TOI−757 b: an eccentric transiting mini−Neptune on a 17.5−d orbit

A Alqasim,N Grieves,N M Rosário,D Gandolfi,J H Livingston,S Sousa,K A Collins,J K Teske,M Fridlund,J A Egger,J Cabrera,C Hellier,A F Lanza,V Van Eylen,F Bouchy,R J Oelkers,G Srdoc,S Shectman,M Günther,E Goffo,T Wilson,L M Serrano,A Brandeker,S X Wang,A Heitzmann,A Bonfanti,L Fossati,Y Alibert,L Delrez,R Sefako,S Barros,K I Collins,O D S Demangeon,S H Albrecht,R Alonso,J Asquier,T Barczy,D Barrado,W Baumjohann,T Beck,W Benz,N Billot,L Borsato,C Broeg,E M Bryant,R P Butler,W D Cochran,A Collier Cameron,A C M Correia,J D Crane,Sz Csizmadia,P E Cubillos,M B Davies,T Daylan,M Deleuil,A Deline,B-O Demory,A Derekas,B Edwards,D Ehrenreich,A Erikson,Z Essack,A Fortier,K Gazeas,M Gillon,M Gudel,J Hasiba,A P Hatzes,Ch Helling,T Hirano,S B Howell,S Hoyer,K G Isaak,J M Jenkins,S Kanodia,L L Kiss,J Korth,K W F Lam,J Laskar,A Lecavelier Des Etangs,M Lendl,M B Lund,R Luque,A W Mann,D Magrin,P F L Maxted,C Mordasini,N Narita,V Nascimbeni,G Nowak,G Olofsson,H P Osborn,H L M Osborne,D Osip,R Ottensamer,I Pagano,E Palle,G Peter,G Piotto,D Pollacco,D Queloz,R Ragazzoni,N Rando,H Rauer,S Redfield,I Ribas,M Rice,G R Ricker,M Rieder,S Salmon,N C Santos,G Scandariato,S Seager,D Segransan,A Shporer,A E Simon,A M S Smith,M Stalport,Gy M Szabo,I Thompson,J D Twicken,S Udry,R Vanderspek,V Van Grootel,J Venturini,E Villaver,J Villaseñor,V Viotto,I Walter,N A Walton,J N Winn,S W Yee

doi:10.1093/mnras/stae1767

Abstract

Abstract We report the spectroscopic confirmation and fundamental properties of TOI−757 b, a mini−Neptune on a 17.5−day orbit transiting a bright star (V = 9.7 mag) discovered by the TESS mission. We acquired high−precision radial velocity measurements with the HARPS, ESPRESSO, and PFS spectrographs to confirm the planet detection and determine its mass. We also acquired space−borne transit photometry with the CHEOPS space telescope to place stronger constraints on the planet radius, supported with ground−based LCOGT photometry. WASP and KELT photometry were used to help constrain the stellar rotation period. We also determined the fundamental parameters of the host star. We find that TOI−757 b has a radius of Rp = 2.5 ± 0.1R⊕ and a mass of $M_{\mathrm{p}} = 10.5^{+2.2}_{-2.1} M_{\oplus }$, implying a bulk density of ρp = 3.6 ± 0.8 g cm−3. Our internal composition modeling was unable to constrain the composition of TOI−757 b, highlighting the importance of atmospheric observations for the system. We also find the planet to be highly eccentric with e = 0.39$^{+0.08}_{-0.07}$, making it one of the very few highly eccentric planets among precisely characterized mini−Neptunes. Based on comparisons to other similar eccentric systems, we find a likely scenario for TOI−757 b’s formation to be high eccentricity migration due to a distant outer companion. We additionally propose the possibility of a more intrinsic explanation for the high eccentricity due to star−star interactions during the earlier epoch of the Galactic disk formation, given the low metallicity and older age of TOI−757.

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