p-winds: An open-source Python code to model planetary outflows and upper atmospheres

Astronomy and Astrophysics, vol. 659, pp. A62

Abstract

Atmospheric escape is considered to be one of the main channels for evolution in sub-Jovian planets, particularly in their early lives. While there are several hypotheses proposed to explain escape in exoplanets, testing them with atmospheric observations remains a challenge. In this context, high-resolution transmission spectroscopy of transiting exoplanets for the metastable helium triplet (He 2 3 S) at 1083 nm has emerged as a reliable technique for observing and measuring escape. To aid in the prediction and interpretation of metastable He transmission spectroscopy observations, we developed the code p-winds . This is an open-source, fully documented, scalable Python implementation of the one-dimensional, purely H+He Parker wind model for upper atmospheres coupled with ionization balance, ray-tracing, and radiative transfer routines. We demonstrate an atmospheric retrieval by fitting p-winds models to the observed metastable He transmission spectrum of the warm Neptune HAT-P-11 b and take the variation in the in-transit absorption caused by transit geometry into account. For this planet, our best fit yields a total atmospheric escape rate of approximately 2.5 × 10 10 g s −1 and an outflow temperature of 7200 K. The range of retrieved mass loss rates increases significantly when we let the H atom fraction be a free parameter, but its posterior distribution remains unconstrained by He observations alone. The stellar host limb darkening does not have a significant impact on the retrieved escape rate or outflow temperature for HAT-P-11 b. Based on the non-detection of escaping He for GJ 436 b, we are able to rule out total escape rates higher than 3.4 × 10 10 g s −1 at 99.7% (3 σ ) confidence.

Authors 9

1. Leonardo A. Dos Santos

   University of Geneva · Space Telescope Science Institute

   Affiliation as printed

   Observatoire astronomique de l’Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland

   Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA

   Observatoire astronomique de l'Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland

2. A. A. Vidotto Aachen Leiden Observatory

   Leiden University · Trinity College Dublin · Leiden Observatory

   Affiliation as printed

   Leiden Observatory, Leiden University, Postbus 9513, 2300 RA Leiden, The Netherlands

   School of Physics, Trinity College Dublin, the University of Dublin, College Green, Dublin-2, Ireland

3. Shreyas Vissapragada

   California Institute of Technology

   Affiliation as printed

   Division of Geological and Planetary Sciences, California Institute of Technology, 1200 East California Blvd, Pasadena, CA 91125, USA

4. Munazza K. Alam

   Carnegie Institution for Science · Center for Astrophysics Harvard & Smithsonian · Earth and Planets Laboratory

   Affiliation as printed

   Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA

   Earth and Planets Laboratory, The Carnegie Institution for Science, 5241 Broad Branch Road, Washington, DC 20015, USA

5. Romain Allart

   Université de Montréal

   Affiliation as printed

   Department of Physics, and Institute for Research on Exoplanets, Université de Montréal, Montréal H3T 1J4, Canada

6. Vincent Bourrier

   University of Geneva

   Affiliation as printed

   Observatoire astronomique de l’Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland

   Observatoire astronomique de l'Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland

7. James A. Kirk

   Center for Astrophysics Harvard & Smithsonian

   Affiliation as printed

   Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA

8. J. V. Seidel

   University of Geneva · European Southern Observatory · European Southern Observatory

   Affiliation as printed

   European Southern Observatory, Alonso de Córdova 3107, Santiago, Casilla 19001, Chile

   Observatoire astronomique de l’Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland

   Observatoire astronomique de l'Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland

9. David Ehrenreich

   University of Geneva

   Affiliation as printed

   Observatoire astronomique de l’Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland

   Observatoire astronomique de l'Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland