Draft version February 5, 2021 Typeset using LATEX twocolumn style in AASTeX63 The Habitable-zone Planet Finder Detects a Terrestrial-mass Planet Candidate Closely Orbiting Gliese 1151: The Likely Source of Coherent Low-frequency Radio Emission from an Inactive Star Suvrath Mahadevan ,1, 2 Guðmundur Stefánsson ,3, ∗ Paul Robertson ,4 Ryan C. Terrien ,5 Joe P. Ninan ,1, 2 Rae J. Holcomb ,4 Samuel Halverson ,6 William D. Cochran ,7, 8 Shubham Kanodia ,1, 2 Lawrence W. Ramsey ,1, 2 Alexander Wolszczan ,1, 2 Michael Endl ,7, 8 Chad F. Bender ,9 Scott A. Diddams ,10, 11 Connor Fredrick ,10, 11 Fred Hearty ,1, 2 Andrew Monson ,1, 2 Andrew J. Metcalf ,12, 10, 11 Arpita Roy ,13 and Christian Schwab 14 1Department of Astronomy & Astrophysics, The Pennsylvania State University, 525 Davey Laboratory, University Park, PA, 16802, USA 2Center for Exoplanets and Habitable Worlds, 525 Davey Laboratory, University Park, PA, 16802, USA 3Princeton University, Department of Astrophysical Sciences, 4 Ivy Lane, Princeton, NJ 08540, USA 4Department of Physics and Astronomy, The University of California, Irvine, Irvine, CA 92697, USA 5Department of Physics and Astronomy, Carleton College, One North College Street, Northfield, MN 55057, USA 6Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, USA 7McDonald Observatory and Department of Astronomy, The University of Texas at Austin, 2515 Speedway, Austin, TX 78712, USA 8Center for Planetary Systems Habitability, The University of Texas at Austin, 2515 Speedway, Austin, TX 78712, USA 9Steward Observatory, The University of Arizona, 933 N. Cherry Ave, Tucson, AZ 85721, USA 10Time and Frequency Division, National Institute of Standards and Technology, 325 Broadway, Boulder, CO 80305, USA 11Department of Physics, University of Colorado, 2000 Colorado Avenue, Boulder, CO 80309, USA 12Space Vehicles Directorate, Air Force Research Laboratory, 3550 Aberdeen Ave. SE, Kirtland AFB, NM 87117, USA 13Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218, USA 14Department of Physics and Astronomy, Macquarie University, Balaclava Road, North Ryde, NSW 2109, Australia ABSTRACT The coherent low-frequency radio emission detected by LOFAR from Gliese 1151, a quiescent M4.5 dwarf star, has radio emission properties consistent with theoretical expectations of star-planet in- teractions for an Earth-sized planet on a 1-5 day orbit. New near-infrared radial velocities from the Habitable-zone Planet Finder (HPF) spectrometer on the 10 m Hobby-Eberly Telescope at McDonald Observatory, combined with previous velocities from HARPS-N, reveal a periodic Doppler signature consistent with an m sin i = 2:5 ± 0:5M⊕ exoplanet on a 2.02-day orbit. Precise photometry from the Transiting Exoplanet Survey Satellite (TESS) shows no flares or activity signature, consistent with a quiescent M dwarf. While no planetary transit is detected in the TESS data, a weak photometric modulation is detectable in the photometry at a ∼ 2 day period. This independent detection of a candidate planet signal with the Doppler radial-velocity technique adds further weight to the claim of the first detection of star-exoplanet interactions at radio wavelengths, and helps validate this emerging technique for the detection of exoplanets. 1. INTRODUCTION ney et al. 2018). This incoming energy can heat up the Planets in close-in orbits orbit are embedded in a mag- chromosphere of the star, causing a hot spot on the sur- arXiv:2102.02233v1 [astro-ph.EP] 3 Feb 2021 netized stellar wind from the expanding stellar corona. face of the star, which can cause variability that is mod- As they orbit, short-period planets can perturb the flow ulated by the orbital period of the planet. Shkolnik et al. of the magnetized wind, which can carry substantial (2005, 2008) and Cauley et al.(2019) detected chromo- amounts of energy towards the host star via sub-Alfvénic spheric modulations in hot Jupiter systems, which they interactions (Neubauer 1980; Saur et al. 2013; Turnpen- interpret as evidence for Star-Planet Interactions (SPI), and Turner et al.(2020) discuss promising candidate de- tections of circularly polarized emission from τ Boötis Corresponding author: Suvrath Mahadevan and ν Andromedae. In addition to hot Jupiters, M- [email protected] dwarf planet systems with close-in rocky planets—such ∗ Henry Norris Russell Fellow as the TRAPPIST-1 system (Gillon et al. 2017)—have also been suggested as capable of producing SPI (Pineda 2 Mahadevan et al. & Hallinan 2018; Turnpenney et al. 2018; Fischer & Saur discuss the energetics of the sub-Alfvénic interactions. 2019). Recently, Pérez-Torres et al.(2021) announced We summarize our key findings in Section5. the detection of circularly polarized 1.6 GHz radio emis- sion from Proxima Centauri that could be consistent 2. DATA with sub-Alfvénic interactions with its 11.2-day planet. 2.1. HARPS-N Radial Velocities Continued monitoring at radio wavelengths will help to Pope et al.(2020) obtained time-series Doppler spec- distinguish between sub-Alfvénic interactions and other troscopy of GJ 1151 using the HARPS-N spectrometer mechanisms (eg. Zic et al. 2020). (Cosentino et al. 2012) at Telescopio Nazionale Galileo Vedantham et al.