1. Detection of Na in WASP-21b's lower and upper atmosphere
- Author
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Nikku Madhusudhan, Guo Chen, Enric Palle, Rafael Luque, Felipe Murgas, Núria Casasayas-Barris, Luis Welbanks, Welbanks Camarena, Luis [0000-0003-0156-4564], Nikku, Madhusudhan [0000-0002-4869-000X], and Apollo - University of Cambridge Repository
- Subjects
Gran Telescopio Canarias ,010504 meteorology & atmospheric sciences ,FOS: Physical sciences ,Astrophysics ,01 natural sciences ,Atmosphere ,planets and satellites: individual: WASP-21b ,Planet ,0103 physical sciences ,Hot Jupiter ,010303 astronomy & astrophysics ,planetary systems ,Solar and Stellar Astrophysics (astro-ph.SR) ,0105 earth and related environmental sciences ,Line (formation) ,Earth and Planetary Astrophysics (astro-ph.EP) ,Physics ,planets and satellites: atmospheres ,Astronomy and Astrophysics ,Planetary system ,Radial velocity ,Atmosphere of Earth ,Astrophysics - Solar and Stellar Astrophysics ,13. Climate action ,Space and Planetary Science ,Astrophysics::Earth and Planetary Astrophysics ,techniques: spectroscopic ,Astrophysics - Earth and Planetary Astrophysics - Abstract
Optical transmission spectroscopy provides crucial constraints on the reference pressure levels and scattering properties for hot Jupiter atmospheres. For certain planets, where alkali atoms are detected in the atmosphere, their line profiles could serve as a good probe to link upper and lower atmospheric layers. WASP-21b is a Saturn-mass hot Jupiter orbiting a thick disc star, with a low density and an equilibrium temperature of 1333 K, which makes it a good target for transmission spectroscopy. Here, we present a low-resolution transmission spectrum for WASP-21b based in one transit observed by the OSIRIS spectrograph at the 10.4 m Gran Telescopio Canarias (GTC), and a high-resolution transmission spectrum based in three transits observed by HARPS-N at Telescopio Nazinale Galileo (TNG) and HARPS at the ESO 3.6 m telescope. We performed spectral retrieval analysis on GTC's low-resolution transmission spectrum and report the detection of Na at a confidence level of $>$3.5-$\sigma$. The Na line exhibits a broad line profile that can be attributed to pressure broadening, indicating a mostly clear planetary atmosphere. The spectrum shows a tentative excess absorption at the K D$_1$ line. Using HARPS-N and HARPS, we spectrally resolved the Na doublet transmission spectrum. An excess absorption at the Na doublet is detected during the transit, and shows a radial velocity shift consistent with the planet orbital motion. We proposed a metric to quantitatively distinguish hot Jupiters with relatively clear atmospheres from others, and WASP-21b has the largest metric value among all the characterized hot Jupiters. The detection of Na at both lower and upper atmosphere of WASP-21b reveals that it is an ideal target for future follow-up observations, providing the opportunity to understand the nature of its atmosphere across a wide range of pressure levels., Comment: 16 pages, 14 figures, accepted for publication in A&A on July 27, 2020
- Published
- 2020