TOI-1231 b: A Temperate, Neptune-sized Planet Transiting the Nearby M3 Dwarf NLTT 24399

TOI-1231 was first reported as a high proper motion star (035 yr⁻¹) by Luyten (1957, as LTT 3840 and L 248-27), and later in the Revised NLTT catalog (Luyten 1979, as NLTT 24399). Over the past decade, the star has appeared in several surveys of high proper motion Two Micron All Sky Survey (2MASS) and Wide-field Infrared Survey Explorer (WISE) stars and nearby M dwarfs (e.g., Lépine & Gaidos 2011; Frith et al. 2013; Kirkpatrick et al. 2014; Schneider et al. 2016). The only previous spectral characterization of TOI-1231 was by Gaidos et al. (2014) in the CONCH-SHELL survey, who reported a spectral type of M3V. Previous estimates of the basic stellar parameters were reported by Gaidos et al. (2014), Muirhead et al. (2018), and Stassun et al. (2019).

Stellar astrometry and visible and infrared photometry are compiled in Table 1. The positions, proper motions, parallax, and Gaia photometry are from Gaia EDR3 (van Leeuwen et al. 2021), whereas the radial velocity is from Gaia DR2 (Gaia Collaboration et al. 2018). We convert the astrometry to Galactic velocities following ESA (1997). ⁵⁰ Photometry is reported from AAVSO Photometric All-Sky Survey (APASS) Data Release 10 (Henden et al. 2016), ⁵¹ the TESS Input Catalog (TIC8), 2MASS (Cutri et al. 2003), and WISE (Cutri et al. 2012).

Table 1. Astrometry and Photometry for TOI-1231

Parameter	Value	Source
Designations	TIC 447061717	Stassun et al. (2019)
	NLTT 24399	Luyten (1979)
R.A. (ICRS, J2000)	10:26:59.34	Gaia EDR3
decl. (ICRS, J2000)	−52 28 04.16	Gaia EDR3
μ R.A. (mas yr−1)	−89.394 ± 0.019	Gaia EDR3
μ decl. (mas yr−1)	361.546 ± 0.015	Gaia EDR3
Parallax (mas)	36.3726 ± 0.0163	Gaia EDR3 a
Distance (pc)	27.4932 ± 0.0123	Gaia EDR3 a
vR (km s−1)	70.48 ± 0.54	Gaia DR2
SpT	M3V	Gaidos et al. (2014)
B	13.739 ± 0.028	APASS/DR10
V	12.322 ± 0.023	APASS/DR10
$g^{\prime}$	12.997 ± 0.044	APASS/DR10
$r^{\prime}$	11.806 ± 0.070	APASS/DR10
$i^{\prime}$	10.754 ± 0.175	APASS/DR10
TESS	10.2565 ± 0.007	TIC8
G	11.3612 ± 0.0009	Gaia EDR3
GBP	12.5440 ± 0.0022	Gaia EDR3
GRP	10.2735 ± 0.0015	Gaia EDR3
J	8.876 ± 0.027	2MASS
H	8.285 ± 0.038	2MASS
Ks	8.069 ± 0.026	2MASS
W1	7.922 ± 0.024	WISE
W2	7.826 ± 0.020	WISE
W3	7.732 ± 0.017	WISE
W4	7.515 ± 0.091	WISE
U (km s−1)	−20.25 ± 0.24	This work
V (km s−1)	−73.37 ± 0.36	This work
W (km s−1)	39.45 ± 0.33	This work
Stot(km s−1)	85.73 ± 0.35	This work

Note.

^a:Correction of −17 μas applied to the Gaia parallax following the prescription in Lindegren et al. (2021).

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We analyzed TOI-1231's broadband spectral energy distribution (SED) alongside its Gaia EDR3 parallax to determine an empirical measurement of the star's radius, following Stassun & Torres (2016) and Stassun et al. (2017, 2018). Together, the available photometry described here and listed in Table 1 cover the full stellar SED from 0.4 to 22 μm (see Figure 1). We exclude the APASS/DR10 data from the SED fit in favor of the Gaia EDR3 passbands, which cover the same wavelength range and have smaller errors, but note that we adopt an error floor of 0.03 mag on the photometry because the systematics on the absolute flux calibration between photometric systems is 2%–3%.

