A Transiting, Temperate Mini-Neptune Orbiting the M Dwarf TOI-1759 Unveiled by TESS

A Transiting, Temperate Mini-Neptune Orbiting the M Dwarf TOI-1759 Unveiled by TESS Néstor Espinoza 1 , Enric Pallé 2 , Jonas Kemmer 3 , Rafael Luque 4 , José A. Caballero 5 , Carlos Cifuentes 5 , Enrique Herrero 6,7 , Víctor J. Sánchez Béjar 2,8 , Stephan Stock 3 , Karan Molaverdikhani 3,9,10,11 , Giuseppe Morello 2,8 , Diana Kossakowski 9 , Martin Schlecker 9 , Pedro J. Amado 4 , Paz Bluhm 3 , Miriam Cortés-Contreras 5 , Thomas Henning 9 , Laura Kreidberg 9 , Martin Kürster 9 , Marina Lafarga 6,7 , Nicolas Lodieu 2,8 , Juan Carlos Morales 6,7 , Mahmoudreza Oshagh 2,8 , Vera M. Passegger 12,13 , Alexey Pavlov 9 , Andreas Quirrenbach 3 , Sabine Reffert 3 , Ansgar Reiners 14 , Ignasi Ribas 6,7 , Eloy Rodríguez 4 , Cristina Rodríguez López 4 , Andreas Schweitzer 13 , Trifon Trifonov 9 , Priyanka Chaturvedi 15 , Stefan Dreizler 14 , Sandra V. Jeffers 16 , Adrian Kaminski 3 , María José López-González 4 , Jorge Lillo-Box 5 , David Montes 17 , Grzegorz Nowak 2,8 , Santos Pedraz 18 , Siegfried Vanaverbeke 19,20 , Maria R. Zapatero Osorio 21 , Mathias Zechmeister 14 , Karen A. Collins 22 , Eric Girardin 23 , Pere Guerra 24 , Ramon Naves 24 , Ian J. M. Crossfield 25 , Elisabeth C. Matthews 26 , Steve B. Howell 27 , David R. Ciardi 28 , Erica Gonzales 29 , Rachel A. Matson 30 , Charles A. Beichman 28 , Joshua E. Schlieder 31 , Thomas Barclay 31,32 , Michael Vezie 33 , Jesus Noel Villaseñor 33 , Tansu Daylan 33 , Ismael Mireies 34 , Diana Dragomir 34 , Joseph D. Twicken 27,35 , Jon Jenkins 27 , Joshua N. Winn 36 , David Latham 22 , George Ricker 33 , and Sara Seager 33 1 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA; [email protected] 2 Instituto de Astrofísica de Canarias (IAC), E-38205 La Laguna, Tenerife, Spain 3 Landessternwarte, Zentrum für Astronomie der Universität Heidelberg, Königstuhl 12, D-69117 Heidelberg, Germany 4 Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomía s/n, E-18008 Granada, Spain 5 Centro de Astrobiología (CSIC-INTA), ESAC, Camino bajo del castillo s/n, E-28692 Villanueva de la Cañada, Madrid, Spain 6 Institut de Ciències de l’Espai (ICE, CSIC), Campus UAB, c/de Can Magrans s/n, E-08193 Cerdanyola del Vallès, Barcelona, Spain 7 Institut d’Estudis Espacials de Catalunya (IEEC),c/Gran Capità 2–4, E-08034 Barcelona, Spain 8 Departamento de Astrofísica, Universidad de La Laguna, E-38206 La Laguna, Tenerife, Spain 9 Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany 10 Universitäts-Sternwarte, Ludwig-Maximilians-Universität München, Scheinerstrasse 1, D-81679 München, Germany 11 ORIGINS: Exzellenzcluster Origins, Boltzmannstrae 2, D-85748 Garching, Germany 12 Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, 440 West Brooks Street, Norman, OK 73019, USA 13 Hamburger Sternwarte, Gojenbergsweg 112, D-21029 Hamburg, Germany 14 Institut für Astrophysik, Georg-August-Universität, Friedrich-Hund-Platz 1, D-37077 Göttingen, Germany 15 Thüringer Landessternwarte Tautenburg, Sternwarte 5, D-07778 Tautenburg, Germany 16 Max-Planck-Institute für Sonnensystemforschung, Justus-von-Liebig-Weg 3, D-37075 Göttingen, Germany 17 Departamento de Física de la Tierra y Astrofísica & IPARCOS-UCM (Instituto de Física de Partículas y del Cosmos de la UCM), Facultad de Ciencias Físicas, Universidad Complutense de Madrid, E-28040 Madrid, Spain 18 Centro Astronómico Hispano-Alemán, Observatorio de Calar Alto, Sierra de los Filabres, E-04550 Gérgal, Almería, Spain 19 Vereniging Voor Sterrenkunde, Brugge & Centre for Mathematical Plasma-Astrophysics, Department of Mathematics, KU Leuven, Celestijnenlaan 200B, B-3001 Heverlee, Belgium 20 AstroLAB IRIS, Provinciaal Domein “De Palingbeek”, Verbrandemolenstraat 5, B-8902 Zillebeke, Ieper, Belgium 21 Centro de Astrobiología (CSIC-INTA), Carretera de Ajalvir km 4, E-28850 Torrejón de Ardoz, Madrid, Spain 22 Center for Astrophysics |Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA 23 Grand Pra Observatory, Switzerland 24 Observatori Astronòmic Albanyà, Camí de Bassegoda s/n, Albanyà E-17733, Girona, Spain 25 Department of Physics and Astronomy, University of Kansas, Lawrence, KS 66045, USA 26 Observatoire de l’Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland 27 NASA Ames Research Center, Moffett Field, CA 94035, USA 28 NASA Exoplanet Science Institute, Caltech-IPAC, MC 314-6, 1200 E California Boulevard, Pasadena, CA 91125, USA 29 Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95060, USA 30 U.S. Naval Observatory, Washington, D.C. 20392, USA 31 NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA 32 University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA 33 Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA 34 Department of Physics and Astronomy, University of New Mexico, 210 Yale Boulevard NE, Albuquerque, NM 87106, USA 35 SETI Institute, Mountain View, CA 94043, USA 36 Department of Astrophysical Sciences, Princeton University, 4 Ivy Lane, Princeton, NJ 08544, USA Received 2021 August 14; revised 2022 January 3; accepted 2022 January 5; published 2022 February 16 Abstract We report the discovery and characterization of TOI-1759 b, a temperate (400 K)sub-Neptune-sized exoplanet orbiting the M dwarf TOI-1759 (TIC 408636441). TOI-1759 b was observed by TESS to transit in Sectors 16, 17, and 24, with only one transit observed per sector, creating an ambiguity regarding the orbital period of the planet candidate. Ground-based photometric observations, combined with radial-velocity measurements obtained with the CARMENES spectrograph, confirm an actual period of 18.85019 ±0.00014 days. A joint analysis of all available The Astronomical Journal, 163:133 (15pp), 2022 March https://doi.org/10.3847/1538-3881/ac4af0 © 2022. