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Home All issues Volume 666 (October 2022) A&A, 666 (2022) A10 Full HTML
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A&A
Volume 666, October 2022
Article Number A10
Number of page(s) 13
Section Planets and planetary systems
DOI https://doi.org/10.1051/0004-6361/202243731
Published online 29 September 2022

© E. Esparza-Borges et al. 2022

Licence Creative CommonsOpen Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

This article is published in open access under the Subscribe-to-Open model. Subscribe to A&A to support open access publication.

1 Introduction

Since the beginning of its observations in 2018, the Transiting Exoplanet Survey Satellite (TESS, Ricker et al. 2015) has provided nearly 5500 objects of interest and 204 confirmed planets1. Although many of these objects of interest may be consistent with a planetary-like transit signal, not all of them have a planetary nature. Several astrophysical objects – such as a brown dwarf or a low-mass star transiting a binary companion, a grazing binary stellar system, or a pair of blended binaries – are able to mimic the signal of a planetary transit. Although these systems would produce deep eclipses, we could be observing their photometry diluted by a bright neighbor star (Cameron 2012; Ciardi et al. 2015). Therefore, the nature of each object of interest needs to be determined by ground-based follow-up observations, which play a supporting role by being able to confirm whether the candidate is a planet or not.

Moreover, although the most reliable method to confirm a planet candidate is the mass determination through radial velocity (RV) measurements, this procedure is very difficult for those candidates orbiting a faint, active, or fast rotating star. Hence, to determine the nature of these candidates it is necessary to use other methods to validate them as planets.

In this context, ground-based multicolor transit photometry is a useful method to validate planet candidates (Drake 2003; Tingley 2004; Parviainen et al. 2019, 2020, 2021; Fukui et al. 2022). It allows us to account for the light contamination from unresolved sources and estimate the uncontaminated radius ratio of the transiting candidate. With an estimate of the stellar radius, combined with the uncontaminated radius ratio, the radius of the candidate can be obtained. Consequently, if the radius of the candidate is significantly below the theoretical radius limit of a brown dwarf, the candidate can be validated as a planet (Parviainen et al. 2020).

Here, we used the approach of Parviainen et al. (2020) to validate the substellar nature of two TESS objects of interest orbiting faint (V = 13.369 mag and V = 15.180 mag) M dwarfs: the super-Earth-sized TOI-2081b and the sub-Neptune-sized TOI-4479b. We find TOI-4479b to be a rare target that lies on the Neptune desert.

Our analysis is performed over space-based TESS photometry, ground-based datasets of multicolor photometry in the g, r, i, and zs bands obtained through MuSCAT2 and MuS-CAT3 multicolor imagers, and complementary ground-based single-passband photometry from the SINISTRO camera. We also obtained low-resolution optical spectra for the stellar characterization. In addition, high angular resolution observations are used to visually discard a binary stellar companion.

In Sects. 2 and 3, we describe the observations used in our study. In Sect. 5, we explain the methodology followed for the light-curve analysis and the validation procedure. In Sect. 6, we present and discuss our results, which confirm the planetary nature of TOI-2081b and TOI-4479b. Finally, we conclude our study in Sect. 7.

2 TESS photometry

TOI-4479 (TIC 126606859) was observed with two-minute cadence by TESS (Ricker et al. 2015) in Sector 41 over 29 days (from UTC 2021 July 23 to UTC 2021 August 20) during Cycle 4, obtaining 22 full transits in total. Full frame image (FFI) observations are also available in Sector 15, but they have not been used here. In this case, a transit signal with a 1.159 day orbital period and S/N = 9.5 was identified in the TESS Science Processing Operations Center (SPOC, Jenkins et al. 2016) transiting planet search (Jenkins 2002; Jenkins et al. 2010, 2020) of the two-minute data from Sector 41. The threshold crossing event was promoted to TESS Object of Interest (TOI) planet candidate status and designated TOI 4479.01 (Guerrero et al. 2021) based on a SPOC data validation report (Twicken et al. 2018; Li et al. 2019) showing clean transiting planet model fit and diagnostic test results.

TOI-2081 (TIC 321669174) was observed by TESS with two-minute cadence in Sectors 14, 17, 20, 21, 24, 25, 26, 40, 41, and 47 (a total duration of 196 days during Cycle 2 and 59 days during Cycle 4), obtaining 22 full transits in total. A transit signal with a 10.504 day orbital period and S/N = 9.0 was identified in the TESS SPOC of the combined two-minute data from Sectors 14, 17, 20, and 21. The threshold crossing event was promoted to TOI planet candidate status and designated TOI 2081.01.

The TESS images around the position of TOI-4479 and TOI-2081 in Sector 41 are shown in Fig. 1. The TESS images in the rest of sectors where TOI-2081 was observed are shown in Fig. A.1.