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We performed the fit using NextGen stellar atmosphere models, placing a prior on the star's surface gravity ($\mathrm{log}g$) from the TESS Input Catalog (TIC8). We set the stellar effective temperature equal to the result from the Cool Dwarf Catalog (T_eff = 3557 ± 82 K, Muirhead et al. 2018) and the extinction to zero due to the star's proximity (within the Local Bubble), which is consistent with the reddening value of E_B−V = 0.001 ± 0.015 from Lallement et al. (2018). ⁵² The resulting SED fit is quite good (Figure 1, top) with a reduced χ² of 1.9. The best-fit stellar metallicity is [Fe/H] = 0.0 ± 0.3. Integrating the model SED results in the bolometric flux at Earth being F_bol = (1.345 ± 0.047) × 10⁻⁹ erg s⁻¹ cm⁻² (m_bol = 10.682 ± 0.032 on IAU 2015 bolometric magnitude scale). ⁵³ Taking the F_bol and T_eff together with the Gaia EDR3 parallax gives the stellar radius as R_⋆ = 0.466 ± 0.021 R_⊙ and the stellar mass as M_⋆ = 0.485 ± 0.024 M_⊙ based on the empirical M_Ks versus mass relations from Mann et al. (2019). ⁵⁴

2.3.1. Speckle Observations

Using the astrometry and radial velocity data from Gaia Collaboration et al. (2018), we calculate the heliocentric Galactic velocity for TOI-1231 to be (U, V, W) = (−20.25, −73.37, 39.45 ± 0.24, 0.36, 0.33) km s⁻¹, with total velocity S_tot = 85.73 ± 0.35 km s⁻¹. Compared with the local standard of rest (LSR) of Schönrich et al. (2010), we estimate velocities of (U, V, W) = (−10.2, −62.4, 46.5) km s⁻¹, with S_LSR = 78.4 km s⁻¹. We use the BANYAN Σ (Bayesian Analysis for Nearby Young AssociatioNs Σ; Gagné et al. 2018) tool to estimate membership probabilities to nearby young associations within 150 pc; however, the probabilities are ≪0.1% for any of the known nearby stellar groups (all with ages <1 Gyr), and the star is classified as "field." Following Bensby et al. (2014), we use the Galactic velocity to estimate kinematic membership probabilities to the Milky Way's principal populations, using a 4-population model for the thin disk, thick disk, halo, and Hercules stream. ⁵⁶ We estimate kinematic membership probabilities of P(thin) = 16.7%, P(thick) = 64.5%, P(Hercules) = 18.7%, and P(halo) = 0.06%. However, the star's LSR velocity places it among the Hercules stream member in Figure 29 of Bensby et al. (2014). Mackereth & Bovy (2018) calculated parameters of the star's Galactic orbit using Stäckel approximation with the Gaia Collaboration et al. (2018) astrometry, and found an eccentricity of 0.332, z_max = 0.905 kpc, a perigalacticon of r_peri = 4.02 kpc, and an apogalacticon of r_ap = 8.03 kpc, ⁵⁷ i.e., we are catching the star near its apogalacticon.

We also searched for companions of TOI-1231 in the Gaia Collaboration et al. (2018) catalog via the 50 pc sample of Torres et al. (2019). Given the mass of 0.46 M_⊙, we estimate the tidal radius for TOI-1231 (where bound companions would likely be found) to be 1.04 pc (Mamajek et al. 2013), which corresponds to a projected radius of ∼22. Querying the Torres et al. (2019) catalog for stars with proper motions and parallaxes within 20% of that of the star within 2 tidal radii (44) yielded no candidate companion. Therefore, we conclude TOI-1231 to be a single star.

The V − K_s versus absolute magnitude position of an M dwarf can be used to infer a photometric metallicity estimate (e.g., Johnson & Apps 2009). TOI-1231's combination of V − K_s color (4.25) and absolute magnitude (M_Ks = 5.86) are consistent with it being 0.17 mag brighter than the locus for nearby M dwarfs, which Schlaufman & Laughlin (2010) estimate represents an isometallicity trend of [Fe/H] = −0.14. Using the calibrations of Johnson & Apps (2009) and Schlaufman & Laughlin (2010), this offset is consistent with a predicted metallicity of [Fe/H] = +0.05 and −0.03, respectively, i.e., approximately solar.

Gaia DR2 (Gaia Collaboration et al. 2018) has an estimate of [Fe/H] = −1.5, but for an unrealistic giant-like surface gravity of $\mathrm{log}g$ = 3.0 and hot T_eff of 4000 K. Taken at face value, the Gaia DR2 metallicity would predict that the star's V − K_s versus M_Ks position should be more than a magnitude below the main sequence (extrapolating the metallicity versus ΔM_Ks relations of Schlaufman & Laughlin 2010), well below where it is observed (∼0.2 mag above the MS).

