighting how certain parameters are difficult to constrain and how differential rotation affects period detection.

Contribution

The study systematically assesses parameter degeneracies in starspot models using synthetic data, emphasizing the challenges in uniquely determining stellar inclination and spot latitudes.

Findings

Spot latitude and stellar inclination are hard to determine without additional constraints.

Differential rotation can be inferred when spots are on opposite hemispheres.

Dominant spots can mask differential rotation signals in periodograms.

Abstract

We present the results of numerical experiments to assess degeneracies in lightcurve models of starspots. Using synthetic lightcurves generated with the Cheetah starspot modeling code, we explore the extent to which photometric light curves constrain spot model parameters, including spot latitudes and stellar inclination. We also investigate the effects of spot parameters and differential rotation on one's ability to correctly recover rotation periods and differential rotation in the Kepler lightcurves. We confirm that in the absence of additional constraints on the stellar inclination, such as spectroscopic measurements of vsini or occultations of starspots by planetary transits, the spot latitude and stellar inclination are difficult to determine uniquely from the photometry alone. We find that for models with no differential rotation, spots that appear on opposite hemispheres of the star may cause one to interpret the rotation period to be half of the true period. When differential rotation is included, the changing longitude separation between spots breaks the symmetry of the hemispheres and the correct rotation period is more likely to be found. The dominant period found via periodogram analysis is typically that of the largest spot. Even when multiple spots with periods representative of the star's differential rotation exist, if one spot dominates the lightcurve the signal of differential rotation may not be detectable from the periodogram alone. Starspot modeling is applicable to stars with a wider range of rotation rates than other surface imaging techniques (such as Doppler imaging), allows subtle signatures of differential rotation to be measured, and may provide valuable information on the distribution of stellar spots. However, given the inherent degeneracies and uncertainty present in starspot models, caution should be exercised in their interpretation.