3 Ground-based follow-up observations

3.1 MuSCAT2 photometry

We observed a full transit of TOI-4479b on UTC 2021 October 17 and a full transit of TOI-2081b on UTC 2020 July 22 with the MuSCAT2 instrument (Narita et al. 2019) mounted on the Telescopio Carlos Sánchez (TCS) at the Teide Observatory, Spain. MuSCAT2 is a multicolor imager capable of performing simultaneous photometry in the g, r, i, and zs photometric bands using four independent CCDs. The exposure times used for TOI-4479 and TOI-2081 observations were set independently for each CCD (g, r, i, zs): (15, 30, 25, 20) and (10, 25, 25, 15) s, respectively. As the objects are red, the exposure times were shorter in redder filters to avoid saturation. The g filter is an exception; since the flux there is small, we set a short integration time and used this channel’s images to auto-guide the instrument, for which we want an exposure time typically <15s.

A dedicated MuSCAT2 pipeline, described by Parviainen et al. (2020), was used to perform the data reduction and to extract the photometry. The pipeline performs aperture photometry for a set of comparison stars and aperture sizes (see Fig. 2). The final relative light curves are obtained through the global optimization of a model, which aims to find the optimal comparison stars and aperture sizes, while the transit and baseline variations are simultaneously modeled using a linear combination of covariates.

thumbnail Fig. 1

TESS target pixel file images of TOI-4479 (top) and TOI-2081 (bottom) observed in Sector 41. The red circles show the sources in the field identified by the Gaia DR2 catalogue (Gaia Collaboration 2018) with scaled magnitudes. The position of the targets are indicated by white crosses and the mosaic of orange squares show the mask used by the pipeline to extract photometry. These plots were made using tpfplotter (Aller et al. 2020).

3.2 MuSCAT3 photometry

A full transit of TOI-4479b was observed simultaneously in Sloan g, r, i, and Pan-STARRS z-short bands on UTC 2021 October 21 using the Las Cumbres Observatory Global Telescope (LCOGT; Brown et al. 2013) 2 m Faulkes Telescope North at Haleakala Observatory on Maui, Hawai’i. The telescope is equipped with the MuSCAT3 multiband imager (Narita et al. 2020). We used the TESS Transit Finder to schedule our transit observations. The images were calibrated using the standard LCOGT BANZAI pipeline, and the differential photometric data were extracted in each band using AstroImageJ (Collins et al. 2017) with circular apertures with a radius of 2.″7. The apertures exclude virtually all of the flux from the nearest Gaia EDR3 (Gaia Collaboration 2021) neighbor (TIC 1951208113) 8″.7 east of the target. The transit was detected on-target in all four filter bands.

thumbnail Fig. 2

MuSCAT2 field in the r band during the TOI-4479 (left) and TOI-2081 (right) observations. The cross indicates the position of the target, and the dotted circle marks the 2.5′-radius region centered around it.

3.3 LCOGT 1 m photometry

We observed a full transit of TOI-4479b from the Las Cumbres Observatory Global Telescope LCOGT 1.0 m network on UTC 2021 October 11 in Sloan i′ band. We used the TESS Transit Finder (Jensen 2013) to schedule our transit observations. The 1 m telescopes are equipped with 4096 × 4096 SINISTRO cameras having an image scale of 0″.389 per pixel, resulting in a 26′ × 26′ field of view. The images were calibrated by the standard LCOGT BANZAI pipeline (McCully et al. 2018). The differential photometric data were extracted using AstroImageJ (Collins et al. 2017) with circular photometric apertures having radius 4″.7. The target star aperture excludes most of the flux of the nearest Gaia EDR3 and TESS Input Catalog neighbor (TIC 1951208113) 8″.7 east of the target. The transit was detected on-target.

3.4 High-resolution imaging of TOI-2081

We observed TOI-2081 on UTC 2021 March 29 using the ShARCS camera on the Shane 3-meter telescope at the Lick Observatory (Kupke et al. 2012; Gavel et al. 2014; McGurk et al. 2014). Observations were taken with the Shane adaptive optics system in natural guide star mode in order to search for nearby, unresolved stellar companions. We collected one sequence of observations using a Ks filter (λ0 = 2.150 µm, ∆λ = 0.320 µm) and reduced the data using the publicly available SImMER pipeline (Savel et al. 2020)2. Our reduced images and corresponding contrast curves are shown in the top panel of Fig. 3. We find no nearby stellar companions within our detection limits.