Anders et al. (2019) uses the StarHorse code to fit G_B , G, G_R , J, H, Ks, W1, and W2 photometry to solve for a photometric metallicity estimate consistent with $[\mathrm{Fe}/{\rm{H}}]\,={0.095}_{-0.061}^{+0.028}$.

Taking the mean of our independent photometric metallicity estimates and that of Anders et al. (2019), we adopt a metallicity of [Fe/H] = +0.05 ± 0.08.

TOI-1231 was selected for transit detection observations by TESS from two input lists. It was included in the exoplanet candidate target list (CTL) that accompanied version 8 of the TESS Input Catalog (TIC; Stassun et al. 2019), and also in the Cool Dwarf Catalog (Muirhead et al. 2018). Its CTL observing priority was 0.00734, placing it among the top 3% of targets selected for transit detection by the mission, due to its brightness and small estimated stellar radius (see Sections 3.1 and 3.3 of Stassun et al. (2019) for details on the prioritization process). It was also selected for observations by TESS Guest Investigator proposal GO11180 (C. Dressing). TOI-1231 was observed by TESS from UT 2019 February 28 through 2019 March 26 as part of the Sector 9 campaign and again from UT 2019 March 26 through 2019 April 22 as part of Sector 10. The star fell on Camera 3 in both sectors, but shifted from CCD 1 in Sector 9 to CCD 2 in Sector 10.

The SPOC data for TOI-1231 can be accessed at the Mikulski Archive for Space Telescopes (MAST) website, ⁵⁸ and includes both the simple aperture photometry (SAP) flux measurements (Twicken et al. 2010; Morris et al. 2017) and the presearch data conditioned simple aperture photometry (PDCSAP) flux measurements (Smith et al. 2012; Stumpe et al. 2012, 2014). These data products differ in that the instrumental variations present in the SAP measurements are removed from the PDCSAP data. The main variations are due to thermal effects and strong scattered light present at the start of each orbit, which affect the systematic error removal in PDC (see TESS data release notes ⁵⁹ DRN16 and DRN17). We therefore use the quality flags provided by SPOC to mask out unreliable segments of the time series before executing the global fitting process in Section 4. We further detrend the TESS data by separating the individual spacecraft orbits (two in each sector) and fitting each orbit's flux measurements with a low order spline in order to mitigate residual trends in the photometry (Figure 2).

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3.1.1. TESS Transit Detection

We acquired ground-based time series follow-up photometry of TOI-1231 during the times of transit predicted by the TESS data. We used the TESS Transit Finder, which is a customized version of the Tapir software package (Jensen 2013), to schedule our transit observations.

3.2.1. LCO 1 m Observations

3.2.2. ASTEP 0.4 m Observations

3.2.3. ExTrA 0.6 m Observations

The ExTrA facility (Bonfils et al. 2015), located at La Silla observatory, consists of a near-infrared (0.85–1.55 μm) multiobject spectrograph fed by three 60 cm telescopes. At the focal plane of each telescope, 5 fiber positioners pick the light from the target and 4 comparison stars. We observed 2 full transits of TOI-1231 b on UTC 2020 March 18 and 2021 January 27 (Figure 3). The first night we observed with 3 telescopes using the fibers with 8'' apertures. The second night we observed with 2 telescopes and 4'' aperture fibers. Both nights we used the high-resolution mode of the spectrograph (R ∼ 200) and 60 s exposures. We also observed 2MASS J10283882-5234151, 2MASS J10285562-5220140, 2MASS J10273532-5206553, and 2MASS J10271569-5239330, with J magnitude (Skrutskie et al. 2006) and T_eff (Gaia Collaboration et al. 2018) similar to TOI-1231, for use as comparison stars. The resulting ExTrA data were analyzed using custom data reduction software.

