# Rapid rotators revisited: absolute dimensions of KOI-13

## TL;DR

This study models the KOI-13 exoplanet system using Kepler data and a Roche model to precisely determine stellar and planetary dimensions, rotation, and orbital alignment without extensive assumptions.

## Contribution

It introduces a minimal-parameter Roche-model approach to derive absolute system dimensions from light-curves and spectroscopic data, improving understanding of rapidly rotating star-planet systems.

## Key findings

- Planet radius: 1.33 R(Jup)
- Stellar radius: 1.55 R(sun)
- Orbital and stellar angular momentum offset: 60.25 degrees

## Abstract

We analyse Kepler light-curves of the exoplanet KOI-13b transiting its moderately rapidly rotating (gravity-darkened) parent star. A physical model, with minimal ad hoc free parameters, reproduces the time-averaged light-curve at the ca. 10 parts per million level. We demonstrate that this Roche-model solution allows the absolute dimensions of the system to be determined from the star's projected equatorial rotation speed, v(e)sin(i), without any additional assumptions; we find a planetary radius 1.33+/-0.05 R(Jup), stellar polar radius 1.55+/-0.06 R(sun), combined mass M(*) + M(P) (\simeq M*) = 1.47 +/- 0.17 M(sun), and distance d \simeq 370+/-25 pc, where the errors are dominated by uncertainties in relative flux contribution of the visual-binary companion KOI-13B. The implied stellar rotation period is within ca. 5% of the non-orbital, 25.43-hr signal found in the Kepler photometry. We show that the model accurately reproduces independent tomographic observations, and yields an offset between orbital and stellar-rotation angular-momentum vectors of 60.25+/-0.05 degrees.