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s)and the title of the work, journal citation and DOI. 1 photometry and radial velocities reveals a radius of 3.17 ±0.10 R ⊕ and a mass of 10.8 ±1.5 M ⊕ . Combining this with the stellar properties derived for TOI-1759 (R å =0.597 ±0.015 R e ;M å =0.606 ±0.020 M e ;T eff =4065 ±51 K), we compute a transmission spectroscopic metric (TSM)value of over 80 for the planet, making it a good target for transmission spectroscopy studies. TOI-1759 b is among the top five temperate, small exoplanets (T eq <500 K, R p <4R ⊕ )with the highest TSM discovered to date. Two additional signals with periods of 80 days and >200 days seem to be present in our radial velocities. While our data suggest both could arise from stellar activity, the later signal’s source and periodicity are hard to pinpoint given the ∼200 days baseline of our radial-velocity campaign with CARMENES. Longer baseline radial-velocity campaigns should be performed in order to unveil the true nature of this long-period signal. Unified Astronomy Thesaurus concepts: Exoplanet astronomy (486) Supporting material: machine-readable tables 1. Introduction One of the most exciting astronomical developments in the last decade, triggered by improved instrumentation and survey designs, is the detection and characterization of small (R p <4R ⊕ )exoplanets. The study of transiting, relatively low-temperature small worlds, in particular, promises to provide key information to understand how different environments (e.g., incident stellar fluxes or initial composition)might impact their bulk properties, and how those might in turn change their atmospheric and interior structures (Dorn et al. 2017; Neil & Rogers 2020; Ma & Ghosh 2021). In addition, these cooler worlds allow us to make connections with the planets in our own solar system, all of which have equilibrium temperatures smaller than 500 K (hereon referred to as “temperate”exoplanets). These connections, in turn, have key implications for the search for life outside the solar system, and have the potential to help us improve and refine the concept of planetary habitability itself (Tasker et al. 2017; Meadows & Barnes 2018; Seager et al. 2021). Detecting these small, temperate exoplanets is, however, challenging. The relatively longer orbital periods needed to have small irradiation levels makes them difficult to detect from the ground using the transit technique, which is why most of the known temperate worlds were detected by the transit survey with the longest continuous time baseline: the Kepler mission (Borucki et al. 2010). While revolutionary in the search and discovery of small worlds—revealing that they are, in fact, among the most abundant population of exoplanets in our Galaxy (at least for close-in exoplanets; Fulton & Petigura 2018; Hsu et al. 2019)—the mission provided few systems amenable for further radial-velocity and/or atmospheric characterization, due to the inherent faintness of the stars it surveyed. This detailed characterization is fundamental to understand the overall makeup of these small, distant worlds, and helps us understand and uncover their different subpopulations (see, e.g., Zeng et al. 2019; Gupta & Schlichting 2021; Schlichting & Young 2021; Yu et al. 2021). It is also important to understand fundamental exoplanet demographic questions such as why these small worlds are the most numerous in our Galaxy (see, e.g., Kite et al. 2019). The Transiting Exoplanet Survey Satellite (Ricker et al. 2015; TESS)has been crucial to the search for small transiting exoplanets amenable for detailed characterization. To date, it has already doubled the known sample of small, temperate worlds for which masses have been measured with follow-up observations. And after three years of operation, the mission is just starting to exploit its long-time baselines, allowing for the discovery of exoplanets on long orbital periods. In this work, we present the detection and characterization of one such system: TOI-1759 b, an 18.85 days sub-Neptune (R p =3.14 R ⊕ ,M p =10.8 M ⊕ ), orbiting a M-dwarf star. This paper is structured as follows. In Section 2we describe the data that were obtained to understand this new exoplanetary system, which includes photometric, spectroscopic, and highresolution imaging data. In Section 3we present the analysis of these data, including the stellar and planetary properties of the system. We discuss our results in Section 4, and summarize our main conclusions from this work in Section 5. 