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The ExTrA light curves are affected by systematic effects that are currently under investigation. To account for them, we modeled the transits observed by ExTrA with juliet (Kreidberg 2015; Espinoza et al. 2019; Speagle 2020), and included the quasi-periodic kernel Gaussian process implemented in celerite (Foreman-Mackey et al. 2017), with different kernel hyperparameters for each ExTrA telescope and for each night. We used a prior for the stellar density of ρ_⋆ = 6.48 ± 0.30 g cm⁻³ (Stassun et al. 2019), and non-informative priors for the rest of the parameters. The posterior provides the timings of the observed transits: 2458927.5716 ± 0.0013 BJD_TDB, and 2459242.7657 ±0.0013 BJD_TDB, a planet to star radius ratio of ${R}_{P}/{R}_{* }={0.0660}_{-0.0086}^{+0.0072}$, and an impact parameter of $b={0.201}_{-0.12}^{+0.091}$. These transit light curves are not used in the global EXOFATv2 fit described in Section 4 in order to avoid biasing the final results due to the systematics present in the ExTrA data. However, the timing of the second transit is used as a prior for the time of conjunction (T_C ) to better constrain the period of the planet.

Shortly after the discovery of the planet candidate by the TSTPC WG, we began radial velocity (RV) follow-up efforts using the Planet Finder Spectrograph (PFS) on Las Campanas Observatory's 6.5 m Magellan Clay telescope (Crane et al. 2006, 2008, 2010). PFS is an iodine cell-based precision RV spectrograph with an average resolution of R ≃ 130,000. RV values are measured by placing a cell of gaseous I₂, which has been scanned at a resolution of 1 million using the National Institute of Standards and Technology Fourier transform spectrometer (Nave 2017), in the converging beam of the telescope. This cell imprints the 5000–6200 Å region of the incoming stellar spectra with a dense forest of I₂ lines that act as a wavelength calibrator and provide a proxy for the point-spread function (PSF) of the spectrometer (Marcy & Butler 1992).

The spectra are split into 2 Å chunks, each of which is analyzed using the spectral synthesis technique described in Butler et al. (1996), which deconvolves the stellar spectrum from the I₂ absorption lines and produces an independent measure of the wavelength, instrument PSF, and Doppler shift. The final Doppler velocity from a given observation is the weighted mean of the velocities of all the individual chunks (∼800 for PFS). The final internal uncertainty of each velocity is the standard deviation of all 800 chunk velocities about that mean.

A total of 28 PFS radial observations were obtained from 2019 May to 2020 February, binned into 14 velocity measurements, with a mean internal uncertainty of 1.22 m s⁻¹(Table 2). A generalized Lomb–Scargle (GLS) periodogram of the PFS RV data shows a significant peak at 24 days (Figure 4), which matches the orbital period determined from the TESS data.

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To estimate how well the atmospheric properties could be extracted with HST, we used the open-source petitRADTRANS package (Mollière et al. 2019) to derive transmission spectra of TOI-1231 b, based on a simple atmospheric model. The atmosphere probed by the observations was assumed to be isothermal, at the equilibrium temperature derived for the planet in this work. Next, equilibrium chemistry was used to calculate the absorber abundances in the atmosphere, obtained with the chemistry model that is part of petitCODE (Mollière et al. 2017). We assumed two different compositional setups, 3 and 100× solar (Jupiter and Neptune-like, respectively), at a solar C/O. In addition, we introduced a gray cloud deck and modeled its effect on the spectrum when placing it between 100 and 10⁻⁶ bar, in 1 dex steps. The model with the highest cloud pressure was assumed to be our cloud-free model, because the atmosphere will become optically thick at lower pressures. We included the gas opacities of the following line absorbers: H₂O, CH₄, CO, CO₂, Na, and K. In addition to the gray cloud, continuum opacity sources arising from H₂, He, CO, H₂O, CH₄, and CO₂ Rayleigh scattering, as well as H₂–H₂ and H₂–He collision-induced absorption, were included. We refer the reader to Mollière et al. (2019) for the references used for the opacities.

We generated mock observations for all cases described above and retrieved them with petitRADTRANS, using the PyMultiNest package (Buchner et al. 2014). The latter uses the nested sampling implementation MultiNest (Feroz et al. 2009). The synthetic HST WCF3 observations were created assuming a wavelength range of 1.12–1.65 μm, with 12 points spaced equidistantly in wavelength space. We estimated the uncertainties on the spectroscopic transit depths using the Pandexo_HST tool. ⁶³ Assuming 4 HST transit observations, we expect uncertainties of 18 ppm on the transit in each spectral channel. This corresponds to ∼0.6 times the transit signal of the planet's scale height, when assuming a 100× solar composition. For these retrievals we placed special emphasis on the detectability of H₂O and CH₄, for which we implemented the method described in Benneke & Seager (2013): the abundances of all metal absorbers were retrieved freely, assuming vertically constant abundance profiles. The abundance of H₂ and He was found by requiring that the mass fractions of all species (metals + H₂ and He) add up to unity, with an abundance ratio of 3:1 between H₂ and He. Three retrievals were run for every synthetic observation: (i) nominal model, retrieving the abundances of all metal absorbers, as well as the cloud deck pressure; (ii) same as (i), but neglecting the CH₄ opacity and CH₄ abundance as a free parameter; (iii) same as (i), but neglecting the H₂O opacity and H₂O abundance as a free parameter.