We also observed TOI-2081 on UTC 2021 March 03 with the Speckle Polarimeter (SPP, Safonov et al. 2017) on the 2.5 m telescope at the Caucasian Observatory of the Sternberg Astronomical Institute (SAI) of Lomonosov Moscow State University. SPP uses the electron multiplying CCD Andor iXon 897 as a detector. The atmospheric dispersion compensator allowed observations of this relatively faint target through the wideband Ic filter. The power spectrum was estimated from 4000 frames with 30 ms exposure. The detector has a pixel scale of 20.6 mas pixel−1, and the angular resolution was 89 mas. We did not detect any stellar companions brighter than ∆IC = 2.4 and 3.1 at ρ = 0″.25 and 1″.0, respectively, where p is the separation between the source and the potential companion.

Nearby faint companions of the host star may remain undetected through seeing-limited photometry, but could contribute to a contamination of the flux of the target and lead to incorrect estimations of the planetary radius. However, the Shane-AO and SAI-Speckle (Fig. 3) observations allow us to rule out this scenario for TOI-2081 in the Ks and I bands. We do not detect any nearby visual companion, and TOI-2081 seems to be an isolated star in the data.

3.5 ALFOSC spectroscopy

TOI-2081 and TOI-4479 were spectroscopically observed with the Alhambra Faint Object Spectrograph and Camera (ALFOSC), mounted on the Cassegrain focus of the 2.5-m Nordic Optical Telescope (NOT) on the Observatorio del Roque de los Muchachos (La Palma, Spain), on 2022 April 8 and 10 UT, respectively. ALFOSC is equipped with a monolithic 2048 × 2048 E2V detector that has a pixel size of 0″.2138 on the sky. On both nights, we used a long slit with a slit width of 170 and the grating number 5. This instrumental configuration yields low-resolution spectra covering the optical wavelength interval from 500 through 1050 nm with a nominal dispersion of 3.38 Å pixel−1 (or resolving power R = 610 at 725 nm). However, fringing is strong (≥4%) redwards of ≈900 nm; therefore, we discarded all data at longer wavelengths. Two exposures of 900 s each were acquired for TOI-2081 and TOI-4479 at an air mass of 1.10 and 1.58, respectively. Together with the main targets, we also observed the spectroscopic standard star BD+26 2606 at different air masses for a proper correction of the instrumental response and telluric absorption. Three exposures of 15 s each were acquired on each night for the stardard star. BD+26 2606 is an early-type star with known fluxes published in Oke (1990).

All observations were acquired at a parallactic angle to minimize light losses on the slit. We windowed the ALFOSC detector along the spatial axis (perpendicularly to the dispersion axis) to a size of 500 pixels.

The ALFOSC spectra of the targets and the standard star were reduced and optimally extracted following standard steps within the IRAF environment (Tody 1993). First, we removed the detector bias at the same time we subtracted “the sky” contribution using the region on both sides of the spectral trace of the stars. The spectra were calibrated in wavelength with a precision of about 1.5 Å using observations of He+Ne lamps acquired immediately after observing the main targets. The ALFOSC spectra of TOI-2081 and TOI-4479 were corrected for instrumental response using the observations of the standard star. The final step was to mask out the hydrogen lines intrinsic to the standard star and to normalize its spectra to the continuum for division with the target’s spectrum (in this way, we removed the Earth’s telluric lines from the spectrum). The telluric-free spectra of TOI-2081 and TOI-4479 are shown in Fig. 4.

thumbnail Fig. 3

High-resolution imaging of TOI-2081. Top: adaptive optics images of TOI-2081 taken with the ShARCS camera on the Shane 3-meter telescope at the Lick Observatory. For each image, we also present a contrast curve generated by calculating the median values (solid lines) and root-mean-square errors (blue, shaded regions) in annuli centered on each target, where the bin width of each annulus is equal to the full width at half maximum of the point spread function. The observations rule out the presence of a possible nearby contaminant companion. Bottom: high angular resolution speckle imaging of TOI-2081 in I filter using SAI-2.5 m telescope. The observation rules out the presence of a possible nearby contaminant companion. The structure in this contrast curve is due to the first derivative discontinuity in the frequency mask that is used to compute the autocorrelation. This discontinuity generates rings at characteristic distances from the center, which then have a minor effect on the detection limit.
thumbnail Fig. 4

ALFOSC spectra (red) of TOI-4479 (top panel) and TOI-2081 (bottom panel) are shown in comparison with solar metallicity templates of known spectral type from the library of Kesseli et al. (2017).

