The EVIL-MC Model for Ellipsoidal Variations of Planet-Hosting Stars and Applications to the HAT-P-7 System

TL;DR

The paper introduces the EVIL-MC model for efficiently analyzing ellipsoidal variations in star-planet systems, enabling rapid and accurate determination of planetary parameters from photometric data, exemplified by the HAT-P-7 system.

Contribution

The EVIL-MC model offers a computationally efficient and accurate method for modeling stellar ellipsoidal variations induced by low-mass companions, improving analysis of exoplanet systems.

Findings

Confirmed the planet-star mass ratio for HAT-P-7

Revised the planet's dayside brightness temperature to 2680 K

Found a significant difference between day- and nightside planetary flux

Abstract

We present a new model for Ellipsoidal Variations Induced by a Low-Mass Companion, the EVIL-MC model. We employ several approximations appropriate for planetary systems to substantially increase the computational efficiency of our model relative to more general ellipsoidal variation models and improve upon the accuracy of simpler models. This new approach gives us a unique ability to rapidly and accurately determine planetary system parameters. We use the EVIL-MC model to analyze Kepler Quarter 0-2 (Q0-2) observations of the HAT-P-7 system, an F-type star orbited by a nearly Jupiter-mass companion. Our analysis corroborates previous estimates of the planet-star mass ratio q = (1.10 +/- 0.06) x 10^(-3), and we have revised the planet's dayside brightness temperature to 2680 +10/-20 K. We also find a large difference between the day- and nightside planetary flux, with little nightside emission. Preliminary dynamical+radiative modeling of the atmosphere indicates this result is qualitatively consistent with high altitude absorption of stellar heating. Similar analyses of Kepler and CoRoT photometry of other planets using EVIL-MC will play a key role in providing constraints on the properties of many extrasolar systems, especially given the limited resources for follow-up and characterization of these systems. However, as we highlight, there are important degeneracies between the contributions from ellipsoidal variations and planetary emission and reflection. Consequently, for many of the hottest and brightest Kepler and CoRoT planets, accurate estimates of the planetary emission and reflection, diagnostic of atmospheric heat budgets, will require accurate modeling of the photometric contribution from the stellar ellipsoidal variation.