Characterisation of 92 Southern TESS Candidate Planet Hosts and a New Photometric [Fe/H] Relation for Cool Dwarfs

MNRAS 000,1–21 (2015) Preprint 15 July 2021 Compiled using MNRAS LATEX style file v3.0 Characterisation of 92 Southern TESS Candidate Planet Hosts and a New Photometric [Fe/H] Relation for Cool Dwarfs Adam D. Rains,1¢ Maruša Žerjal,1 Michael J. Ireland,1 Thomas Nordlander,1,2 Michael S. Bessell,1 Luca Casagrande,1,2 Christopher A. Onken,1,3 Meridith Joyce,1,2 Jens Kammerer,1,4 and Harrison Abbot1 1Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia 2ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D) 3Centre for Gravitational Astrophysics, Research Schools of Physics, and Astronomy and Astrophysics, Australian National University 4European Southern Observatory, Karl-Schwarzschild-Str 2, 85748, Garching, Germany Last updated 2015 May 22; in original form 2013 September 5 ABSTRACT We present the results of a medium resolution optical spectroscopic survey of 92 cool (3, 000 . )eff . 4, 500 K) southern TESS candidate planet hosts, and describe our spectral fitting methodology used to recover stellar parameters. We quantify model deficiencies at predicting optical fluxes, and while our technique works well for )eff, further improvements are needed for [Fe/H]. To this end, we developed an updated photometric [Fe/H] calibration for isolated main sequence stars built upon a calibration sample of 69 cool dwarfs in binary systems, precise to ±0.19 dex, from super-solar to metal poor, over 1.51 < Gaia ¹퐵% − '%º < 3.3. Our fitted )eff and '¢ have median precisions of 0.8% and 1.7%, respectively and are consistent with our sample of standard stars. We use these to model the transit light curves and determine exoplanet radii for 100 candidate planets to 3.5% precision and see evidence that the planet- radius gap is also present for cool dwarfs. Our results are consistent with the sample of confirmed TESS planets, with this survey representing one of the largest uniform analyses of cool TESS candidate planet hosts to date. Key words: stars: low-mass, stars: fundamental parameters, planets and satellites: fundamental parameters, techniques: spectroscopic, 1 INTRODUCTION 4,000 confirmed planets orbiting stars other than our own (over- whelmingly discovered by transiting exoplanet surveys), almost an Low mass stars are the most common kind of star in the Galaxy, equal number await confirmation1. Exoplanet transit surveys like comprising more than two thirds of all stars (Chabrier 2003), and Kepler and TESS (Ricker et al. 2015) are able to place tight con- dominating the Solar Neighbourhood population (e.g. Henry et al. straints on planetary radii given a known stellar radius, but follow- 1994, 2006; Winters et al. 2015; Henry et al. 2018). This abun- up precision radial velocity observations are required to provide dance alone makes them prime targets for planet searches, with mi- planetary mass constraints. This is the second reason why planet crolensing surveys, which have very little bias on host star masses, searches around low mass stars are critical: their smaller radii and arXiv:2102.08133v2 [astro-ph.EP] 14 Jul 2021 revealing that there is at least one bound planet per Milky Way star lower masses make the transit signals and radial velocities of higher (Cassan et al. 2012). Results from the Kepler Mission (Borucki et al. amplitudes for any planets they host as compared to the same planets 2010) also bear this out, showing that a large number of planets re- around more massive host stars. This is especially important when main undiscovered around cool dwarfs (Morton & Swift 2014), and looking for planets with terrestrial radii or masses respectively. that such cool stars are actually more likely to host small planets Many planet host stars have never been targeted by a spec- (2 < ' < 4 ' , where ' and ' are the planet and earth radius % ⊕ % ⊕ troscopic survey, leaving their properties to be estimated through respectively) than their hotter counterparts (Howard et al. 2012; photometry alone. For instance, the TESS input catalogue (Stassun Dressing & Charbonneau 2015). et al. 2018, 2019) based its stellar parameters primarily on photome- However, the inherent faintness of these stars complicates the try, having spectroscopic properties for only about 4 million stars of study of both them and their planets. While we now know of over the nearly 700 million with photometrically estimated equivalents. ¢ E-mail: [email protected] (ADR) 1 https://exoplanetarchive.ipac.caltech.edu/ © 2015 The Authors 2 Adam D. Rains et al. While stars warmer than 4,000 K are well suited to bulk estima- these features is a function of both temperature and [Fe/H], making tion of properties from photometry (see e.g. Carrillo et al. 2020), it difficult to ascribe a unique )eff-[Fe/H] pair to a given star. special care must be taken for cool dwarfs whose faintness and complex atmospheres make such relations more complex to