4 Stellar parameters

In Fig. 4, the ALFOSC spectra are compared to spectral standard stars from the library of empirical stellar spectra of Kesseli et al. (2017). Our data are characterized by the presence of TiO absorption over the optical wavelengths, which is a signpost of M spectral classification. Using templates of solar metallicity, we derived the following spectral types: M1.0 ± 0.5 (TOI-2081) and M3.0 ± 0.5 (TOI-4479). The ALFOSC spectra are well reproduced by the templates with no significant deviations. Hα is not in emission in any of the two dwarfs. Template spectra from Kesseli et al. (2017) are binned by metallicity from −2.0dex through +1.0 dex, and are separated into main-sequence (dwarf) stars and giant stars. We also compared the ALFOSC data to the sets of different metallicities and gravities, finding that TOI-2081 and TOI-4479 are better described by high-gravity surfaces and atmospheric metallicities of [Fe/H] ≥ −0.5 and ≥0.0 dex, respectively, thus supporting the solar-to-metal-rich nature of both stars. At low resolution, we cannot better constrain the metallicity or surface gravity of the targets.

To derive the stellar mass and radius, we first built the stars’ spectral energy distribution (SED) by combining the ALFOSC data and all available broad-band photometry from Gaia DR3 (Gaia Collaboration 2021), the JHK magnitudes from 2MASS (Skrutskie et al. 2006), and the W1 – W4 magnitudes from WISE (Wright et al. 2010). All these data were converted into the absolute fluxes by using the Gaia trigonometric distances. The extension of the SEDs toward bluer and redder wavelengths was done with the BT-Settl model (Allard et al. 2012) that best reproduces the observations. The integration of the SEDs yields the bolometric luminosities provided in Table 1. We then employed the bolometric luminosities and the mass-radius-luminosity relations of Cifuentes et al. (2020) to derive the TOI-2081 and TOI-4479 stellar parameters, which are listed in Table 1 and was used in our analysis of the planetary systems.

Table 1

TOI-4479 and TOI-2081 identifiers, coordinates, magnitudes, and stellar parameters.

5 Light-curve analysis

5.1 Multicolor transit analysis

The TESS3 and ground-based light curves were analyzed individually and jointly following the procedure described in Parviainen et al. (2019, 2020, 2021), which performs an exoplanet-orientated Bayesian parameter estimation (Parviainen 2018). The multicolor analysis procedure can be summarized in the following steps. In the first step a flux model is generated to fit the light curves, accounting for both the transit signal and the systematic effects present in the time series. In the second step a noise model is defined to account for the stochastic variability in the data. In the third step, The likelihood is obtained combining the flux model, the noise model and the observations. Finally, a Markov chain Monte Carlo (MCMC) sampling is performed to obtain the joint parameter posterior distribution based on the priors defined from the model parameters.

The pipeline used to perform the multicolor analysis makes use of PHOENIX (Husser et al. 2013) for physics-based contamination modeling; LDTk (Parviainen & Aigrain 2015) for limb-darkening estimations; PyTRANSIT (Parviainen 2015), which provides flux and noise models; and emcee (Foreman-Mackey et al. 2013) to perform the MCMC sampling.

We applied the multicolor analysis to the complete set of photometric data of both targets. The TOI-4479 dataset is composed of 31 transits (22 TESS transits, one MuSCAT2 transit observed simultaneously in g, r, i, and zs bands, one MuSCAT3 transit observed simultaneously in gp, Rp, ip, and zs bands, and one SINISTRO transit in the ip band). The TOI-2081 dataset is composed of 26 transits in total (22 TESS transits and one MuS-CAT2 transit observed simultaneously in g, r, i, zs bands). In our calculations, we adopted the values for the stellar parameters shown in Table 1, which were derived from the analysis of ALFOSC spectra.

5.2 Contamination analysis

The possible multiplicity of the system and the subsequent presence of an unresolved companion entails a flux contamination of the host star that may affect the observed transit depth and lead to erroneous parameters of the planetary system (Daemgen et al. 2009). Single-passband photometry is not able to constrain such contamination because of the degeneracy with orbital geometry, limb darkening and planet-to-star radius ratio. Nevertheless, as described in Parviainen et al. (2019), some effects of the flux contamination are color-dependent (Rosenblatt 1971; Drake 2003; Tingley 2004), making multicolor photometry a valuable tool to constrain the degree of contamination and estimate the true planet-to-star radius ratio. On the one hand, color differences between the host star and the companion may lead to significant variations in the transit depth in different passbands. On the other hand, the transiting object produces a color-dependent signal, leaving a distinctive signature that relies either on the radius of the transiting object and the nature of the transiting object. Both effects allow us to determine whether a transiting planet candidate is actually a planet or, conversely, a mimicked signal by flux contamination.