2. Observations 2.1. TESS Observations from TESS for TOI-1759 (TYC 4266-736-1, TIC408636441)were obtained during its second year of operation in its high-cadence, 2 minutes exposure mode in Sectors 16 (2019 September to October),17(2019 October to November), and 24 (2020 April to May—see Figures 1and 2; the data are also presented in Table 6). The 2 minutes cadence data were processed in the TESS Science Processing Operations Center (SPOC; Jenkins et al. 2016)photometry and transit search pipelines (Jenkins 2002; Jenkins et al. 2010)at NASA Ames Research Center. The TESS data validation reports (Twicken et al. 2018; Li et al. 2019)on TOI-1759 (Guerrero et al. 2021)show detections of a transiting exoplanet candidate at a 37.7 days period (although the data were also consistent with a planet at half this period, i.e., 18.85 days)and a transit depth of about 2700 ppm. To perform further analyses on this target, we retrieved the PreData-Conditioning (PDC)−corrected photometry (Smith et al. 2012; Stumpe et al. 2012,2014)from all sectors from the Mikulski Archive for Space Telescopes archive 37 , as this is the highest-quality photometry from the three TESS sectors mentioned above. After removing the transits of the planet candidate, we ran the Transit Least Squares (TLS; Hippke & Heller 2019)algorithm on these photometric time series and found no extra significant signals (i.e., signals with a signal-tonoise ratio >5)in the data. 2.2. CARMENES Spectroscopy We monitored TOI-1759 with the CARMENES 38 instrument located on the 3.5 m telescope at the Calar Alto Observatory in Almería, Spain, from 2020 July 24 to 2021 January 17. Our data covered a time span of about 175 days, over which we 37 https://archive.stsci.edu/ 38 Calar Alto high-Resolution search for M dwarfs with Exo-earths with Nearinfrared and optical Échelle Spectrographs, http://carmenes.caha.es. 2 The Astronomical Journal, 163:133 (15pp), 2022 March Espinoza et al. were able to detect significant radial-velocity signals. The spectra were processed following the standard CARMENES data flow (Caballero et al. 2016)that has been extensively used by previous works (e.g., Zechmeister et al. 2018; Morales et al. 2019; Trifonov et al. 2020). In our analyses, we only used radial velocities from the visual (VIS)channel that had mean errors of 2.6 m s −1 . A total of 57 radial-velocity data points were used for our analysis, which are presented in Table 5. The spectra used to derive those have a median signal-to-noise ratio of 95 at 840 nm. Data from our infrared channel were not used, as their precision (mean error of 10 m s −1 )was not high enough to put meaningful constraints on the radial-velocity variations observed in the VIS channel. Figure 3(a)shows the radial velocity as a function of time as observed through the VIS channel, which covers the spectral range 520–960 nm with a spectral resolution of = R 94, 600 (Quirrenbach et al. 2014,2018). Our campaign allowed us to clearly detect a signal at about 18.5 days (consistent with half the period of the transiting exoplanet detected in the TESS photometry, already discussed in Section 2.1)on top of an additional long-term radial-velocity signal. We discuss the details of our analysis of these signals in Section 3. 2.3. Ground-based Photometry Ground-based photometric follow-up observations were performed as part of the TESS Follow-up Program’s(TFOP) Subgroup 1 (SG1). Among the observations, a transit of TOI1759 b in 2020 May 21 was captured by three independent telescopes/observatories: the OAA telescope of the Observatori Astronòmic Albanyà (Albanyà Spain; 4 hr of total observing time, per-point precision of 1140 ppm at 1 minutes cadence; R-filter observations), the RCO telescope of the Grand-Pra Observatory (Valais Sion, Switzerland; 6 hr of total observing time, per-point precision of 1080 ppm at 1.3 minutes cadence; i p -filter observations), and the OMC telescope of the Montcabrer Observatory (Barcelona, Spain; 5 hr of total observing time, per-point precision of 1500 ppm at 1.9 minutes cadence; I c -filter observations). Data reduction for the OAA and OMC observations was performed in a two-step process: the MaximDL image processing software was used to perform image calibration (bias, darks, flats), while differential photometry was obtained using the AstroImageJ software (Collins et al. 2017). For the RCO observations, image calibration and differential photometry were both performed using AstroImageJ. The data, along with a best-fit model transit after subtracting the best-fit systematics model for each data set (see Section 3for details), are presented in Figure 4. The data are also presented in Table 6. The observed transits by these three independent observatories on 2020 May 21 not only confirmed that the event observed by TESS was on target (i.e., it happened on TOI1759), but in practice confirmed that the real period of the event was 18.85 days (i.e., half the period proposed by the TESS data validation reports), with the duration and depth detected by those observatories being consistent with the duration and depth observed in the TESS transit events. Long-term photometric monitoring was also performed from the ground using the 0.8 m Joan Oró telescope (TJO; Colomé et al. 2010)at the Montsec Observatory in Lleida, Spain and the 90 cm telescope at the Sierra Nevada Observatory (SNO; Amado et al. 2021). For the TJO observations, the data were obtained from 2020 June to 2021 April, spanning more than 300 days and covering 107 different nights. We obtained a total of 1331 images with an exposure time of 40 s using the Johnson Rfilter of the LAIA imager, a 4k ×4k CCD with a field of view of 30′and a scale of 0 4 pixel −1 . The SNO data were obtained from 2021 April to August, spanning 135 days and collecting observations on 55 different nights. Each night, 20 exposures per filter were obtained using both Johnson Vand Rfilters, with exposure times of 60 and 40 s, respectively. The photometry from these exposures was averaged to obtain a single photometric value per filter each night. These data were obtained with a VersArray 2k ×2k CCD camera with a field