Exoplanets & Exomoons

Retrieving Exoplanet Atmospheric Winds From High-resolution Spectroscopy

By Keith Cowing
Press Release
December 6, 2019
Filed under
Retrieving Exoplanet Atmospheric Winds From High-resolution Spectroscopy
Illustration of the implementation of the day-to-night side wind pattern with a 1D calculation of the atmosphere. A polar view is shown on the left and an equatorial view on the right. The wind is indicated by the dotted blue arrows. Left: The extinction coefficient is calculated in altitude along the z axis and then transposed in the x direction along the LOS (in dark green). The LOS is then iterated upward in z until the top of the atmosphere is reached and all values saved in a 2D grid (here visualised as a slice in light green). In each bin of the 2D grid the velocity and the broadened profile are calculated and stored. This is only necessary on one side of the atmosphere due to the symmetry of the problem. Right: The calculated slice is then rotated to create the full atmosphere. The line of sight is shown as a dark green box in the main illustration, where the reader is in the position of the observer. Points indicate a flow towards the reader. In the day-to-night side case, the morning and evening limb winds point towards the observer and only one atmospheric slice has to be calculated. The slice is then rotated by 2π to create the full atmosphere. In this simplification of the atmosphere the wind will not go parallel to the equator at all times, but point towards the center of the night side. This reduces calculation time significantly, given that it reduces the problem from 3D to 2D, with the extinction coefficient only calculated in 1D.

The atmosphere of exoplanets has been studied extensively in recent years, using numerical models to retrieve chemical composition, dynamical circulation or temperature from data.

One of the best observational probes in transmission is the sodium doublet, due to its large cross section.

However, modelling the shape of the planetary sodium lines has proven to be challenging. Models with different assumptions regarding the atmosphere have been employed to fit the lines in the literature, yet statistically sound direct comparisons of different models are needed to paint a clear picture. Aims. We will compare different wind and temperature patterns and provide a tool to distinguish them driven by their best fit for the sodium transmission spectrum of the hot Jupiter HD 189733b. We parametrise different possible wind patterns already tested in literature and introduce the new option of an upwards driven vertical wind. Methods.

We construct a forward model where the wind speed, wind geometry and temperature are injected into the calculation of the transmission spectrum. We embed this forward model in a nested sampling retrieval code to rank the models via their Bayesian evidence. Results. We retrieve a best-fit to the HD 189733b data for vertical upward winds |v⃗ ver(mean)|=40±4 km/s at altitudes above 10−6 bar. With the current data from HARPS, we cannot distinguish wind patterns for higher pressure atmospheric layers. Conclusions. We show that vertical upwards winds in the upper atmosphere are a possible explanation for the broad sodium signature in hot Jupiters. We highlight other influences on the width of the doublet and explore strong magnetic fields acting on the lower atmosphere as one possible origin of the retrieved wind speed.

J. V. Seidel (1), D. Ehrenreich (1), L. Pino (2), V. Bourrier (1), B. Lavie (1), R. Allart (1), A. Wyttenbach (3), C. Lovis (1) (Observatoire astronomique de l’Université de Genève, CH (1), Anton Pannekoek Institute for Astronomy, University of Amsterdam, NL (2), Leiden Observatory, Leiden University, NL (3))
(Submitted on 5 Dec 2019)

Comments: 17 pages, 30 figures, accepted for publication in Astronomy & Astrophysics (04.12.2019)
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:1912.02787 [astro-ph.EP] (or arXiv:1912.02787v1 [astro-ph.EP] for this version)

Submission history
From: J.V. Seidel
[v1] Thu, 5 Dec 2019 18:28:28 UTC (9,044 KB)

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