TOI-431/HIP 26013: a super-Earth and a sub-Neptune transiting a bright, early K dwarf, with a third RV planet
Creators: name: Osborn, Ares
name: Armstrong, David J.
name: Cale, Bryson
name: Brahm, Rafael
name: Wittenmyer, Robert A.
name: Dai, Fei
name: Crossfield, Ian J. M.
name: Bryant, Edward M.
name: Adibekyan, Vardan
name: Cloutier, Ryan
name: Collins, Karen A.
name: Mena, E. Delgado
name: Fridlund, Malcolm
name: Hellier, Coel
name: Howell, Steve B.
name: King, George W.
name: Lillo-Box, Jorge
name: Otegi, Jon
name: Sousa, S.
name: Stassun, Keivan G.
name: Matthews, Elisabeth C.
name: Ziegler, Carl
name: Ricker, George
name: Vanderspek, Roland
name: Latham, David W.
name: Seager, S.
name: Winn, Joshua N.
name: Jenkins, Jon M.
name: Acton, Jack S.
name: Addison, Brett C.
name: Anderson, David R.
name: Ballard, Sarah
name: Barrado, David
name: Barros, Susana C. C.
name: Batalha, Natalie
name: Bayliss, Daniel
name: Barclay, Thomas
name: Benneke, Björn
name: Berberian, John
name: Bouchy, Francois
name: Bowler, Brendan P.
name: Briceño, César
name: Burke, Christopher J.
name: Burleigh, Matthew R.
name: Casewell, Sarah L.
name: Ciardi, David
name: Collins, Kevin I.
name: Cooke, Benjamin F.
name: Demangeon, Olivier D. S.
name: Díaz, Rodrigo F.
name: Dorn, C.
name: Dragomir, Diana
name: Dressing, Courtney
name: Dumusque, Xavier
name: Espinoza, Néstor
name: Figueira, P.
name: Fulton, Benjamin
name: Furlan, E.
name: Gaidos, E.
name: Geneser, C.
name: Gill, Samuel
name: Goad, Michael R.
name: Gonzales, Erica J.
name: Gorjian, Varoujan
name: Günther, Maximilian N.
name: Helled, Ravit
name: Henderson, Beth A.
name: Henning, Thomas
name: Hogan, Aleisha
name: Hojjatpanah, Saeed
name: Horner, Jonathan
name: Howard, Andrew W.
name: Hoyer, Sergio
name: Huber, Dan
name: Isaacson, Howard
name: Jenkins, James S.
name: Jensen, Eric L. N.
name: Jordán, Andrés
name: Kane, Stephen R.
name: Kidwell, Richard C.
name: Kielkopf, John
name: Law, Nicholas
name: Lendl, Monika
name: Lund, M.
name: Matson, Rachel A.
name: Mann, Andrew W.
name: McCormac, James
name: Mengel, Matthew W.
name: Morales, Farisa Y.
name: Nielsen, Louise D.
name: Okumura, Jack
name: Osborn, Hugh P.
name: Petigura, Erik A.
name: Plavchan, Peter
name: Pollacco, Don
name: Quintana, Elisa V.
name: Raynard, Liam
name: Robertson, Paul
name: Rose, Mark E.
name: Roy, Arpita
name: Reefe, Michael
name: Santerne, Alexandre
name: Santos, Nuno C.
name: Sarkis, Paula
name: Schlieder, J.
name: Schwarz, Richard P.
name: Scott, Nicholas J.
name: Shporer, Avi
name: Smith, A. M. S.
name: Stibbard, C.
name: Stockdale, Chris
name: Strøm, Paul A.
name: Twicken, Joseph D.
name: Tan, Thiam-Guan
name: Tanner, A.
name: Teske, J.
name: Tilbrook, Rosanna H.
name: Tinney, C. G.
name: Udry, Stephane
name: Villaseñor, Jesus Noel
name: Vines, Jose I.
name: Wang, Sharon X.
name: Weiss, Lauren M.
name: West, Richard G.
name: Wheatley, Peter J.
name: Wright, Duncan J.
name: Zhang, Hui
name: Zohrabi, F.
Publisher: name: arXiv
Year: 2021
Subjects: subject: Earth and Planetary Astrophysics (astro-ph.EP)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: We present the bright (V$_{mag} = 9.12$), multi-planet system TOI-431, characterised with photometry and radial velocities. We estimate the stellar rotation period to be $30.5 \pm 0.7$ days using archival photometry and radial velocities. TOI-431b is a super-Earth with a period of 0.49 days, a radius of 1.28 $\pm$ 0.04 R$_{\oplus}$, a mass of $3.07 \pm 0.35$ M$_{\oplus}$, and a density of $8.0 \pm 1.0$ g cm$^{-3}$; TOI-431d is a sub-Neptune with a period of 12.46 days, a radius of $3.29 \pm 0.09$ R$_{\oplus}$, a mass of $9.90^{+1.53}_{-1.49}$ M$_{\oplus}$, and a density of $1.36 \pm 0.25$ g cm$^{-3}$. We find a third planet, TOI-431c, in the HARPS radial velocity data, but it is not seen to transit in the TESS light curves. It has an $M \sin i$ of $2.83^{+0.41}_{-0.34}$ M$_{\oplus}$, and a period of 4.85 days. TOI-431d likely has an extended atmosphere and is one of the most well-suited TESS discoveries for atmospheric characterisation, while the super-Earth TOI-431b may be a stripped core. These planets straddle the radius gap, presenting an interesting case-study for atmospheric evolution, and TOI-431b is a prime TESS discovery for the study of rocky planet phase curves.
description: 21 pages, 11 figures, 3 appendices, accepted for publication in MNRAS