The TESS-Keck Survey. II. An Ultra-short-period Rocky Planet and Its Siblings Transiting the Galactic Thick-disk Star TOI-561

Creators: Weiss, Lauren M.; Dai, Fei; Huber, Daniel; Brewer, John; Collins, Karen A.; Ciardi, David R.; Matthews, Elisabeth C.; Ziegler, Carl; Howell, Steve B.; Batalha, Natalie M.; Crossfield, Ian J. M.; Dressing, Courtney; Fulton, Benjamin J.; Howard, Andrew W.; Isaacson, Howard; Kane, Stephen R.; Petigura, Erik A.; Robertson, Paul; Roy, Arpita; Rubenzahl, Ryan A.; Claytor, Zachary R.; Stassun, Keivan G.; Chontos, Ashley; Giacalone, Steven; Dalba, Paul A.; Mocnik, Teo; Hill, Michelle L.; Beard, Corey; Murphy, Joseph M. Akana; Rosenthal, Lee J.; Behmard, Aida; Van Zandt, Judah; Lubin, Jack; Kosiarek, Molly R.; Lund, Michael B.; Christiansen, Jessie L.; Matson, Rachel A.; Beichman, Charles A.; Schlieder, Joshua E.; Gonzales, Erica J.; Briceño, César; Law, Nicholas; Mann, Andrew W.; Collins, Kevin I.; Evans, Phil; Fukui, Akihiko; Jensen, Eric L. N.; Murgas, Felipe; Narita, Norio; Palle, Enric; Parviainen, Hannu; Schwarz, Richard P.; Tan, Thiam-Guan; Jenkins, Jon; Ricker, George; Winn, Joshua N.

Abstract

We report the discovery of TOI-561, a multiplanet system in the galactic thick disk that contains a rocky, ultra-short-period planet. 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_p = 1.45 ± 0.11 R_⊕), c (TOI-561.01, P = 10.8 days, R_p = 2.90 ± 0.13 R_⊕), and d (TOI-561.03, P = 16.3 days, R_p = 2.32 ± 0.16 R_⊕). The star is chemically ([Fe/H] = −0.41 ± 0.05, [α/Fe] = +0.23 ± 0.05) and kinematically consistent with the galactic thick-disk population, making TOI-561 one of the oldest (10 ± 3 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 ± 0.8 M_⊕ 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 ± 2.3 M_⊕ and 1.6 ± 0.6 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.