(2020) reported evidence for low- (TNG). The HARPS-N time series consists of 21 obser- frequency highly circularly polarized radio emission at vations compiled over a time baseline of approximately 150 MHz in GJ 1151 using the LOFAR Telescope Array 3 months between December 2018 and February 2019. (the LOw-Frequency ARray; van Haarlem et al. 2013). The standard HARPS-N RV pipeline did not offer suf- GJ 1151 is a nearby bright (J = 8:5) M4.5 dwarf with ficient Doppler precision for a star as cool as GJ 1151, a quiet chromosphere. The origin of the radio emission, so Pope et al.(2020) extracted their own RVs using the which is observed to be transient and highly circularly wobble (Bedell et al. 2019) spectral analysis code. From polarized (circular polarization fraction of 64 ± 6%), is the 21 HARPS-N spectra, Pope et al.(2020) provided most consistent with sub-Alfvénic star-planet interac- 19 wobble RVs; one spectrum (BJD=2458475.75494602) tions between a rocky planet in a 1 to 5 day orbit around was explicitly excluded for having low signal-to-noise, the host star. while another (BJD=2458491.64178017) was excluded To search for the presence of a potential rocky planet for unspecified reasons. The wobble RVs exhibit companion and exclude more massive companions, Pope an RMS scatter of 3.8 m s−1, with a mean single- et al.(2020) obtained precise radial velocities (RVs) us- measurement error of 2.9 m s−1. ing the HARPS-N spectrograph. Their observations, In our analysis, we have incorporated the wobble RVs yielding 19 RVs over a span of three months, allowed as presented by Pope et al.(2020). For our analysis of them to place upper limits on the mass of a potential stellar magnetic activity (§3.2), we have also used the planetary companion of m sin i < 5:6 M with periods ⊕ 1D extracted HARPS-N spectra from the TNG archive between 1 and 5 days. to derive the Ca II H&K (S ; Vaughan et al. 1978; We report on a rocky planet candidate revealed in HK Gomes da Silva et al. 2011) and Hα (I ; Robertson additional precise RVs obtained in the near-infrared us- Hα et al. 2013) activity indices. ing the Habitable-zone Planet Finder (Mahadevan et al. 2012, 2014) on the 10m Hobby-Eberly Telescope. To- 2.2. HPF Radial Velocities gether, the HPF RVs and the HARPS-N RVs from HPF is a stabilized fiber-fed near-infrared (NIR) spec- Pope et al.(2020) reveal a planet with an orbital pe- trograph on the 10 m Hobby-Eberly Telescope (HET) riod of P = 2:02 days and an RV semi-amplitude of covering the z, Y , and J bands from 810 − 1280 nm at a K = 4:1 ± 0:8 m s−1, translating to a mass of m sin i = spectral resolution of R ∼ 55000. To enable precise RVs 2:5±0:5M , where i is the inclination of the orbit. Given ⊕ in the NIR, HPF is actively temperature stabilized to its short period, the planet is capable of sub-Alfvénic in- the milli-Kelvin level (Stefansson et al. 2016). HET is teractions with its host star, and is likely source of the a fully queue-scheduled telescope (Shetrone et al. 2007), coherent radio emission observed by Vedantham et al. and all of the observations presented here were executed (2020). Photometric data from the Transiting Exo- as part of the HET queue. planet Survey Satellite (TESS; Ricker et al. 2015) con- HPF uses a Laser-Frequency Comb (LFC) calibrator fidently rules out transits of the planet candidate, but (Metcalf et al. 2019) to provide long-term accurate and the data show hints of modulation at ∼2 days at the precise instrument drift correction and monitoring. We 100 ppm level, which could constitute the photometric did not obtain simultaneous LFC observations to mini- signature of the SPI interaction. mize the possibility of contaminating the target spec- This paper is structured as follows. Section2 details trum, but performed the drift correction using LFC the observations. Section3 presents the analysis of the frames obtained throughout the night. We have previ- HPF and HARPS-N RVs along with the TESS photo- ously demonstrated that we can achieve a drift-corrected metric data. In Section4, using the SPI model from precision at the < 30 cm s−1 level (Stefansson et al. Saur et al.(2013) and Turnpenney et al.(2018), we 2020) using this technique. demonstrate that the planet candidate satisfies the sub- We used the (Ninan et al. 2018) package to Alfvénic criterion and is capable of SPI, and we further HxRGproc perform bias, non-linearity, and cosmic-ray corrections, A terrestrial mass planet candidate orbiting GJ 1151 3 b) Periodogram Focus 0.70 2.02 a) RV Periodogram 10 10 5 8 ln(BF) 6 0 4 0 1 2 3 20 10 10 10 10 2 ln(BF) 10 0 Window 0 2 0 1 2 3 10 10 10 10 4 1.90 1.95 2.00 2.05 2.10 Period (d) Period (d) c) Time-Series RVs d) Phase-folded RVs HPF 10 HARPS-N (2019) HPF (2020) 10 HARPS-N 5 5 0 RV (m/s) 5 0 RV (m/s) 10 470 480 490 500 510 520 530 540 5 10 5 10 0 0.0 0.2 0.4 0.6 0.8 1.0 10 RV (m/s) HPF RMS = 3.4 m/s HARPS-N RMS = 3.0 m/s 5 0 10 Residual (m/s) 10 900 920 940 960 980 1000 1020 0.0 0.2 0.4 0.6 0.8 1.0 BJD - 2458000 Phase (Per = 2.018d) Figure 1.