develop and Despite this complexity, it is possible to take advantage of the implement (e.g. see Muirhead et al. 2018 for the K and M dwarf relative [Fe/H]-insensitivity of NIR band magnitudes alongside specific approach taken from the TESS input catalogue). [Fe/H]-sensitive optical photometry to probe cool dwarf [Fe/H]. NASA’s TESS Mission, by virtue of being all sky, has given us This was predicted by theory (see e.g. Allard et al. 1997, Baraffe a wealth of bright candidates which are now being actively followed et al. 1998, and Chabrier & Baraffe 2000 for summaries), confirmed up by ground based spectroscopic surveys. While multi-epoch radial observationally (Delfosse et al. 2000), and later formalised into velocity observations are required to determine planetary masses, various empirical calibrations (Bonfils et al. 2005; Johnson & Apps these surveys are typically biased towards the brightest stars and 2009; Schlaufman & Laughlin 2010; Neves et al. 2012; Hejazi et al. smallest planets. As such, there remains a need for single-epoch 2015; Dittmann et al. 2016). spectroscopic follow-up of fainter targets to provide reliable host star properties (primarily )eff, log 6, [Fe/H], and the stellar radius The last decade has seen a number of studies using low-medium '¢) and allow radial constraints to be placed on transiting planet resolution (mostly NIR) spectra, often focused on the development candidates. Indeed, the LAMOST Survey (Zhao et al. 2012) under- of [Fe/H] relations based on spectral indices (e.g. optical-NIR: Mann took targeted low resolution spectroscopic follow-up of stars in the et al. 2013b,c, 2015; Kuznetsov et al. 2019; NIR: Newton et al. Kepler field (De Cat et al. 2015) with the goal of deriving spec- 2014; H band: Terrien et al. 2012; K band: Rojas-Ayala et al. 2010, troscopic stellar properties. Considering the goal of planet radii 2012). Other studies have opted to use high-resolution spectra which determination specifically, Dressing et al.(2019) used medium- gives access to unblended atomic lines that are not accessible to resolution near-infrared (NIR) spectra, and Wittenmyer et al.(2020) lower resolution observations (e.g. optical: Bean et al. 2006a,b; high-resolution optical spectra to follow-up K2 (Howell et al. 2014) Rajpurohit et al. 2014; Passegger et al. 2016; Y band: Veyette et al. transiting planet candidate hosts and place radius constraints on 2017; optical-NIR: Woolf & Wallerstein 2005, 2006; Passegger both planets and their hosts. et al. 2018; J band: Önehag et al. 2012; H band: Souto et al. 2017). Even without mass estimates, much can be learned about exoplanet demographics from their radii alone. As demonstrated by Ful- Finally, on the point of M-dwarf evolutionary models (and low- ton et al.(2017), Fulton & Petigura(2018), Van Eylen et al.(2018), mass, cool main sequence stars more generally), there has long been Kruse et al.(2019), Hardegree-Ullman et al.(2020), Cloutier & contention between model radii and observed radii (e.g. Kraus et al. Menou(2020), and Hansen et al.(2021), having a large sample 2015). This is often attributed to magnetic fields (and/or the mixing of precise planet radii allows insight into the exoplanet radius dis- length parameter, which simplistically parameterizes the effects of tribution, which appears to be bimodal with an observable gap in magnetic fields among other energy transport mechanisms in 1D the super-Earth regime (∼ 1.8 '⊕). This is thought to be the result stellar structure and evolution programs) and is related to the diffi- of physical phenomena like photoevaporation (where flux from the culty in accurately modelling convection (e.g. Feiden & Chaboyer parent star strips away weakly held atmospheres, e.g. Owen & Wu 2012; Joyce & Chaboyer 2018). Fortunately, due to the aforemen- 2013; Lee et al. 2014; Lopez & Fortney 2014; Lee & Chiang 2016; tioned insensitivity of NIR band photometry to [Fe/H], empiri- Owen & Wu 2017; Lopez & Rice 2018), or core-powered mass loss cal mass and radius relations have been developed and calibrated (where a cooling rocky core erodes light planetary atmospheres via on interferometric diameters and dynamical masses (e.g. Henry & its cooling luminosity, e.g. Ikoma & Hori 2012; Ginzburg et al. McCarthy 1993; Delfosse et al. 2000; Benedict et al. 2016; Mann 2018; Gupta & Schlichting 2019, 2020), and its location likely has et al. 2015, 2019). a dependence on stellar host mass (e.g. Cloutier & Menou 2020). As such, improving the sample of planets with radius measurements allows us to place observational constraints on planet formation Here we conduct a moderate resolution spectroscopic sur- ) , channels and the mechanisms that sculpt planets throughout their vey of 92 southern cool ( eff . 4 500 K) TESS candidate planet lives. hosts with the WiFeS instrument (Dopita et al. 2007) on the ANU 2.3 m Telescope at Siding Spring Observatory (NSW, Australia).

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