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Characterizing a world within the hot-neptune desert: Transit observations of LTT 9779 b with the Hubble space telescope/WFC3

Edwards, Billy, Changeat, Quentin, Tsiaras, Angelos, Allan, Andrew, Behr, Patrick, Hagey, Simone R., Himes, Michael D., Ma, Sushuang, Stassun, Keivan G., Thomas, Luis, Thompson, Alexandra, Boley, Aaron, Booth, Luke, Bouwman, Jeroen, France, Kevin, Lowson, Nataliea, Meech, Annabella, Phillips, Caprice L., Vidotto, Aline A., Yip, Kai Hou, Bieger, Michelle, Gressier, Amélie, Janin, Estelle, Jiang, Ing-Guey, Leonardi, Pietro, Sarkar, Subhajit, Skaf, Nour, Taylor, Jake, Yang, Ming and Ward-Thompson, Derek ORCID: 2023. Characterizing a world within the hot-neptune desert: Transit observations of LTT 9779 b with the Hubble space telescope/WFC3. Astronomical Journal 166 (4) , 158. 10.3847/1538-3881/acea77

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We present an atmospheric analysis of LTT 9779 b, a rare planet situated in the hot-Neptune desert, that has been observed with Hubble Space Telescope (HST)/WFC3 with G102 and G141. The combined transmission spectrum, which covers 0.8–1.6 μm, shows a gradual increase in transit depth with wavelength. Our preferred atmospheric model shows evidence for H2O, CO2, and FeH with a significance of 3.1σ, 2.4σ, and 2.1σ, respectively. In an attempt to constrain the rate of atmospheric escape for this planet, we search for the 1.083 μm helium line in the G102 data but find no evidence of excess absorption that would indicate an escaping atmosphere using this tracer. We refine the orbital ephemerides of LTT 9779 b using our HST data and observations from TESS, searching for evidence of orbital decay or apsidal precession, which are not found. The phase-curve observation of LTT 9779 b with JWST NIRISS should provide deeper insights into the atmosphere of this planet and the expected atmospheric escape might be detected with further observations concentrated on other tracers such as Lyα.

Item Type: Article
Date Type: Published Online
Status: Published
Schools: Physics and Astronomy
Additional Information: License information from Publisher: LICENSE 1: URL:, Type: cc-by
Publisher: IOP Publishing
ISSN: 0004-6256
Date of First Compliant Deposit: 18 September 2023
Date of Acceptance: 21 June 2023
Last Modified: 26 Sep 2023 17:07

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