Our multicolor transit analysis pipeline accounts for the effects of flux contamination and estimates, among the system parameters, the posterior distributions of the apparent radius ratio, the true radius ratio (free of the contribution of the contaminant), and the effective temperature of the possible contaminant companion. By analyzing these three parameters, we are able to evaluate the flux contamination and to validate the nature of the companion.

thumbnail Fig. 5

MuSCAT2 (top) and MUSCAT3 (bottom) detrended light curves of TOI-4479b in g, r, i, and zs passbands. The dots show the MuSCAT2 and MUSCAT3 relative flux and the lines show the best light-curve model from the MuSCAT2 pipeline for each band.
thumbnail Fig. 6

MuSCAT2 detrended light curves of TOI-2081b in g, r, i, and zs passbands. The dots show the MuSCAT2 relative flux and the lines show the best light-curve model.
thumbnail Fig. 7

TESS PDCSAP folded relative flux from the combination of all the transits and best light-curve model of TOI-4479b (left) and TOI-2081b (right).

6 Results and discussion

In this section, we validate the planetary nature and discuss the properties of TOI-2081b and TOI-4479b. We evaluate the possible flux contamination in both systems by studying the posterior distributions of the true and apparent planet-to-star radius ratio (ktrue, kapp) and the difference in effective temperature between the possible contaminant and the host star (∆Teff) parameters.

6.1 TOI-4479

We show the phase-folded combined MuSCAT2, TESS, and SINISTRO photometric datasets of TOI-4479b with the best light-curve models in Figs. 5, 7, and 8, respectively. Also, in Table 2 we show the derived stellar and planetary parameters from our multicolor validation pipeline. The corner plot showing the parameter posterior distributions can be found in Fig. A.2.

To evaluate the possible flux contamination, in Fig. 9 we show the posterior distribution of the true radius ratio (ktrue) as a function of the difference in effective temperature between the contaminant and the host star (∆Teff), the apparent radius ratio (kapp), the impact parameter, and the stellar density. We also show a comparison among the posterior distributions of the apparent and true radius ratio as well as the effective temperatures of the host star and contaminant for the TOI-4479 system in Fig. 10.

For TOI-4479, we found ktrue to be close in value to kapp (Fig. 9), implying a very low degree of flux contamination from a possible companion. Thus, considering the contamination negligible, we derived the size of the companion from the kapp, leading to a R⊕ sized object. Moreover, the posterior distribution of ∆Teff is centered around 0, meaning that the effective temperature of the possible contaminant would be the same as that of the host star, and the posterior distribution of the impact parameter implies a non-grazing transit. Thus, we can validate TOI-4479b as a sub-Neptune-sized planet orbiting an M dwarf with a period of days.

In Fig. 11, we compare TOI-4479b with the sample of confirmed planets around M dwarfs known to date in the periodradius plane. We also compare TOI-4479b with the entire population of confirmed planets with a radius uncertainty below 10%, showing that TOI-4479b lies in an underpopulated region of the period-radius plane known as the Neptune desert. This deserted region contrasts with the highly populated regions of hot-Jupiters (Rp > 10 R⊕) and ultra-short-period (USP) rocky planets (Rp < 2 R⊕) located above and below the Neptune desert, respectively. In Fig. 11, we plot the lower and upper boundaries among these regions as derived by Mazeh et al. (2016) with dashed-dotted lines. The dearth of short-period Neptune-sized planets has been widely studied in the literature and several formation mechanisms (e.g., photo-evaporation, high-eccentricity migration, and in situ formation) have been considered to explain the causes of the Neptune desert region (Sanchis-Ojeda et al. 2014; Mazeh et al. 2016; Lundkvist et al. 2016; Lopez 2017; Owen & Wu 2017; Owen & Lai 2018).

Although TOI-4479b has a slightly longer period that the common definition of USP planets (Porb < 1 day), given their shared properties we associate it here to this population. TOI-4479b joins a small group of known USP intermediate-sized planets inhabiting the Neptune desert, namely LTT 9779b (Jenkins et al. 2020), LP 714-47 b (Dreizler et al. 2020), HATS-37Ab (Jordán et al. 2020), HATS-38b (Jordán et al. 2020), TOI-824 b (Burt et al. 2020), TOI-849 b (Armstrong et al. 2020), TOI-132 b (Díaz et al. 2020), and TOI-674 b (Murgas et al. 2021). The existence of such uncommon planets has been interpreted as a consequence of the photo-evaporation produced over short timescales by the strong stellar irradiation experienced by some low-mass planets, which are unable to retain the H/He envelope (Jenkins et al. 2020). However, TOI-4479b is among the biggest USP planets in the desert, meaning that it still retains an appreciable fraction of its volatile envelope in an early stage of the stripping process. We find TOI-4479b to be the biggest USP planet orbiting an M dwarf known to date. We used Forecaster4 (Chen & Kipping 2017) to predict the plausible mass of TOI-4479b, which is Mp = M⊕. We note that the error bars on the mass are dominated by the intrinsic spread of the mass-radius distribution for Neptunian planets relatively to the simple power-law relation. We evaluated the prospects to spectroscopically investigate the atmosphere of TOI-4479b by computing the transmission spectroscopy metric (TSM), as defined by Kempton et al. (2018). The tSm is inversely proportional to the planetary mass, and using the forecasted mass range the TSM ranges between 26 and 198 with a peak at TSM ~ 75. According to Kempton et al. (2018), Neptune-sized planets with TSM ≳ 90 are the highest-priority candidates for transmission spectroscopy with the JWST, where the ranking is based on the predicted signal-to-noise (S/N) of atmospheric detections. However, individual planets with TSM < 90 can still be extremely suitable candidates for transmission spectroscopy with the JWST, based on scientific merit. This applies to TOI-4479b due to the fact that it orbits an M star and is located in the Neptune desert.

