Revisiting TOI-4438 and TOI-442 planetary systems with new observations from SPIRou and TESS

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Abstract
We present a comprehensive reanalysis of two star-planet systems: TOI-4438, an M3.5V star hosting a mini-Neptune in a 7.4-day orbit, and TOI-442, an M1V star with a 4-day period planet located within the hot Neptune desert. Both systems were originally identified as transiting planet candidates by the Transiting Exoplanet Survey Satellite (TESS) and subsequently validated through the radial velocity (RV) method. Our work incorporates new TESS transit data and high-resolution spectroscopy from the SPIRou near-infrared spectropolarimeter. We detected a persistent and relatively strong Zeeman signature in TOI-442, while TOI-4438 exhibits weaker and intermittent magnetic activity, and we inferred the stellar rotation periods of both stars from the variability of the longitudinal magnetic field. We jointly fit photometry and RV models for each system. For TOI-4438 b we combined archival CARMENES data with 81 SPIRou observations and five TESS sectors. This yielded a refined planetary mass of M p = 4.11 +0.40 −0.38 M ⊕ and a radius of R p = 2.40 +0.09 −0.10 R ⊕ , consistent with the previous estimate within 1.1 σ while improving by 53% the precision on the mass and 22% on the radius. For TOI-442 b, we added 29 SPIRou RV measurements to an extensive archival dataset, significantly extending the temporal baseline. Incorporating this with a new TESS sector, we tighten the constraints on the planetary mass to M p = 28.38 +0.77 −0.73 M ⊕ and radius to R p = 4.25 +0.10 −0.08 R ⊕ , which agrees with the previous values within 1.5 σ and improves the precisions by 46 and 67%, respectively. We found no clear signs of additional planets in the currently available RV data, although we detected a single-transit event in the TOI-4438 light curve. We compared various RV models and found that those accounting for stellar variability-induced signals yield improved constraints on the planetary parameters.

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