Selection and characterisation of the M-dwarf targets in the PLATO Input Catalogue

The ESA’s PLAnetary Transits and Oscillations of Stars (PLATO) mission aims to detect planets orbiting around dwarfs and subgiant stars with spectral type F5 or later, including M-dwarfs. The PLATO Input Catalogue (PIC) contains all targets available for observation by the nominal science. The latest version, PIC2.1.0.1, focuses on the Southern PLATO field, named LOPS2, selected as the first long observation field, and includes the P4 sample, one of the four target samples outlined in the Science Requirement Document. P4 includes the M-dwarfs with magnitudes V < 16 located within LOPS2. A characterisation of the M-dwarfs in the PIC is essential for assessing their potentiality to host exoplanets, and eventually estimate the hosted planet(s) properties. The purpose of this paper is to describe how we selected the P4 M-dwarf targets, and obtained their fundamental parameters and properties. In this work, we introduce the P4 sample and detail the methodologies adopted for the measurement of their stellar parameters. Based on a statistical analysis of the P4 sample, we assess both the photometric and volume completeness, and classify the stellar populations according to their Galactic spatial-velocity components. The adopted stellar parameters are validated by comparison with independent methods from the literature used to estimate stellar radii. The P4 sample is compliant with the PLATO science requirements. Being magnitude limited, its volume completeness decreases going towards distances larger than 30 pc, where late-type targets are progressively less covered. The observed large spread in the colour-magnitude diagram is likely due to the combination of several effects such as metallicity, age, binarity and activity. The strategy we adopted for deriving stellar parameters provides results consistent with those obtained in the literature with different and independent methods.

💡 Research Summary

The PLAnetary Transits and Oscillations of Stars (PLATO) mission, scheduled for launch in late 2026, will search for transiting exoplanets around bright dwarf and sub‑giant stars of spectral type F5 or later, with a particular emphasis on low‑mass M‑dwarfs. To enable this science, the PLATO Input Catalogue (PIC) has been compiled, and its most recent release, PIC 2.1.0.1, focuses on the Southern long‑duration observation field (LOPS2), the first field to be observed continuously for several years. Within PIC 2.1.0.1 the P4 sample is defined as all M‑dwarfs with visual magnitude V ≤ 16 that lie inside the LOPS2 footprint.

The authors describe a complete pipeline for selecting these targets and deriving their fundamental stellar parameters. The primary astrometric and photometric data come from Gaia DR3, with distances taken from the Bailer‑Jones (2021) Bayesian estimates, and near‑infrared photometry from 2MASS (K_S band). Because most M‑dwarfs lack published Johnson V magnitudes, the team constructed a new colour transformation from Gaia (G, G_BP, G_RP) to Johnson V. Using synthetic colours from six stellar atmosphere libraries (MPSA, MARCS, POLLUX/AMBRE, POLLUX/BT‑Dusty, POLLUX/CMFGEN, COELHO), they fitted a sixth‑order polynomial relating (G − V)_0 to (G_BP − G_RP)_0 over the range –0.51 ≤ (G_BP − G_RP)_0 ≤ 5.75. The resulting coefficients (Table 1) give residuals of only a few millimagnitudes, ensuring that the V‑band estimates are accurate enough for the magnitude cut.

Target selection was guided by TRILEGAL v1.6 simulations of the LOPS2 sky region. Two synthetic populations were generated: (i) genuine M‑dwarfs (Teff < 3850 K, log g > 3.5) and (ii) contaminants (FGK dwarfs or giants) that also satisfy V ≤ 16. By maximizing the metric S = (N_target − N_contaminant) in the colour‑absolute magnitude diagram (CAMD), the authors derived a blue boundary line: M_G,0 = –8.62·(G_BP − G_RP)_0 + 24.96. The red boundary follows a 10 Myr solar‑metallicity isochrone from the PARSEC database: M_G,0 = 2.334·(G_BP − G_RP)_0 + 2.259. Together with the V ≤ 16 cut, these criteria yielded 15 140 P4 candidates. An additional 17 M‑dwarfs with V > 16 but known or candidate planets (from Exo‑MerCat) were appended, giving a total of 15 157 M‑dwarfs in the extended sample.

Statistical properties of the P4 sample show a mean distance of 135 pc (median 133 pc, σ ≈ 59 pc), with the nearest star being Kapteyn’s star (≈ 4 pc) and the farthest ≈ 409 pc. The distribution of G and V magnitudes reflects the intrinsic faintness of later‑type M‑dwarfs, which are only detectable at short distances. Completeness analysis indicates that the sample is essentially 100 % complete in both photometry and volume out to ≈ 30 pc; beyond this radius the magnitude limit causes a rapid decline in completeness, especially for spectral types later than M4.

Kinematic classification was performed using Gaia proper motions, radial velocities (when available), and the Bailer‑Jones distances to compute Galactic space velocities (U, V, W) and positions (X, Y, Z). A Toomre diagram separates thin‑disk, thick‑disk, and halo populations; the overwhelming majority of P4 stars belong to the thin disk, consistent with their relatively high metallicities and young to intermediate ages.

Stellar masses and radii were derived from absolute K_S magnitudes via empirical mass‑luminosity relations calibrated on well‑studied nearby M‑dwarfs. The resulting radii were validated against independent determinations from interferometry (e.g., Boyajian et al. 2012) and recent literature (e.g., Kesseli et al. 2018). The agreement is within 5 %, confirming that the purely photometric method adopted here is reliable. The authors also discuss the “radius inflation” problem: their sample shows only modest inflation (0–7 %) compared with the larger (5–20 %) discrepancies reported for active, rapidly rotating eclipsing binaries. This suggests that magnetic activity, spots, metallicity, and age all contribute, but the effect is limited for the mostly inactive field M‑dwarfs selected for PLATO.

The colour‑absolute magnitude diagram exhibits a pronounced spread, which the authors attribute to a combination of metallicity variations, age spread, unresolved binaries, and magnetic activity (starspots). They note that higher metallicity tends to shift stars to brighter absolute magnitudes at a given colour, while spots reduce the effective temperature and increase the apparent radius, both of which broaden the observed sequence.

In conclusion, the P4 sample satisfies all PLATO science requirements: it contains well over the minimum 5 000 cool dwarfs, all are brighter than V = 16, and imagettes can be obtained with the required 25 s cadence for up to 5 000 targets. The catalogue provides robust stellar parameters, validated against independent methods, and includes a thorough assessment of completeness and Galactic population. While volume completeness declines beyond 30 pc, the sample remains a powerful resource for PLATO’s primary goal of detecting and characterising terrestrial planets around low‑mass stars, and it will serve as a benchmark for future statistical studies of M‑dwarf planetary systems.