Time-resolved Rotational Velocities in the Upper Atmosphere of WASP-33 b
Abstract
While steady empirical progress has been made in understanding the structure and composition of hot-planet atmospheres, direct measurements of velocity signatures, including winds, rotation, and jets, have lagged behind. Quantifying atmospheric dynamics of hot planets is critical for a complete understanding of their atmospheres, and such measurements may even illuminate other planetary properties, such as magnetic field strengths. In this manuscript we present the first detection of the Balmer lines Hα and Hβ in the atmosphere of the ultra-hot Jupiter WASP-33 b. Using atmospheric models that include the effects of atmospheric dynamics, we show that the shape of the average Balmer line transmission spectrum is consistent with rotational velocities in the planet's thermosphere of ${v}_{\mathrm{rot}}={10.1}_{-1.0}^{+0.8}$ km s-1. We also measure a low-significance day- to nightside velocity shift of $-{4.6}_{-3.4}^{+3.4}$ km s-1 in the transmission spectrum, which is naturally explained by a global wind across the planet's terminator. In a separate analysis the time-resolved velocity centroids of individual transmission spectra show unambiguous evidence of rotation, with a best-fit velocity of ${10.0}_{-2.0}^{+2.4}$ km s-1, consistent with the value of vrot derived from the shape of the average Balmer line transmission spectrum. Our observations and analysis confirm the power of time-resolved transmission spectra with a high signal-to-noise ratio to measure the velocity structures in exoplanet atmospheres. The high rotational and wind velocities we measure highlight the need for more detailed 3D global climate simulations of the rarefied upper atmospheres of ultra-hot gas giants. * Based on data acquired with PEPSI using the Large Binocular Telescope (LBT). The LBT is an international collaboration among institutions in the United States, Italy, and Germany. LBT Corporation partners are the University of Arizona on behalf of the Arizona university system; Istituto Nazionale di Astrofisica, Italy; LBT Beteiligungsgesellschaft, Germany, representing the Max-Planck Society, the Leibniz-Institute for Astrophysics Potsdam (AIP), and Heidelberg University; the Ohio State University; and the Research Corporation, on behalf of the University of Notre Dame, University of Minnesota and University of Virginia.
- Publication:
-
The Astronomical Journal
- Pub Date:
- March 2021
- DOI:
- 10.3847/1538-3881/abde43
- arXiv:
- arXiv:2010.02118
- Bibcode:
- 2021AJ....161..152C
- Keywords:
-
- Exoplanet atmospheres;
- Exoplanets;
- Hot Jupiters;
- 487;
- 498;
- 753;
- Astrophysics - Earth and Planetary Astrophysics
- E-Print:
- Accepted to AAS Journals. This re-submission updates the original version with changes made during the referee process. None of the changes affect the conclusions of the paper, although a few specific numbers have new values. We also note that during the review process we became aware of a contemporaneous detection of the Balmer lines in WASP-33 b's atmosphere by Yan et al. 2020, A&