TOI-1728b: The Habitable-zone Planet Finder Confirms a Warm Super-Neptune Orbiting an M-dwarf Host

Creators: Kanodia, Shubham; Cañas, Caleb I.; Stefansson, Gudmundur; Ninan, Joe P.; Hebb, Leslie; Lin, Andrea S. J.; Baran, Helen; Maney, Marissa; Terrien, Ryan C.; Mahadevan, Suvrath; Cochran, William D.; Endl, Michael; Dong, Jiayin; Bender, Chad F.; Diddams, Scott A.; Ford, Eric B.; Fredrick, Connor; Halverson, Samuel; Hearty, Fred; Metcalf, Andrew J.; Monson, Andrew; Ramsey, Lawrence W.; Robertson, Paul; Roy, Arpita; Schwab, Christian; Wright, Jason T.

Abstract

We confirm the planetary nature of TOI-1728b using a combination of ground-based photometry, near-infrared Doppler velocimetry and spectroscopy with the Habitable-zone Planet Finder. TOI-1728 is an old, inactive M0 star with T_(eff) = 3980⁺³¹₋₃₂ K, which hosts a transiting super-Neptune at an orbital period of ~3.49 days. Joint fitting of the radial velocities and TESS and ground-based transits yields a planetary radius of 5.05^(+0.16)_(-0.17) R_⊕, mass 26.78^(+5.43)_(-5.13) M_⊕, and eccentricity 0.057^(+0.054)_(-0.039). We estimate the stellar properties, and perform a search for He 10830 Å absorption during the transit of this planet and claim a null detection with an upper limit of 1.1% with 90% confidence. A deeper level of He 10830 Å absorption has been detected in the planet atmosphere of GJ 3470b, a comparable gaseous planet. TOI-1728b is the largest super-Neptune—the intermediate subclass of planets between Neptune and the more massive gas-giant planets—discovered around an M dwarf. With its relatively large mass and radius, TOI-1728 represents a valuable data point in the M-dwarf exoplanet mass–radius diagram, bridging the gap between the lighter Neptune-sized planets and the heavier Jovian planets known to orbit M dwarfs. With a low bulk density of 1.14^(+0.26)_(-0.24) g cm⁻³, and orbiting a bright host star (J ~ 9.6, V ~ 12.4), TOI-1728b is also a promising candidate for transmission spectroscopy both from the ground and from space, which can be used to constrain planet formation and evolutionary models.

Additional Information

© 2020 The American Astronomical Society. Received 2020 May 20; revised 2020 June 25; accepted 2020 June 25; published 2020 August 10. The authors thank the referee for providing insightful comments, which improved this work and made the results clearer. This work was partially supported by funding from the Center for Exoplanets and Habitable Worlds. The Center for Exoplanets and Habitable Worlds is supported by the Pennsylvania State University, the Eberly College of Science, and the Pennsylvania Space Grant Consortium. These results are based on observations obtained with the Habitable-zone Planet Finder Spectrograph on the HET. We acknowledge support from NSF grants AST 1006676, AST 1126413, AST 1310875, AST 1310885, and the NASA Astrobiology Institute (NNA09DA76A) in our pursuit of precision radial velocities in the NIR. We acknowledge support from the Heising-Simons Foundation via grant 2017-0494. The HET is a joint project of the University of Texas at Austin, the Pennsylvania State University, Ludwig-Maximilians-Universität München, and Georg-August Universität Gottingen. The HET is named in honor of its principal benefactors, William P. Hobby and Robert E. Eberly. This is University of Texas Center for Planetary Systems Habitability contribution 0012. The HET collaboration acknowledges the support and resources from the Texas Advanced Computing Center. We thank the Resident Astronomers and Telescope Operators at the HET for the skilful execution of our observations of our observations with HPF. We acknowledge support from NSF grant AST-1909506, AST-1910954, and the Research Corporation for precision photometric observations with diffuser-assisted photometry. This research has made use of the NASA Exoplanet Archive, which is operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program. This work includes data collected by the TESS mission, which are publicly available from MAST. Funding for the TESS mission is provided by the NASA Science Mission directorate. Some of the data presented in this paper were obtained from MAST. Support for MAST for non-HST data is provided by the NASA Office of Space Science via grant NNX09AF08G and by other grants and contracts. This research has made use of NASA's Astrophysics Data System Bibliographic Services. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (NASA). Computations for this research were performed on the Pennsylvania State University's Institute for Computational and Data Sciences Advanced CyberInfrastructure (ICDS-ACI), including the CyberLAMP cluster supported by NSF grant MRI-1626251. This work includes data from 2MASS, which is a joint project of the University of Massachusetts and IPAC at Caltech funded by NASA and the NSF. We acknowledge with thanks the variable star observations from the AAVSO International Database contributed by observers worldwide and used in this research. 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. Some observations were obtained with the Samuel Oschin 48 inch Telescope at the Palomar Observatory as part of the ZTF project. ZTF is supported by the NSF under grant No. AST-1440341 and a collaboration including Caltech, IPAC, the Weizmann Institute for Science, the Oskar Klein Center at Stockholm University, the University of Maryland, the University of Washington, Deutsches Elektronen-Synchrotron and Humboldt University, Los Alamos National Laboratories, the TANGO Consortium of Taiwan, the University of Wisconsin at Milwaukee, and Lawrence Berkeley National Laboratories. Operations are conducted by COO, IPAC, and UW. We gathered data using the Planewave CDK 24 Telescope operated by the Penn State Department of Astronomy & Astrophysics at Davey Lab Observatory. S.K. would like to acknowledge useful discussions with Dan Foreman-Mackey regarding the exoplanet package and Theodora for help with this project. C.I.C. acknowledges support by NASA Headquarters under the NASA Earth and Space Science Fellowship Program through grant 80NSSC18K1114. This research made use of exoplanet(Foreman-Mackey et al. 2020) and its dependencies (Astropy Collaboration et al. 2013, 2018; Kipping 2013; Salvatier et al. 2016; Theano Development Team 2016; Foreman-Mackey et al. 2017; Foreman-Mackey 2018; Agol et al. 2020; Luger et al. 2019). Facilities: Gaia - , HET (HPF) - , TESS - , Perkin 17'' - , Penn State CDK0.6 m - , Exoplanet Archive. - Software: AstroImageJ (Collins et al. 2017), astroquery (Ginsburg et al. 2019), astropy (Astropy Collaboration et al. 2013, 2018), barycorrpy (Kanodia & Wright 2018), batman (Kreidberg 2015), celerite (Foreman-Mackey et al. 2017), corner.py (Foreman-Mackey 2016), dustmaps (Green 2018), dynesty (Speagle 2020), EXOFASTv2 (Eastman et al. 2019), exoplanet (Foreman-Mackey et al. 2020), HxRGproc (Ninan et al. 2018), GALPY (Bovy 2015), GNU Parallel (Tange 2011), juliet (Espinoza et al. 2019), lightkurve (Lightkurve Collaboration et al. 2018), matplotlib (Hunter 2007), MRExo (Kanodia et al. 2019), numpy (van der Walt et al. 2011), pandas (McKinney 2010), Photutils (Bradley et al. 2019), PyMC3(Salvatier et al. 2016), radvel(Fulton et al. 2018), scipy (Virtanen et al. 2020), SERVAL (Zechmeister et al. 2018), starry(Luger et al. 2019), Theano(Theano Development Team 2016).

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