The TESS-Keck Survey II: An Ultra-Short Period Rocky Planet and its Siblings Transiting the Galactic Thick-Disk Star TOI-561
Creators: name: Weiss, Lauren M.
name: Dai, Fei
name: Huber, Daniel
name: Brewer, John M.
name: Collins, Karen A.
name: Ciardi, David R.
name: Matthews, Elisabeth C.
name: Ziegler, Carl
name: Howell, Steve B.
name: Batalha, Natalie M.
name: Crossfield, Ian J. M>
name: Dressing, Courtney
name: Fulton, Benjamin
name: Howard, Andrew W.
name: Isaacson, Howard
name: Kane, Stephen R.
name: Petigura, Erik A.
name: Robertson, Paul
name: Roy, Arpita
name: Rubenzahl, Ryan A.
name: Twicken, Joseph D.
name: Claytor, Zachary R.
name: Stassun, Keivan G.
name: MacDougall, Mason G.
name: Chontos, Ashley
name: Giacalone, Steven
name: Dalba, Paul A.
name: Mocnik, Teo
name: Hill, Michelle L.
name: Beard, Corey
name: Murphy, Joseph M. Akana
name: Rosenthal, Lee J.
name: Behmard, Aida
name: van Zandt, Judan
name: Lubin, Jack
name: Kosiarek, Molly R.
name: Lund, Michael B.
name: Christiansen, Jessie L.
name: Matson, Rachel A.
name: Beichman, Charles A.
name: Schlieder, Joshua E.
name: Gonzales, Erica J.
name: Briceño, César
name: Law, Nicholas
name: Mann, Andrew W.
name: Collins, Kevin I.
name: Evans, Phil
name: Fukui, Akihiko
name: Jensen, Eric L. N.
name: Murgas, Felipe
name: Narita, Norio
name: Palle, Eric
name: Parviainen, Hannu
name: Schwarz, Richard P.
name: Tan, Thiam-Guan
name: Acton, Jack S.
name: Bryant, Edwarf M.
name: Chaushev, Alexander
name: Eigmüller, Philipp
name: Gill, Sam
name: Jenkins, Jon
name: Ricker, George
name: Seager, Sara
name: Winn, Joshua N.
Publisher: name: arXiv
Year: 2020
Subjects: subject: Earth and Planetary Astrophysics (astro-ph.EP)
subject: Astrophysics of Galaxies (astro-ph.GA)
subject: Solar and Stellar Astrophysics (astro-ph.SR)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: We report the discovery of TOI-561, a multi-planet system in the galactic thick disk that contains a rocky, ultra-short period planet (USP). This bright ($V=10.2$) star hosts three small transiting planets identified in photometry from the NASA TESS mission: TOI-561 b (TOI-561.02, P=0.44 days, $R_b = 1.45\pm0.11\,R_\oplus$), c (TOI-561.01, P=10.8 days, $R_c=2.90\pm0.13\,R_\oplus$), and d (TOI-561.03, P=16.3 days, $R_d=2.32\pm0.16\,R_\oplus$). The star is chemically ([Fe/H]$=-0.41\pm0.05$, [$α$/H]$=+0.23\pm0.05$) and kinematically consistent with the galactic thick disk population, making TOI-561 one of the oldest ($10\pm3\,$Gyr) and most metal-poor planetary systems discovered yet. We dynamically confirm planets b and c with radial velocities from the W. M. Keck Observatory High Resolution Echelle Spectrometer. Planet b has a mass and density of $3.2\pm0.8\,M_\oplus$ and $5.5^{+2.0}_{-1.6}\,$g$\,$cm$^{-3}$, consistent with a rocky composition. Its lower-than-average density is consistent with an iron-poor composition, although an Earth-like iron-to-silicates ratio is not ruled out. Planet c is $7.0\pm2.3\,M_\oplus$ and $1.6\pm0.6\,$g$\,$cm$^{-3}$, consistent with an interior rocky core overlaid with a low-mass volatile envelope. Several attributes of the photometry for planet d (which we did not detect dynamically) complicate the analysis, but we vet the planet with high-contrast imaging, ground-based photometric follow-up and radial velocities. TOI-561 b is the first rocky world around a galactic thick-disk star confirmed with radial velocities and one of the best rocky planets for thermal emission studies.
description: Accepted at The Astronomical Journal; 25 pages, 10 figures