Identifying and Fitting Eclipse Maps of Exoplanets with Cross-Validation

arXiv (Cornell University)

Abstract

Eclipse mapping uses the shape of the eclipse of an exoplanet to measure its two-dimensional structure. Light curves are mostly composed of longitudinal information, with the latitudinal information only contained in the brief ingress and egress of the eclipse. This imbalance can lead to a spuriously confident map, where the longitudinal structure is constrained by out-of-eclipse data and the latitudinal structure is wrongly determined by the priors on the map. We present a new method to address this issue. The method tests for the presence of an eclipse mapping signal by using k-fold cross-validation to compare the performance of a simple mapping model to the null hypothesis of a uniform disk. If a signal is found, the method fits a map with more degrees of freedom, optimising its information content. The information content is varied by penalising the model likelihood by a factor proportional to the spatial entropy of the map, optimised by cross-validation. We demonstrate this method for simulated datasets then apply it to three observational datasets. The method identifies an eclipse mapping signal for JWST MIRI/LRS observations of WASP-43b but does not identify a signal for JWST NIRISS/SOSS observations of WASP-18b or Spitzer Space Telescope observations of HD 189733b. It is possible to fit eclipse maps to these datasets, but we suggest that these maps are overfitting the eclipse shape. We fit a new map with more spatial freedom to the WASP-43b dataset and show a flatter east-west structure than previously derived.

Authors 13

1. Mark Hammond

   University of Oxford

   Affiliation as printed

   Atmospheric , Oceanic and Planetary Physics , Department of Physics , University of Oxford , Oxford , UK

2. Neil T. Lewis

   University of Exeter

   Affiliation as printed

   Department of Physics and Astronomy , University of Exeter , Exeter , UK

3. Sasha Boone

   University of Oxford

   Affiliation as printed

   Atmospheric , Oceanic and Planetary Physics , Department of Physics , University of Oxford , Oxford , UK

4. Xueqing Chen

   Royal Observatory · University of Edinburgh

   Affiliation as printed

   Institute for Astronomy , University of Edinburgh , Royal Observatory , Blackford Hill , Edinburgh , EH9 3HJ , UK

5. João M. Mendonça
6. Vivien Parmentier

   Université Côte d'Azur

   Affiliation as printed

   Université Côte d'Azur , Av. Valrose , 06000 Nice , France

7. Jake Taylor

   University of Oxford

   Affiliation as printed

   Astrophysics , Department of Physics , University of Oxford , Oxford , UK

8. Taylor James Bell

   Ames Research Center

   Affiliation as printed

   BAER Institute , NASA Ames Research Center , Moffett Field , CA , USA

   Space Science and Astrobiology Division , NASA Ames Research Center , Moffett Field , CA , USA

9. Leonardo A. Dos Santos
10. Nicolas Crouzet Aachen Leiden Observatory

    Leiden University · Leiden Observatory

    Affiliation as printed

    Leiden Observatory , Leiden University , P.O. Box 9513 , 2300 RA Leiden , The Netherlands

11. Laura Kreidberg

    Max Planck Institute for Astronomy

    Affiliation as printed

    Max Planck Institute for Astronomy , Heidelberg , Germany

12. Michael Radica

    Université de Montréal

    Affiliation as printed

    Trottier Institute for Research on Exoplanets , Université de Montréal , Montréal , Québec , Canada

13. Michael Zhang

    University of Chicago

    Affiliation as printed

    Department of Astronomy and Astrophysics , The University of Chicago , Chicago , IL 60637 , USA