For every synthetic observation we then derived three evidences for models (i), (ii), and (iii), using nested sampling retrievals. The Bayes factor B, which is the ratio of these evidences, then allows us to assess how strongly models including CH₄ or H₂O are preferred (Bayes factor of models (i) and (ii) or models (i) and (iii), respectively). We used a boundary value of B > 3 to express strong preference for a given model, following Kass & Raftery (1995).

We show the synthetic HST observation of the clear, 100× solar case in Figure 7. Because the 100× solar case has a larger mean molecular weight, it is the more challenging of the two enrichment cases. In addition to the synthetic observation, the 16–18 and 2–98 percentile envelopes of the retrieved transit depth distribution are shown for the retrievals with models (i), (ii), and (iii), that is, the full model and the model neglecting the CH₄ or H₂O opacity, respectively.

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For these cases, we find very strong preference for including CH₄ in the model (B = 10⁸) and no preference for including H₂O (H₂O and CH₄ have roughly equal abundances in the input model, but the CH4 opacity is larger than the H₂O opacity at all wavelengths). Clouds are a possibility for such planets (Crossfield & Kreidberg 2017) and we find that from P_cloud < 1 mbar on it becomes challenging to detect atmospheric features at all, and the 3 models (i), (ii), and (iii) become indistinguishable. In summary, we conclude that TOI-1231 b is an excellent target for atmospheric characterization. With a few transit observations, it will be possible to detect spectral features in an atmosphere similar to that of K2-18 b (Benneke et al. 2019b; Tsiaras et al. 2019), enabling the first comparative planetology in the temperature range 250–350 Kelvin.

With just 1 transit observed with JWST's Near Infrared Imager and Slitless Spectrograph (NIRISS), for a clear, solar composition atmosphere, we expect to detect TOI-1231 b's spectrum (dominated by water at NIRISS wavelengths) with 90σ significance (Kempton et al. 2018).

We also investigate prospects for a cloudy atmosphere, using PLATON (Zhang et al. 2019) to generate a solar composition model spectrum with a cloud deck pressure of 10 mbar (similar to GJ 3470b; Benneke et al. 2019a). We then used PandEXO (Batalha et al. 2017) to simulate a transmission spectrum for such an atmosphere. We find that even in this scenario, 1 transit with the NIRISS instrument would be sufficient to detect water absorption with 7.5σ significance. For reference, this is 2σ higher than the detection significance obtained with 6 HST WFC3 transits for GJ 3470b (Benneke et al. 2019a), a planet with a similar size but much lower density (and also orbiting a 0.5 R_⊙ star).

Given TOI-1231 b's low gravitational potential and expected X-ray and ultraviolet (XUV) instellation, we consider the likelihood that atmospheric escape is occurring and traceable with H i Lyman α (Lyα; 1216 Å) and the meta-stable He i line (10830 Å). TOI-1231 b's bulk density is similar to that of GJ 436 b (1.80 ± 0.29 g m⁻³; Maciejewski et al. 2014), a planet well known for its vigorously escaping atmosphere (Kulow et al. 2014; Ehrenreich et al. 2015). TOI-1231's fundamental stellar properties are very similar to GJ 436's, and our PFS spectra indicate that TOI-1231's log₁₀ ${R}_{\mathrm{HK}}^{{\prime} }=-5.06$ is nearly equivalent to GJ 436's (−5.09; Boro Saikia et al. 2018), indicating the same level of magnetic activity. Because ${R}_{\mathrm{HK}}^{{\prime} }$ is known to correlate well with UV emission (Youngblood et al. 2017), we assume GJ 436's synthetic X-ray and UV spectrum from Peacock et al. (2019) as a proxy for TOI-1231's. The integrated flux from 100 to 912 Å at 0.13 au is 172 erg cm⁻² s⁻¹ but could be as low as ∼53 erg cm⁻² s⁻¹, according to the estimates from the MUSCLES Treasury Survey (France et al. 2016; Loyd et al. 2016; Youngblood et al. 2016) based on Linsky et al. (2014). In the energy-limited approximation (Salz et al. 2016), the mass-loss rate scales inversely with the planet's bulk density and inversely with the square of the orbital distance. Thus, under this approximation we expect a mass-loss rate about 14 times lower for TOI-1231 b than for GJ 436 b.