thumbnail Fig. 8

SINISTRO detrended light-curve of TOI-4479b in the ip pass-band. The dots show the SINISTRO relative flux and the line shows the best light-curve model.
Table 2

Planetary parameters of TOI-2081b and TOI-4479b derived through the multicolor validation pipeline.

6.2 TOI-2081

We show the phase-folded combined MuSCAT2 and TESS photometric datasets of TOI-2081b together with the best light-curve models in Figs. 6 and 7, respectively. Also, in Table 2 we show the derived stellar and planetary parameters from our multicolor validation pipeline. The corner-plot showing the parameter posterior distributions can be found in Fig. A.3.

To evaluate the possible flux contamination, in Fig. 9 we show the posterior distribution ktrue as a function of the difference in ∆Teff, kapp, the impact parameter and the stellar density. The comparison among the posterior distributions of the apparent and true radius ratio as well as the effective temperatures of the host star and contaminant for TOI-2081 system is shown in Fig. 10.

For TOI-2081, we found ktrue to be close in value to kapp (Fig. 9), implying a very low degree of flux contamination from the companion. Considering the contamination negligible, we derived the size of the companion from the kapp, leading to a R⊕ sized object. Thus, we can validate TOI-2081b as a super-Earth-sized planet orbiting an M dwarf with a period of days.

TOI-208lb is also included in the radius-period diagram in Fig. 11. We find TOI-2081b to be a temperate super-Earth, in a well-populated parameter space region both around M dwarfs and earlier stellar-type hosts.

TOI-2081b (a ~ 0.07 AU) is orbiting within the inner edge of the habitable zone of its star (we find a conservative habitable zone of [0.16 ± 0.02, 0.34 ± 0.04] AU) and in a tidally locked regime. Assuming a zero Albedo and a cloud-free atmosphere without greenhouse gases, the temperature of the day side is estimated at around Tday ≈ 680K. Using Forecaster, we estimated a mass of Mp = M⊕. The corresponding TSM ranges between 14 and 89 with peak at TSM ~ 35.

thumbnail Fig. 9

From left to right, posterior distributions of ktrue versus ∆Teff, kapp, the impact parameter, and the stellar density from the joint multicolor light-curve analysis, for TOI-4479 (top four panels) and TOI-2081 (bottom four panels).
thumbnail Fig. 10

Comparison among the posterior distributions of the apparent and true radius ratio and the effective temperatures of the host star and contaminant. Upper panels: posterior distributions of the true radius ratio (ktrue) and the apparent radius ratio (kapp) for TOI-2081 (left) and TOI-4479 systems (right). Lower panels: posterior distributions of the effective temperature of the host star (Teff, h) and the effective temperature of the contaminant (Teff, c) for TOI-2081 (left) and TOI-4479 systems (right). The solid lines show the median of each distribution and the dashed lines show the lower and upper 1σ.
thumbnail Fig. 11

TOI-4479b and TOI-2081b in the period-radius diagram compared to all the confirmed planets to date with a radius uncertainty below 10%. The dark blue dots show the planets around M-type stars and the dashed-dotted lines show the Neptune desert boundaries in the period-radius plane derived by Mazeh et al. (2016).

7 Conclusions

By using multicolor photometric observations with MuSCAT2, MuSCAT3, and LCOGT 1m, we determined that the degree of contamination by a possible nearby contaminant is negligible in both the TOI-4479b and the TOI-2081b systems, and validated their planetary nature. TOI-4479b is a sub-Neptune-sized planet (Rp = R⊕) and TOI-2081b is a super-Earth-sized planet (Rp = R⊕). Both planets orbit around M dwarf host stars with orbital periods of 10.50534 ± 0.00007 days and days, respectively.

We also found that the TOI-4479b lies in the Neptune desert and joins a small sample of (~8) short-period, intermediate-sized planets, with TOI-4479b being the biggest USP planet orbiting an M dwarf known to date. Thus, this planet is an interesting target for future radial velocity observations (which will require very large telescope apertures) and atmospheric studies, as its full characterization may provide significant observational constraints for planet formation and evolution theories.