of view of 13.2 ×13.2 arcmin 2 and a scale of 0 4 pixel −1 as well. The TJO CCD images were calibrated with darks, and bias and flat fields with the ICAT pipeline (Colome & Ribas 2006). The differential photometry was extracted with AstroImageJ (Collins et al. 2017)using the aperture size that minimized the rms of the resulting relative fluxes and a selection of the 30 brightest comparison stars in the field that did not show variability. Then, we used our own pipelines to remove outliers and measurements affected by poor observing conditions or presenting a low signal-to-noise ratio. This resulted in a total of 1087 measurements in the final data set with an rms of 6 parts per thousand (ppt). In a similar way, the SNO resulting light curves were obtained by the method of synthetic aperture photometry. Each CCD frame was also corrected in a standard way for bias and flat-fielding. Different aperture sizes were tested in order to choose the best one for our observations. A number of nearby Figure 1. TESS target pixel files (TPFs)for TOI-1759 from the different sectors from which data were gathered with the mission (Sector 16, left; 17, center; 24, right). TOI-1759 is marked with a white cross on top of a red point and is numbered 1. Smaller numbered red points are the closest stars to the target (drawn from Gaia)with Gaia magnitude differences with the target of |ΔG|<6 mag. Contamination is not a problem for TOI-1759, as most nearby targets are very faint. The plot is made using tpfplotter (Aller et al. 2020). 3 The Astronomical Journal, 163:133 (15pp), 2022 March Espinoza et al. and relatively bright stars within the frames were selected as reference stars to produce differential photometry of TOI-1759. Finally, outliers due to poor observing conditions or very high airmass were removed. This resulted in a total of 1029 and 1027 individual data points in filters Vand R, respectively, with rms values of 6.1 and 6.4 ppt. Both the TJO and SNO data sets are presented in Table 7.An analysis of these data sets is presented in Section 3.2. Figure 2. TESS transits of TOI-1759 b. The top panels present the TESS photometry of TOI-1759 in Sectors 16, 17, and 24 as a function of time (black points with error bars), along with the best-fit model, which consists of a transit model plus a Gaussian process (blue curve). Note there is only a single transit observed in each sector. The bottom panels shows a close-up to each of those transits, which have been phased around the time of midtransit (gray points with error bars); the Gaussian process component has been removed from this photometry. The black line in these panels show the best-fit transit model; blue bands represent the 68% and 95% credibility bands of the model. 4 The Astronomical Journal, 163:133 (15pp), 2022 March Espinoza et al. 2.4. High-resolution Imaging To help rule out stellar multiplicity and close blends with nearby stars and to obtain more precise planetary radii by accounting for close-in stellar blends (Ciardi et al. 2015; Schlieder et al. 2021), we observed TOI-1759 with both the ’Alopeke 39 speckle imaging camera (Scott & Howell 2018)on the 8 m Gemini North telescope and the NIRC2 near-infrared (NIR)adaptive-optics fed camera on the 10 m Keck II telescope. The optical and NIR high-resolution imaging complement each other with higher resolution in the optical but deeper sensitivity (especially to low-mass stars)in the infrared. ’Alopeke obtains diffraction-limited imaging in two simultaneously imaged narrow bands centered at 562 and 832 nm. Due to the relative faintness of the target star at these wavelengths, we obtained five exposures in each channel, with integration times of 60 ms each. We reduced the data using standard techniques using the methods described by Matson et al. (2019). The resulting contrast curves and reconstructed 832 nm image are all shown in Figure 5. The optical speckle observations show no evidence of an additional stellar companion. TOI-1759 was observed with the NIRC2 instrument on Keck II behind the natural guide star AO system (Wizinowich et al. 2000). The observations were made on 2020 September 09 UT in the standard three-point dither pattern that is used with NIRC2 to avoid the left lower quadrant of the detector, which is typically noisier than the other three quadrants. The dither pattern step size was 3″and was repeated twice, with each dither offset from the previous dither by 0 5. The camera was in the narrow-angle mode with a full field of view of ∼10″and a pixel scale of approximately 0 0099 pixel −1 . The observations were made in the narrowband Brγfilter (λ o =2.1686; Δλ=0.0326 μm)with an integration time of 1 s with one coadd per frame for a total of 9 s on target. The AO data were processed and analyzed with a custom set of IDL tools. The science frames were flat-fielded and skysubtracted. The flat fields were generated from a median average of dark-subtracted flats taken on-sky, and the flats were normalized such that the median value of the flats is unity. Sky frames were generated from the median average of the nine dithered science frames; each science image was then skysubtracted and flat-fielded. The reduced science frames were combined into a single combined image using a intrapixel interpolation that conserves flux, shifts the individual dithered frames by the appropriate fractional pixels, and median coadds the frames. The final resolution of the combined dithers was determined from the FWHM of