TOI-2109: An Ultrahot Gas Giant on a 16 hr Orbit

Wong, Ian; Shporer, Avi; Zhou, George; Kitzmann, Daniel; Komacek, Thaddeus D.; Tan, Xianyu; Tronsgaard, René; Buchhave, Lars A.; Vissapragada, Shreyas; Greklek-McKeon, Michael; Rodriguez, Joseph E.; Ahlers, John P.; Quinn, Samuel N.; Furlan, Elise; Howell, Steve B.; Bieryla, Allyson; Heng, Kevin; Knutson, Heather A.; Collins, Karen A.; McLeod, Kim K.; Berlind, Perry; Brown, Peyton; Calkins, Michael L.; de Leon, Jerome P.; Esparza-Borges, Emma; Esquerdo, Gilbert A.; Fukui, Akihiko; Gan, Tianjun; Girardin, Eric; Gnilka, Crystal L.; Ikoma, Masahiro; Jensen, Eric L. N.; Kielkopf, John; Kodama, Takanori; Kurita, Seiya; Lester, Kathryn V.; Lewin, Pablo; Marino, Giuseppe; Murgas, Felipe; Narita, Norio; Pallé, Enric; Schwarz, Richard P.; Stassun, Keivan G.; Tamura, Motohide; Watanabe, Noriharu; Benneke, Björn; Ricker, George R.; Latham, David W.; Vanderspek, Roland; Seager, Sara; Winn, Joshua N.; Jenkins, Jon M.; Caldwell, Douglas A.; Fong, William; Huang, Chelsea X.; Mireles, Ismael; Schlieder, Joshua E.; Shiao, Bernie; Noel Villaseñor, Jesus
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The Astronomical Journal

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We report the discovery of an ultrahot Jupiter with an extremely short orbital period of 0.67247414 ± 0.00000028 days (~16 hr). The 1.347 ± 0.047 R Jup planet, initially identified by the Transiting Exoplanet Survey Satellite (TESS) mission, orbits TOI-2109 (TIC 392476080)-a T eff ~ 6500 K F-type star with a mass of 1.447 ± 0.077 M ☉, a radius of 1.698 ± 0.060 R ☉, and a rotational velocity of $v\sin {i}_{* }=81.9\pm 1.7$ km s-1. The planetary nature of TOI-2109b was confirmed through radial-velocity measurements, which yielded a planet mass of 5.02 ± 0.75 M Jup. Analysis of the Doppler shadow in spectroscopic transit observations indicates a well-aligned system, with a sky-projected obliquity of λ = 1.°7 ± 1.°7. From the TESS full-orbit light curve, we measured a secondary eclipse depth of 731 ± 46 ppm, as well as phase-curve variations from the planet's longitudinal brightness modulation and ellipsoidal distortion of the host star. Combining the TESS-band occultation measurement with a K s -band secondary eclipse depth (2012 ± 80 ppm) derived from ground-based observations, we find that the dayside emission of TOI-2109b is consistent with a brightness temperature of 3631 ± 69 K, making it the second hottest exoplanet hitherto discovered. By virtue of its extreme irradiation and strong planet-star gravitational interaction, TOI-2109b is an exceptionally promising target for intensive follow-up studies using current and near-future telescope facilities to probe for orbital decay, detect tidally driven atmospheric escape, and assess the impacts of H2 dissociation and recombination on the global heat transport.
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