Acknowledgements

We thank the anonymous referee for insightful suggestions, which added the clarity of this paper. Funding for the TESS mission is provided by NASA’s Science Mission Directorate. We acknowledge the use of public TESS data from pipelines at the TESS Science Office and at the TESS Science Processing Operations Center. Resources supporting this work were provided by the NASA High-End Computing (HEC) Program through the NASA Advanced Supercomputing (NAS) Division at Ames Research Center for the production of the SPOC data products. We acknowledge the use of public TESS data from pipelines at the TESS Science Office and at the TESS Science Processing Operations Center. This research has made use of the Exoplanet Follow-up Observation Program website, which is operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program. This paper includes data collected by the TESS mission that are publicly available from the Mikulski Archive for Space Telescopes (MAST). This paper is based on observations made with the MuS-CAT2 instrument, developed by ABC, at Telescopio Carlos Sanchez operated on the island of Tenerife by the IAC in the Spanish Observatorio del Teide. This paper is based on observations made with the MuSCAT3 instrument, developed by the Astrobiology Center and under financial supports by JSPS KAKENHI (JP18H05439) and JST PRESTO (JPMJPR1775), at Faulkes Telescope North on Maui, HI, operated by the Las Cumbres Observatory. This work makes use of observations from the LCOGT network. Part of the LCOGT telescope time was granted by NOIRLab through the Mid-Scale Innovations Program (MSIP). MSIP is funded by NSF. Based on observations made with the Nordic Optical Telescope, owned in collaboration by the University of Turku and Aarhus University, and operated jointly by Aarhus University, the University of Turku and the University of Oslo, representing Denmark, Finland and Norway, the University of Iceland and Stockholm University at the Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de Astrofísica de Canarias. The data presented here were obtained in part with ALFOSC, which is provided by the Instituto de Astrofísica de Andalucia (IAA) under a joint agreement with the University of Copenhagen and NOT. This work made use of tpfplotter by J. Lillo-Box (publicly available in www.github.com/jlillo/tpfplotter), which also made use of the python packages astropy, lightkurve, matplotlib and numpy. E.E.-B. acknowledges financial support from the European Union and the State Agency of Investigation of the Spanish Ministry of Science and Innovation (MICINN) under the grant PRE2020-093107 of the Pre-Doc Program for the Training of Doctors (FPI-SO) through FSE funds. G.M. has received funding from the European Union s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 895525. C.D.D. acknowledges support provided by the NASA Exoplanets Research Program (XRP) via grant 80NSSC20K0250. J.K. gratefully acknowledge the support of the Swedish National Space Agency (SNSA; DNR 2020-00104). R.L. acknowledges financial support from the Spanish Ministerio de Ciencia e Innovación, through project PID2019-109522GB-C52, and the Centre of Excellence “Severo Ochoa” award to the Instituto de Astrofísica de Andalucía (SEV-2017-0709). A.A.B. and M.V.G. acknowledge the support of Ministry of Science and Higher Education of the Russian Federation under the grant 075-15-2020-780(N13.1902.21.0039). M.T. is supported by JSPS KAK-ENHI grant Nos.18H05442, 15H02063, and 22000005. M.R.Z.O. acknowledges financial support from the Spanish Ministerio de Ciencia e Innovación through project PID2019-109522GB-C51. This work is partly supported by JSPS KAK-ENHI Grant Numbers 22000005, JP15H02063, JP17H04574, JP18H05439, P18H05442, JP20K14518, JP20K14521, JP21K13975, JP21K20376, Grant-in-Aid for JSPS Fellows Grant Number JP20J21872, JST CREST Grant Number JPMJCR1761, and the Astrobiology Center of National Institutes of Natural Sciences (NINS) (Grant Numbers AB022006, AB031010, AB031014).

Appendix A Additional plots

thumbnail Fig. A.1

TESS target pixel file images of TOI-2081 observed in Sectors 14, 17, 20, 21, 24, 25, 26, 40, 41, and 47. The red circles show the sources in the field identified by the Gaia DR2 catalogue with scaled magnitudes. The position of the targets is indicated by white crosses and the mosaic of orange squares show the mask used by the pipeline to extract photometry. These plots were made using tpfplotter (Aller et al. 2020).
thumbnail Fig. A.2

Corner plot of the posterior distributions obtained through the multicolor validation pipeline for TOI-4479.
thumbnail Fig. A.3

Corner plot of the posterior distributions obtained through the multicolor validation pipeline for TOI-2081.

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1

From the NASA Exoplanet Archive.

3

The TESS SPOC Presearch Data Conditioning Simple Aperture Photometry (PDCDAP; Stumpe et al. 2012, 2014; Smith et al. 2012) light curves.

All Tables

Table 1

TOI-4479 and TOI-2081 identifiers, coordinates, magnitudes, and stellar parameters.