the point-spread function, 0049. The sensitivities of the final combined AO image were determined by injecting simulated sources azimuthally around Figure 3. CARMENES radial-velocity follow-up of TOI-1759. (a)Our CARMENES radial-velocity campaign (white points with black error bars; first point corresponding to 2,459,054.56851)had a total duration of about 6 months, over which we were able to detect both a radial-velocity variation at 18.85 days, along with a long-term signal here modeled as a GP. The blue line with transparent bands around it indicate our full (GP +planetary)median signal, along with its 68%, 95%, and 99% credibility intervals, respectively. (b)Phase-folded radial-velocity curve at the period of the transiting exoplanet TOI-1759 b (P=18.85 days; gray points, binned data points covering about 2 days plotted as white points with black error bars). The semiamplitude we obtain for this signal is 3.64 ±0.50 m s −1 . Figure 4. Ground-based follow-up transit photometry of TOI-1759 b. Transit of TOI-1759 b on 2020 May 21 captured by three different observatories: the OAA telescope of the Observatori Astronòmic Albanyà, the RCO telescope of the Grand-Pra Observatory, and the OMC telescope of the Montcabrer Observatori. The duration and depth of the event are consistent between instruments. 39 https://www.gemini.edu/instrumentation/alopeke-zorro 5 The Astronomical Journal, 163:133 (15pp), 2022 March Espinoza et al. the primary target every 20°at separations of integer multiples of the central source’s FWHM (Furlan et al. 2017). The brightness of each injected source was scaled until standard aperture photometry detected it with 5σsignificance. The resulting brightness of the injected sources relative to the target set the contrast limits at that injection location. The final 5σ limit at each separation was determined from the average of all of the determined limits at that separation and the uncertainty on the limit was set by the rms dispersion of the azimuthal slices at a given radial distance. The final combined image and sensitivity curve are shown in Figure 6. Both the optical speckle and the near-infrared adaptiveoptics observations find no additional stars (down to 0 1, dimmer by about 4–5 mag than the target in the optical and near-infrared)and so further strengthen the case for TOI-1759 b being a bona fide planet. 3. Analysis 3.1. Stellar Parameters We obtained the photospheric parameters T eff ,glog , and [Fe/H]of TOI-1759 following Passegger et al. (2019)by fitting PHOENIX synthetic spectra to the combined (coadded) CARMENES VIS spectrum described in Section 2.2, which has a signal-to-noise ratio in the VIS channel of ≈200. We used =visin 2 km s −1 , which was measured by Marfil et al. (2021)as an upper limit. We derived its luminosity following Cifuentes et al. (2020)by using the latest parallactic distance from Gaia EDR3 (Gaia Collaboration et al. 2021), and by integrating Gaia, 2MASS (Skrutskie et al. 2006), and AllWISE (Cutri et al. 2014)photometry covering the full spectral energy distribution with the Virtual Observatory Spectral energy distribution Analyzer (Bayo et al. 2008). The stellar radius follows from the Stefan−Boltzmann law and the stellar mass was determined using the linear mass–radius relation from Schweitzer et al. (2019). TOI-1759ʼs pseudo equivalent width of the Hαline as defined by Schöfer et al. (2019)is pEW′(Hα)<−0.3 Å(Marfil et al. 2021), classifying it as an Hαinactive star. Furthermore, Marfil et al. (2021)assign it to the Galactic thin disk population, which has a maximum age of about 8 Gyr (Fuhrmann 1998). Using these two properties as age indicators, we conclude that its age is between 1 Gyr (the typical minimum age for field stars)and 8 Gyr without being able to be more precise. All collected and derived parameters are presented in Table 1. 3.2. Radial-velocity Analysis We performed a detailed analysis of the radial velocities described in Section 2.2 in order to constrain the possible signals arising from these data. To this end, we performed a suite of model fits to the radial-velocity data using juliet (Espinoza et al. 2019), in order to measure the evidence for a planet in the data using Bayesian evidences, Z=P (Model |Data). The fits were performed using the Dynamic Nested Sampling algorithm implemented in the dynesty library (Speagle 2020). We considered three main types of radial-velocity models. The first was a “no planet”model, namely, a set of models in which it is assumed there is no planetary signal present in the radial-velocity data, and which thus assumes the data are either consistent with a flat line or with correlated noise modeled through a Gaussian process (GP). The second class were “1 planet”models; these considered the presence of a planetary signal in the radial-velocity data (modeled as a circular orbit), and a suite of possible extra signals, such as linear or quadratic trends, or a (quasi-periodic)GP. Finally, we also considered the possibility that the data were best explained by a “2 planet” model, as a sum of two circular orbits and a suite of possible extra signals, such as a linear, quadratic, or GP trend. We first performed “blind”fits to the data—that is, fits in which we assumed no strong prior knowledge on the signal(s) present on our radial velocities. For our GP, we assumed a quasi-periodic kernel of the form sat pt =--G k tt P ,exp sin, ij GP 222 rot ⎜⎟ ⎛ ⎝⎡ ⎣ ⎢⎤ ⎦ ⎥⎞ ⎠ () where τ=|t i −t j |. We set log-uniform priors for σ GP ,α, and Γ, with lower and upper limits of (0.01, 100)ms −1 ,(10 −10 , 1)day −1 and (0.01, 100)respectively, based on the Figure 5. High spatial resolution imaging of TOI-1759. Two-band speckle imaging observations obtained with the ’Alopeke speckle imaging camera on the 8 m Gemini North telescope for TOI-1759 reveal no close companion down to 0.1″, dimmer than about Δm=4−5 mag than the target. Figure 6. Near-infrared high spatial resolution imaging of TOI-1759. NIRC2 imaging observations obtained for TOI-1759 on Keck II reveals no close companion down to 0.1″, dimmer than about Δm=4−5 mag than the target. 