To better understand the properties of the escaping atmosphere of TOI-1231 b, we constructed a 1D model of this planet's upper atmosphere that solves the hydrodynamics equations for an escaping atmosphere and considers photochemistry at pressures ≲1 μbar, and radial distances from the planet center r/R_P = 1–10. We assume that the planet's bulk composition is dominated by H₂ and He, and set volume mixing ratios at the 1 μbar boundary of 0.9 for H₂ and 0.1 for He. The model is one-dimensional and spherically symmetric, appropriate to the substellar line. See García Muñoz et al. (2020) and references within for more details about the method. The model simulations shown in Figure 8 are specific to the stellar spectrum from Peacock et al. (2019) and assume supersonic conditions at r/R_p ∼ 10. We take the stellar spectrum in its original format, ⁶⁴ correcting only for orbital distance. A one-dimensional approach is expected to give a good representation of the flow within a few planetary radii from the surface, but cannot capture the shape of the flow after its interaction with the stellar wind. In any case, our predictions provide helpful insight to understand the prevalent forms of hydrogen, and the range of temperatures and velocities expected in the vicinity of the planet.

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Figure 8 shows that H₂ remains abundant up to very high altitudes, and that the transition from H to H⁺ also occurs at high altitude, a condition favorable for Lyα transit spectroscopy. Unlike for typical hot Jupiters, ${{\rm{H}}}_{3}^{+}$ remains relatively abundant over an extended column and contributes to cooling of the atmosphere and to reducing the mass-loss rate. The model predicts that the planet is losing 2.3 × 10⁹ g s⁻¹ (integrated over a solid angle 4π).

We use the H i profile predicted by the model to estimate the Lyα transit depth attributable to hydrogen escaping the planet and before interacting with the stellar wind. This "cold" component is typically hidden by interstellar medium (ISM) absorption and has so far remained undetected. The Lyα absorption reported in other systems including GJ 436 b (e.g., Ehrenreich et al. 2015) is attributed to a "hot" component that results from charge exchange of the hydrogen atoms from the planet and the stellar wind protons (Khodachenko et al.2019).

Despite the smaller expected escape rate with respect to other planets, TOI-1231 b's exosphere may be observable during a H i Lyα (1215.67 Å) transit. The host star's radial velocity (+70.5 km s⁻¹) Doppler shifts the entire system partially out of the bulk of ISM's H i attenuation region, allowing access to the core of the Lyα line and therefore to the "cold" component.

To assess the utility of a Lyα transit with HST for studying this planet, we estimate the profiles of the stellar Lyα emission, ISM attenuation, and planetary absorption (Figure 9). We use the reconstructed Lyα profile of GJ 436 (Youngblood et al. 2016) rescaled to match TOI-1231's distance. For the ISM, we assume a velocity centroid for the H i absorbers of −4.3 km s⁻¹ based on a kinematic model for the local ISM (Redfield & Linsky 2008) and a conservative H i column density log₁₀ N(HI) = 18.6 based on measured column densities of nearby sight lines (Wood et al. 2005; Youngblood et al. 2016; Waalkes et al. 2019). Using STScI's online exposure time calculator for STIS with the G140M, 1222 central wavelength, and 52'' × 02 slit, we find that the expected planetary absorption around the Lyα line core (the "cold" component) could be detected at high confidence in a single transit.

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We have not modeled the expected transit signature for meta-stable He i, but note the potential for exploring this planet's upper atmosphere with He i transits. However, as noted above, TOI-1231's XUV spectrum, which is responsible for both driving escape and populating the meta-stable He i state, is likely very similar to GJ 436's, and GJ 436 b's vigorously escaping atmosphere was not detected in He i (Nortmann et al. 2018). TOI-1231 b's larger orbit (a = 0.1266 au) further lowers its likelihood to be a promising He i target, as the farthest planet with a confirmed He i detection thus far is WASP 107 b, which has a semimajor axis of only 0.055 au and orbits a more favorable (to He i excitation) K6V star (Spake et al. 2018). See Oklopčić & Hirata (2018) and Oklopčić (2019) for more details on the prospects tracing atmospheric escape with meta-stable He i.