In the text
Table 2

Planetary parameters of TOI-2081b and TOI-4479b derived through the multicolor validation pipeline.

In the text

All Figures

thumbnail Fig. 1

TESS target pixel file images of TOI-4479 (top) and TOI-2081 (bottom) observed in Sector 41. The red circles show the sources in the field identified by the Gaia DR2 catalogue (Gaia Collaboration 2018) with scaled magnitudes. The position of the targets are indicated by white crosses and the mosaic of orange squares show the mask used by the pipeline to extract photometry. These plots were made using tpfplotter (Aller et al. 2020).
In the text
thumbnail Fig. 2

MuSCAT2 field in the r band during the TOI-4479 (left) and TOI-2081 (right) observations. The cross indicates the position of the target, and the dotted circle marks the 2.5′-radius region centered around it.
In the text
thumbnail Fig. 3

High-resolution imaging of TOI-2081. Top: adaptive optics images of TOI-2081 taken with the ShARCS camera on the Shane 3-meter telescope at the Lick Observatory. For each image, we also present a contrast curve generated by calculating the median values (solid lines) and root-mean-square errors (blue, shaded regions) in annuli centered on each target, where the bin width of each annulus is equal to the full width at half maximum of the point spread function. The observations rule out the presence of a possible nearby contaminant companion. Bottom: high angular resolution speckle imaging of TOI-2081 in I filter using SAI-2.5 m telescope. The observation rules out the presence of a possible nearby contaminant companion. The structure in this contrast curve is due to the first derivative discontinuity in the frequency mask that is used to compute the autocorrelation. This discontinuity generates rings at characteristic distances from the center, which then have a minor effect on the detection limit.
In the text
thumbnail Fig. 4

ALFOSC spectra (red) of TOI-4479 (top panel) and TOI-2081 (bottom panel) are shown in comparison with solar metallicity templates of known spectral type from the library of Kesseli et al. (2017).
In the text
thumbnail Fig. 5

MuSCAT2 (top) and MUSCAT3 (bottom) detrended light curves of TOI-4479b in g, r, i, and zs passbands. The dots show the MuSCAT2 and MUSCAT3 relative flux and the lines show the best light-curve model from the MuSCAT2 pipeline for each band.
In the text
thumbnail Fig. 6

MuSCAT2 detrended light curves of TOI-2081b in g, r, i, and zs passbands. The dots show the MuSCAT2 relative flux and the lines show the best light-curve model.
In the text
thumbnail Fig. 7

TESS PDCSAP folded relative flux from the combination of all the transits and best light-curve model of TOI-4479b (left) and TOI-2081b (right).
In the text
thumbnail Fig. 8

SINISTRO detrended light-curve of TOI-4479b in the ip pass-band. The dots show the SINISTRO relative flux and the line shows the best light-curve model.
In the text
thumbnail Fig. 9

From left to right, posterior distributions of ktrue versus ∆Teff, kapp, the impact parameter, and the stellar density from the joint multicolor light-curve analysis, for TOI-4479 (top four panels) and TOI-2081 (bottom four panels).
In the text
thumbnail Fig. 10

Comparison among the posterior distributions of the apparent and true radius ratio and the effective temperatures of the host star and contaminant. Upper panels: posterior distributions of the true radius ratio (ktrue) and the apparent radius ratio (kapp) for TOI-2081 (left) and TOI-4479 systems (right). Lower panels: posterior distributions of the effective temperature of the host star (Teff, h) and the effective temperature of the contaminant (Teff, c) for TOI-2081 (left) and TOI-4479 systems (right). The solid lines show the median of each distribution and the dashed lines show the lower and upper 1σ.
In the text
thumbnail Fig. 11

TOI-4479b and TOI-2081b in the period-radius diagram compared to all the confirmed planets to date with a radius uncertainty below 10%. The dark blue dots show the planets around M-type stars and the dashed-dotted lines show the Neptune desert boundaries in the period-radius plane derived by Mazeh et al. (2016).
In the text
thumbnail Fig. A.1

TESS target pixel file images of TOI-2081 observed in Sectors 14, 17, 20, 21, 24, 25, 26, 40, 41, and 47. The red circles show the sources in the field identified by the Gaia DR2 catalogue with scaled magnitudes. The position of the targets is indicated by white crosses and the mosaic of orange squares show the mask used by the pipeline to extract photometry. These plots were made using tpfplotter (Aller et al. 2020).
In the text
thumbnail Fig. A.2

Corner plot of the posterior distributions obtained through the multicolor validation pipeline for TOI-4479.
In the text
thumbnail Fig. A.3

Corner plot of the posterior distributions obtained through the multicolor validation pipeline for TOI-2081.
In the text

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