6 The Astronomical Journal, 163:133 (15pp), 2022 March Espinoza et al. experiments performed with this kernel in Stock et al. (2020a) and Stock et al. (2020b), and a uniform prior for P rot between 0.5 (half the best sampling in our radial velocities)and 350 days (two times our time baseline). The linear and quadratic trends both had uniform priors on the coefficients of (10 −3 ,10 3 ). As for the circular orbits, we set a uniform prior on the period of the first one from 0.5 to 50 days (so as to cover the 18 and 36 days periods that could possibly originate from the transiting exoplanet), and a uniform prior on the period of the second one from 50 to 350 days. Uniform priors were set for the time of inferior conjunction to cover the entire time baseline of our observations, the semiamplitude—between 0 and 100 m s −1 —and the systematic radial velocity—between −100 and 100 m s −1 . A jitter term, σ w was added to all of our fits with a log-uniform prior between 0.01 and 100 m s −1 . In our “blind”fits, we found that all models considering a periodic component were consistent with a prominent signal at ∼18.5 days, which corresponds to the transit signal implied by the TESS photometry presented in Section 2.1 and the groundbased transits presented in Section 2.3. The model with the highest evidence in our set of fits was one composed of a sinusoid plus a quasi-periodic GP (D»Zlog 4. 3∣ ∣, compared with the no-planet model). Given the high-resolution imaging data presented in Section 2.4, the ground-based transit detected on target presented in Section 2.3, and the fact that the period of the planetary signal for the model with the highest evidence (- + 18.49 0.21 0.23 days)agrees with the period implied by the photometric data (18.8480 ±0.0010 days), we consider that this 18 days period signal in both photometry and radial velocities is, indeed, a bona fide transiting exoplanet. Having concluded that the 18 days period signal is indeed a bona fide transiting exoplanet, we then focused on finding the best model that explains the radial-velocity data set. We considered the same class of models and priors as the ones presented above, but now for the first planet we fixed the period and transit center to the values defined by a photometric fit made to the data using juliet (see Section 3.3 for details on the priors of that fit): period P=18.85008 ±0.00018 days, and time-of-transit center t 0 =2,458,745.4651 ±0.0015 days. A compilation of the log-evidences for each of the fits we performed is presented in Table 2. As can be seen, the model with the highest evidence is once again a 1 planet +GP model. Interestingly, however, this model is in practice indistinguishable (D<Zlog 2∣ ∣; Trotta 2008)from most of the 2 planet models (except the 2 planet +linear trend model). It is also indistinguishable from all of those models considering either one or both of them having eccentric orbits. It is interesting to note that the posterior distribution function of the GP rotation period of the 1 planet +GP model, P rot , shows a bimodal distribution, with peaks at ∼80 days and >150 days. In the 2 planet +GP model, the latter long periodic signal is picked up by the second sinusoid and is constrained to be about 270 ±60 days, while the quasi-periodic component of the GP shows again an accumulation of samples with a higher likelihood at ∼80 days. A Generalized Lomb−Scargle (GLS; Zechmeister & Kürster 2009)periodogram analysis of the residuals of each of those models is presented in Figure 7,in order to further explore the nature of these long-period signals. If the signal of the transiting planet is subtracted (second panel), a significant power excess is apparent at ∼270 days (which is above the baseline of our observations, which was Table 1 Stellar Properties of TOI-1759 Parameter Value Reference Names TIC 408636441 TIC 2MASS J1472477+6245139 2MASS TYC 4266-00736-1 Tycho-2 WISEA J214724.51+624513.8 AllWISE R.A. (J2000)21 h 47 m 24 39 Gaia EDR3 Decl. (J2000)62°45′13 7 Gaia EDR3 Spectral type M0.0 V Lep13 m d acos (mas yr −1 )−173.425 ±0.012 Gaia EDR3 μ δ (mas yr −1 )−10.654 ±0.011 Gaia EDR3 π(mas)24.922 ±0.010 Gaia EDR3 d(pc)40.112 ±0.016 Gaia EDR3 G BP (mag)11.7164 ±0.0029 Gaia EDR3 G(mag)10.8386 ±0.0028 Gaia EDR3 T(mag)9.9284 ±0.0073 TIC G RP (mag)9.9174 ±0.0038 Gaia EDR3 J(mag)8.771 ±0.043 2MASS H(mag)8.114 ±0.059 2MASS K s (mag)7.930 ±0.020 2MASS W1(mag)7.825 ±0.027 AllWISE W2(mag)7.886 ±0.020 AllWISE W3(mag)7.787 ±0.018 AllWISE W4(mag)7.643 ±0.111 AllWISE L å (10 −4 L e )876.7 ±6.3 This work T eff (K)4065 ±51 This work g log (dex)4.65 ±0.04 This work [Fe/H](dex)0.05 ±0.16 This work visin (km s −1 )2 Mar21 M å (M e )0.606 ±0.020 This work R å (R e )0.597 ±0.015 This work Age (Gyr)1–8 This work ρ å (kg m −3 )3949 ±323 This work References—2MASS (Skrutskie et al. 2006), AllWISE (Cutri et al. 2014), Gaia EDR3 (Gaia Collaboration et al. 2021), Lep13 (Lépine et al. 2013), Mar21 (Marfil et al. 2021), TIC (Stassun et al. 2019), Tycho-2 (Høg et al. 2000). Table 2 Log-evidence Differences ΔZbetween Different Models Considered for Our Radial-velocity-only Analysis Model DZln No planet models Flat line −9.4 GP −6.6 1 planet models 1 planet +linear trend −23.6 1 planet +quadratic trend −9.1 1 planet −8.8 1 planet +GP 0 2 planet models 2 planet +linear trend −12.6 2 planet −1.6 2 planet +GP −1.4 2 planet +quadratic trend −1.3 Note. It is assumed that one of the Keplerian signals has the same ephemerides as those implied by the observed transits. A flat-line model includes only a systematic radial velocity and a jitter term. The GP refers to a Gaussian Process with a quasi-periodic kernel. See text for details on the priors. 