The Habitable Zone Planet Finder Reveals a High Mass and Low Obliquity for the Young Neptune K2-25b

Creators: Stefansson, Gudmundur; Mahadevan, Suvrath; Maney, Marissa; Ninan, Joe P.; Robertson, Paul; Rajagopal, Jayadev; Haase, Flynn; Allen, Lori; Ford, Eric B.; Winn, Joshua; Wolfgang, Angie; Dawson, Rebekah I.; Wisniewski, John; Bender, Chad F.; Cañas, Caleb; Cochran, William; Diddams, Scott A.; Fredrick, Connor; Halverson, Samuel; Hearty, Fred; Hebb, Leslie; Kanodia, Shubham; Levi, Eric; Metcalf, Andrew J.; Monson, Andrew; Ramsey, Lawrence; Roy, Arpita; Schwab, Christian; Terrien, Ryan; Wright, Jason T.

Abstract

Using radial velocity data from the Habitable Zone Planet Finder, we have measured the mass of the Neptune-sized planet K2-25b, as well as the obliquity of its M4.5 dwarf host star in the 600–800 Myr Hyades cluster. This is one of the youngest planetary systems for which both of these quantities have been measured and one of the very few M dwarfs with a measured obliquity. Based on a joint analysis of the radial velocity data, time-series photometry from the K2 mission, and new transit light curves obtained with diffuser-assisted photometry, the planet's radius and mass are 3.44 ± 0.12 R_⊕ and 24.5_(-5.2)^(+5.7) M_⊕. These properties are compatible with a rocky core enshrouded by a thin hydrogen–helium atmosphere (5% by mass). We measure an orbital eccentricity of e = 0.43 ± 0.05. The sky-projected stellar obliquity is λ = 3° ± 16°, compatible with spin–orbit alignment, in contrast to other "hot Neptunes" that have been studied around older stars.

Additional Information

© 2020. The American Astronomical Society. Received 2020 April 20; revised 2020 July 14; accepted 2020 July 20; published 2020 September 30. We thank the anonymous referee for a thoughtful reading of the manuscript and useful suggestions and comments that made for a clearer and stronger manuscript. 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. This work was supported by NASA Headquarters under the NASA Earth and Space Science Fellowship Program through grants NNX16AO28H and 80NSSC18K1114. We acknowledge support from NSF grants AST-1006676, AST-1126413, AST-1310885, AST-1517592, AST-1310875, and AST-1907622; the NASA Astrobiology Institute (NAI; NNA09DA76A); and PSARC in our pursuit of precision radial velocities in the NIR. We acknowledge support from the Heising-Simons Foundation via grants 2017-0494 and 2019-1177. We acknowledge support from NSF grant AST-1909506 and the Research Corporation for precision photometric observations with diffuser-assisted photometry. Computations for this research were performed at the Pennsylvania State University's Institute for Computational & Data Sciences (ICDS). A portion of this work was enabled by support from the Mt. Cuba Astronomical Foundation. R.I.D. is supported by NASA XRP 80NSSC18K0355 and the Alfred P. Sloan Foundation's Sloan Research Fellowship. 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). G.K.S. wishes to thank Kento Masuda for informative discussions on determining stellar inclinations from projected rotational velocities, stellar radii, and rotation periods. These results are based on observations obtained with the Habitable Zone Planet Finder Spectrograph on the Hobby–Eberly Telescope. We thank the resident astronomers and telescope operators at the HET for the skillful execution of our observations with HPF. The Hobby–Eberly Telescope 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. The HET collaboration acknowledges support and resources from the Texas Advanced Computing Center. This is the University of Texas Center for Planetary Systems Habitability Contribution 0001. These results are based on observations obtained with the Apache Point Observatory 3.5-meter telescope which is owned and operated by the Astrophysical Research Consortium. We wish to thank the APO 3.5m telescope operators in their assistance in obtaining these data. Based in part on observations at the Kitt Peak National Observatory, NSF's NOIRLab (Prop. ID 0925-2018B; PI: G. Stefansson), managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. The WIYN 0.9m telescope is operated by WIYN Inc. on behalf of a Consortium of 10 partner Universities and Organizations. WIYN is a joint facility of the University of Wisconsin–Madison, Indiana University, NSF's NOIRLab, the Pennsylvania State University, Purdue University, University of California, Irvine, and the University of Missouri. The authors are honored to be permitted to conduct astronomical research on Iolkam Du'ag (Kitt Peak), a mountain with particular significance to the Tohono O'odham. We wish to dearly thank James Winsky at NSF's NOIRLab for his help to conduct some of the HDI observations. This paper includes data collected by the Kepler telescope. The Kepler and K2 data presented in this paper were obtained from the Mikulski Archive for Space Telescopes (MAST). The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. 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. Funding for the K2 mission is provided by the NASA Science Mission directorate. This research 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 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. Facilities: K2 - , Gaia - , HDI/WIYN 0.9 m - , ARCTIC/ARC 3.5 m - , HPF/HET 10 m - . Software: AstroImageJ (Collins et al. 2017), astroplan (Morris et al. 2018), astropy (Astropy Collaboration et al. 2013), astroquery (Ginsburg et al. 2018), barycorrpy (Kanodia & Wright 2018), batman (Kreidberg 2015), corner.py (Foreman-Mackey 2016), celerite (Foreman-Mackey et al. 2017), dynesty (Speagle 2020), emcee (Foreman-Mackey et al. 2013), everest (Luger et al. 2018), EXOFASTv2 (Eastman 2017), HxRGproc (Ninan et al. 2018), iDiffuse (Stefansson et al. 2018b), Jupyter (Kluyver et al. 2016), juliet (Espinoza et al. 2019), matplotlib (Hunter 2007), numpy (Van Der Walt et al. 2011), MRExo (Kanodia et al. 2019), pandas (McKinney 2010), PyAstronomy (Czesla et al. 2019), pyde (Parviainen 2016), radvel (Fulton et al. 2018), SERVAL (Zechmeister et al. 2018).

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