Additional Information

© 2021 The American Astronomical Society. Received 2020 September 4; revised 2020 December 7; accepted 2020 December 14; published 2021 January 11. We thank the time assignment committees of the University of California, the California Institute of Technology, NASA, and the University of Hawaii for supporting the TESS-Keck Survey with observing time at the W. M. Keck Observatory and on the Automated Planet Finder. We thank NASA for funding associated with our NASA-Keck Key Strategic Mission Support project. We gratefully acknowledge the efforts and dedication of the Keck Observatory staff for support of HIRES and remote observing. We recognize and acknowledge the cultural role and reverence that the summit of Maunakea has within the indigenous Hawaiian community. We are deeply grateful to have the opportunity to conduct observations from this mountain. We thank Ken and Gloria Levy, who supported the construction of the Levy Spectrometer on the Automated Planet Finder. We thank the University of California and Google for supporting Lick Observatory and the UCO staff for their dedicated work scheduling and operating the telescopes of Lick Observatory. This paper is based on data collected by the TESS mission. Funding for the TESS mission is provided by the NASA Explorer Program. We acknowledge the use of public TESS Alert data from pipelines at the TESS Science Office and at the TESS Science Processing Operations Center. We thank David Latham for organizing the TESS community follow-up program, which brought together the widespread authorship and diversity of resources presented in this manuscript. The work includes observations obtained at the international Gemini Observatory, a program of NSF's NOIRLab acquired through the Gemini Observatory Archive at NSF's NOIRLab, which is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation on behalf of the Gemini Observatory partnership: the National Science Foundation (United States), National Research Council (Canada), Agencia Nacional de Investigación y Desarrollo (Chile), Ministerio de Ciencia, Tecnología e Innovación (Argentina), Ministério da Ciência, Tecnologia, Inovações e Comunicações (Brazil), and Korea Astronomy and Space Science Institute (Republic of Korea). Data were collected under program GN-2019A-LP-101. Observations in the paper made use of the High-Resolution Imaging instrument Zorro. Zorro was funded by the NASA Exoplanet Exploration Program and built at the NASA Ames Research Center by Steve B. Howell, Nic Scott, Elliott P. Horch, and Emmett Quigley. Zorro was mounted on the Gemini-South telescope of the international Gemini Observatory. Observations also made use of the NIRI instrument, which is mounted at Gemini-North. The Gemini-North telescope is located within the Maunakea Science Reserve and adjacent to the summit of Maunakea. We are grateful for the privilege of observing the universe from a place that is unique in both its astronomical quality and its cultural significance. This work makes use of observations from the LCOGT network. This article is based on observations made with the MuSCAT2 instrument, developed by ABC, at Telescopio Carlos Sánchez operated on the island of Tenerife by the IAC in the Spanish Observatorio del Teide. This work is partly supported by JSPS KAKENHI grant Nos. JP17H04574, JP18H01265, and JP18H05439, and JST PRESTO grant No. JPMJPR1775. This work makes use of data collected under the NGTS project at the ESO La Silla Paranal Observatory. The NGTS facility is operated by the consortium institutes with support from the UK Science and Technology Facilities Council (STFC) projects ST/M001962/1 and ST/S002642/1. This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. J.S.J. acknowledges support by FONDECYT grant 1201371, and partial support from CONICYT project Basal AFB-170002. J.I.V. acknowledges support of CONICYT-PFCHA/Doctorado Nacional-21191829. This research has made use of the Exoplanet Follow-up Observing Program (ExoFOP), which is operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration. Resources supporting this work were provided by the NASA High-End Computing (HEC) Program through the NASA Advanced Supercomputing (NAS) Division at Ames Research Center for the production of the SPOC data products. L.M.W. is supported by the Beatrice Watson Parrent Fellowship and NASA ADAP Grant 80NSSC19K0597. D.H. acknowledges support from the Alfred P. Sloan Foundation, the National Aeronautics and Space Administration (80NSSC18K1585, 80NSSC19K0379), and the National Science Foundation (AST-1717000). E.A.P. acknowledges the support of the Alfred P. Sloan Foundation. C.D.D. acknowledges the support of the Hellman Family Faculty Fund, the Alfred P. Sloan Foundation, the David & Lucile Packard Foundation, and the National Aeronautics and Space Administration via the TESS Guest Investigator Program (80NSSC18K1583). I.J.M.C. acknowledges support from the NSF through grant AST-1824644. Z.R.C. acknowledges support from the TESS Guest Investigator Program (80NSSC18K18584). A.C. acknowledges support from the National Science Foundation through the Graduate Research Fellowship Program (DGE 1842402). P.D. acknowledges support from a National Science Foundation Astronomy and Astrophysics Postdoctoral Fellowship under award AST-1903811. A.B. is supported by the NSF Graduate Research Fellowship, grant No. DGE 1745301. R.A.R. is supported by the NSF Graduate Research Fellowship, grant No. DGE 1745301. M.R.K. is supported by the NSF Graduate Research Fellowship, grant No. DGE 1339067. J.N.W. thanks the Heising Simons Foundation for support. Facilities: TESS - , Keck I-HIRES - , Gemini-North-NIRI - , Gemini-South-Zorro - , Palomar - , SOAR - , LCOGT - , NGTS - , El Sauce - , PEST - , MuSCAT2. - Software: Astropy (Astropy Collaboration et al. 2013, 2018), radvel (Fulton et al. 2018), emcee (Foreman-Mackey et al. 2013), Spectroscopy Made Easy (Valenti & Piskunov 1996; Piskunov & Valenti 2017), SpecMatch Synth (Petigura 2015), SpecMatch-Emp (Yee et al. 2017), isoclassify (Huber et al. 2017), AstroImageJ (Collins et al. 2017), lightkurve (Lightkurve Collaboration et al. 2018), spock (Tamayo et al. 2020), kiauhoku (Claytor et al. 2020).

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