7 The Astronomical Journal, 163:133 (15pp), 2022 March Espinoza et al. 175 days), and next to it, even though it is insignificant, is a peak at ∼80 days. The latter is independent of the long-term variation, as it is still present after fitting the transiting planet together with the ∼270 days signal (third panel). To find out whether the ∼80 days signal or the ∼270 days power excess could be attributed to stellar activity, we investigated the activity indicators that are routinely derived from the CARMENES spectra (see Zechmeister et al. 2018; Schöfer et al. 2019; Lafarga et al. 2020, for the full list of indicators and how they are calculated). In Figure 8we show the GLS periodograms of several activity indicators and the long-term photometry presented in Section 2.3. Almost all indicators, as well as the photometry, show consistent signals around 80 and/or 40 days, which would explain the ∼80 days seen in the radial velocities (RVs)as the stellar rotation period. If indeed the rotation period of the star is 80 days, the 40 days signal could be interpreted as spots at opposite longitudes and/ or a byproduct of the not strictly periodic nature of the signal. Further, the TJO data, the BIS, and the Ca II IRT b and c also show a long-term trend, which might be related with the ∼270 days power excess. Given that we could not rule out a stellar origin for the ∼80 days signal and the ∼270 days power excess, and that the 1 planet +GP model has the highest evidence for our RV data and can account for all significant signals in the data (see the residuals in the last panel of Figure 7), we consider this model for the global modeling of the data, which we present in the next subsection. We note that we also tested fitting our global model using the rest of the models in Table 2, which are indistinguishable from the one selected here, and all of them gave rise to similar constraints to the final parameters of the transiting exoplanet. Figure 7. Generalized Lomb−Scargle (GLS)periodograms of the CARMENES radial-velocity data after subtracting different models. The power at each frequency corresponds to the one defined in Zechmeister & Kürster (2009; ZK in the labels above). The period of the transiting planet, P=18.85 days, is marked by the red solid line and the stellar rotation period, P≈80 days, and its first harmonic are shown with the blue solid and dashed lines, respectively. Further, the additional present long-term periodicity with P≈270 days is marked by the yellow solid line. The horizontal dashed gray lines show the analytical false-alarm probabilities (FAP)of 10%, 1%, and 0.1%. The top panel corresponds to the mean-subtracted CARMENES radial-velocity data set. Figure 8. GLS periodograms of the TJO photometry, SNO photometry, and the CARMENES spectral activity indicators. Analogous to the periodograms of the RVs, the period of the transiting planet, P=18.85 days, and the additional present long-term periodicity with P≈270 days are marked by the solid red and yellow lines, respectively. The stellar rotation period, P≈80 days, and its first harmonic are indicated by the blue solid and dashed lines. The horizontal dashed gray lines show the analytical false-alarm probabilities (FAP)of 10%, 1%, and 0.1%. 8 The Astronomical Journal, 163:133 (15pp), 2022 March Espinoza et al. 3.3. Global Modeling We performed a global modeling of the radial-velocity and photometric data using the juliet library (Espinoza et al. 2019)in order to jointly constrain the planetary properties from the photometry and radial-velocity data sets outlined in previous sections. As in the radial-velocity analysis presented in Section 3.2, we once again used the Dynamic Nested Sampling algorithm implemented in the dynesty library (Speagle 2020). For the TESS photometry, we decided to use a GP to consider residual systematic trends in the PDC light curves used in this work. We used an Exponential−Matérn kernel (i.e., the product of an exponential and a Matérn 3/2kernel)as implemented in the celerite library (Foreman-Mackey et al. 2017)via juliet, with hyperparameters (GP amplitude, σ GP , and two timescales,: one for the Matèrn 3/2 part of the kernel, ρ,and another for the exponential part of the kernel, T), which are individual to each of the sectors; a jitter term was also added in quadrature to the covariance matrix for each sector. A quadratic law was used to constrain limb darkening, where the coefficients were shared between the different sectors; we used the parameterization of Kipping (2013)instead of fitting for the limb-darkening coefficients directly. For the ground-based photometry, we found that airmass was a very good predictor of