A close-in puffy Neptune with hidden friends: The enigma of TOI 620
Creators: name: Reefe, Michael A.
name: Luque, Rafael
name: Gaidos, Eric
name: Beard, Corey
name: Plavchan, Peter P.
name: Cointepas, Marion
name: Cale, Bryson L.
name: Palle, Enric
name: Parviainen, Hannu
name: Feliz, Dax L.
name: Eastman, Jason
name: Stassun, Keivan
name: Gagné, Jonathan
name: Jenkins, Jon M.
name: Boyd, Patricia T.
name: Kidwell, Richard C.
name: McDermott, Scott
name: Collins, Karen A.
name: Fong, William
name: Guerrero, Natalia
name: Almenara-Villa, Jose-Manuel
name: Bean, Jacob
name: Beichman, Charles A.
name: Berberian, John
name: Bieryla, Allyson
name: Bonfils, Xavier
name: Bouchy, François
name: Brady, Madison
name: Bryant, Edward M.
name: Cacciapuoti, Luca
name: Cañas, Caleb I.
name: Ciardi, David R.
name: Collins, Kevin I.
name: Crossfield, Ian
name: Dressing, Courtney D.
name: Eigmueller, Philipp
name: Mufti, Mohammed El
name: Esparza-Borges, Emma
name: Fukui, Akihiko
name: Gao, Peter
name: Geneser, Claire
name: Gnilka, Crystal L.
name: Gonzales, Erica
name: Gupta, Arvind F.
name: Halverson, Sam
name: Hearty, Fred
name: Howell, Steve B.
name: Irwin, Jonathan
name: Kanodia, Shubham
name: Kasper, David
name: Kodama, Takanori
name: Kostov, Veselin
name: Latham, David W.
name: Lendl, Monika
name: Lin, Andrea
name: Livingston, John H.
name: Lubin, Jack
name: Mahadevan, Suvrath
name: Matson, Rachel
name: Matthews, Elisabeth
name: Murgas, Felipe
name: Narita, Norio
name: Newman, Patrick
name: Ninan, Joe
name: Osborn, Ares
name: Quinn, Samuel N.
name: Robertson, Paul
name: Roy, Arpita
name: Schlieder, Joshua
name: Schwab, Christian
name: Seifahrt, Andreas
name: Smith, Gareth D.
name: Sohani, Ahmad
name: Stefánsson, Guðmundur
name: Stevens, Daniel
name: Stürmer, Julian
name: Tanner, Angelle
name: Terrien, Ryan
name: Teske, Johanna
name: Vermilion, David
name: Wang, Sharon X.
name: Wittrock, Justin
name: Wright, Jason T.
name: Zechmeister, Mathias
name: Zohrabi, Farzaneh
Publisher: name: arXiv
Year: 2022
Subjects: subject: Earth and Planetary Astrophysics (astro-ph.EP)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: We present the validation of a transiting low-density exoplanet orbiting the M2.5 dwarf TOI 620 discovered by the NASA TESS mission. We utilize photometric data from both TESS and ground-based follow-up observations to validate the ephemerides of the 5.09-day transiting signal and vet false positive scenarios. High-contrast imaging data are used to resolve the stellar host and exclude stellar companions at separations $\gtrsim 0.2''$. We obtain follow-up spectroscopy and corresponding precise radial velocities (RVs) with multiple PRV spectrographs to confirm the planetary nature of the transiting exoplanet. We calculate a 5$σ$ upper limit of $M_P &lt; 7.1$ M$_\oplus$ and $ρ_P &lt; 0.74$ g cm$^{-3}$, and we identify a non-transiting 17.7-day candidate. We also find evidence for a substellar (1-20 M$_{\rm J}$) companion with a projected separation $\lesssim 20$ au from a combined analysis of Gaia, AO imaging, and RVs. With the discovery of this outer companion, we carry out a detailed exploration of the possibilities that TOI 620 b might instead be a circum-secondary planet or a pair of eclipsing binary stars orbiting the host in a hierarchical triple system. We find, under scrutiny, that we can exclude both of these scenarios from the multi-wavelength transit photometry, thus validating TOI 620 b as a low-density exoplanet transiting the central star in this system. The low density of TOI 620 b makes it one of the most amenable exoplanets for atmospheric characterization, such as with JWST and Ariel, validated or confirmed by the TESS mission to date.
description: 64 pages, 34 figures, 22 tables. Accepted for publication in the Astronomical Journal