the long-term trends in the data, and so we added this as a linear regressor in our fit—weighted by a coefficient θ, which was different for each instrument and was jointly fit with the rest of the parameters of the global fit. In addition, we observed that this linear regressor was insufficient to model all of the correlated noise leftover on the OMC and RCO data sets. We thus decided to fit those with an additional Matèrn 3/2 kernel. A linear limbdarkening law was used for all ground-based instruments, as a higher-order law was not necessary given the lower photometric precision (see, e.g., Espinoza & Jordán 2016). An individual jitter term was added to the diagonal of the covariance matrix on each of those data sets as well. For the radial velocities, following our results in Section 3.2, we considered a 1 planet model plus a quasi-periodic GP as the model to be fit in our joint analysis. We set a wide prior for the systemic radial velocity, as well as for the jitter term and the hyperparameters of the GP—in particular, for the period of the quasi-periodic GP P rot ,weusea wide period of between 20 and 350 days in order to cover the two possible periods for this parameter observed in Section 3.2. The full definition of the priors and corresponding posteriors of our joint fit are given in Table 3. The resulting best-fit models and corresponding credibility bands are presented in Figure 2for the TESS photometry, in Figure 4for the ground-based photometry, and in Figure 3for the radial velocities. The constraints from our radial-velocity follow-up allowed us to obtain a precise (>5σabove 0) measurement of the semiamplitude imprinted by TOI-1759 b on its star of =- + K3.64 0.51 0.50 ms −1 , which is an over 7σdetection of the semiamplitude. Joining the derived transit and radialvelocity parameters, along with the stellar properties presented in Table 1, we derive the fundamental parameters of TOI1759 b in Table 4. As can be observed, TOI-1759 b is a relatively cool (443 K equilibrium temperature, assuming 0 albedo)sub-Neptune-sized exoplanet (R p =3.14 ±0.10 R ⊕ ). Coupling these numbers with our estimated mass of M p =10.8 ±1.5M ⊕ , we derive a planetary bulk density (ρ p =1.91 ±0.32 g cm −3 )and gravity (g p =10.7 ±1.6 m s −2 ) that are strikingly similar to those of Neptune (1.64 g cm −3 and 11.15 m s −2 , respectively). We discuss the properties of TOI1759 b in the context of other discovered systems in the next section. Table 3 Prior and Posterior Parameters of the Global Fit Performed on TOI-1759 Parameter Prior Posterior Stellar and planetary parameters P 1 (days)N(18.85, 0.1 2 )18.85019 ±0.00013 t 0,1 (BJD)N(2,458,745.45, 0.1 2 )2,458,745.4654 ±0.0011 R p,1 /R å U(0.0, 1.0)0.0483 ±0.0010 = b aR icos 1()() U(0.0, 1.0)- + 0 .21 0.10 0.09 K 1 (ms −1 )U(0, 100)- + 3 .64 0.51 0.50 e 1 fixed 0 ω 1 fixed 90 ρ å (kg m −3 )TN(3949, 323 2 ; 1000, 10,000) - + 3 970 233 218 TESS photometry instrumental parameters q 1,TESSa U(0, 1)- + 0 .32 0.14 0.2 4 q 2,TESSa U(0, 1)- + 0 .46 0.25 0.28 m flux,16 (ppm)U(0, 10 5 )- + 851 7770 7916 m flux,17 (ppm)U(0, 10 5 ) - - + 1598 8089 806 2 m flux,24 (ppm)U(0, 10 5 )- + 428 7797 790 4 σ w,16 (ppm)Ulog 0.1, 105 () - + 1 .8 1.5 7.5 σ w,17 (ppm)Ulog 0.1, 105 () - + 2.9 2.5 15.1 σ w,24 (ppm)Ulog 0.1, 105 () - + 1 .9 1.6 7.7 σ GP,16 (ppm)- Ulog 10 , 10 42 () - + 0 .000104 0.0000026 0.000047 σ GP,17 (ppm)- Ulog 10 , 10 42 () - + 0 .000106 0.0000040 0.000067 σ GP,24 (ppm)- Ulog 10 , 10 42 () - + 0 .000100 0.0000017 0.000030 ρ GP,16 (days)- Ulog 10 , 10 32 () - + 73 17 16 ρ GP,17 (days)- Ulog 10 , 10 32 () - + 74 18 15 ρ GP,24 (days)- Ulog 10 , 10 32 () - + 76 17 1 4 T GP,16 (days)- Ulog 10 , 10 32 () - + 0 .0010 0.000026 0.000047 T GP,17 (days)- Ulog 10 , 10 32 () - + 0 .0011 0.00004 5 0.000081 T GP,24 (days)- Ulog 10 , 10 32 () - + 0 .0010 0.000019 0.00003 4 Ground-based photometry instrumental parameters q 1,OAA U(0, 1)- + 0 .41 0.24 0.27 q 1,OMC U(0, 1)- + 0 .55 0.31 0.27 q 1,RCO U(0, 1)- + 0 .50 0.29 0.30 m flux,OAA (ppm)U(0, 10 5 ) - - + 2148 216 21 4 m flux,OMC (ppm)U(0, 10 5 )- + 7936 6203 6471 m flux,RCO (ppm)U(0, 10 5 ) - - + 979 5945 6296 σ w,OAA (ppm)Ulog 0.1, 105 () - + 3 395 140 146 σ w,OMC (ppm)Ulog 0.1, 105 () - + 3 123 190 202 σ w,RCO (ppm)Ulog 0.1, 105 () - + 4143 158 165 θ OAA (days)U(−10, 10)- + 0 .00112 0.00021 0.00022 θ OMC (days)U(−10, 10) - - + 0.0036 0.015 0.01 5 θ RCO (days)U(−10, 10)- + 0 .013 0.015 0.015 σ GP,OMC (ppm)- Ulog 10 , 10 42 () - 0 .117 0.012 0.022 ρ GP,OMC (days)- Ulog 10 , 10 32 () - + 0 .438 0.059 0.040 σ GP,RCO (ppm)- Ulog 10 , 10 42 () - 0 .117 0.011 0.019 ρ GP,RCO (days)- Ulog 10 , 10 32 () - + 0 .438 0.051 0.039 Radial-velocity instrumental/activity parameters μ CARMENES (ms −1 )U(−100, 100)- + 0 .1 4.0 4.7 σ w,CARMENES (ms −1 )Ulog 0.01, 100() - + 1 .85 0.49 0.50 σ GP,CARMENES (ms −1 )Ulog 0.01, 102 () - + 5.8 2.5 7.3 α GP,CARMENES (days −1 )- Ulog 10 , 1 10 () - + 0 .000011 0.000011 0.000433 Γ GP,CARMENES Ulog 0.01, 100() - + 0 .5 0.5 2.8 P GP,Rot U(20, 350)- + 237 103 67 Note. For the priors, N(μ,σ 2 )stands for a normal distribution with mean μand variance σ 2 ;TN(μ,σ 2 ;a,b)is a truncated normal distribution with lower and upper limits given by aand b, respectively; U(a,b)and Uablog ,( ) stand for a uniform and log-uniform distribution between aand b, respectively. a These parameterize the quadratic limb-darkening law using the transformations in Kipping (2013). 9 The Astronomical Journal, 163:133 (15pp), 2022 March Espinoza et al.