JWST COMPASS: A NIRSpec G395H Transmission Spectrum of Radius Valley Dweller TOI-260 b

Abstract We present a JWST/NIRSpec G395H transmission spectrum of TOI-260 b, a T eq ∼ 490 K, R p = 1.76 R ⊕ planet. The transmission spectrum is derived by combining two transit observations, collected as part of the JWST Compositions of Mini-Planet Atmospheres for Statistical Study program. We achieved the same median transit depth precision of 37 ppm in both visits, as well as a median precision of 26 ppm when combining the spectroscopic light curves from the two visits. Implementing a 30 pixel wide ( R ∼ 200) spectroscopic binning scheme, we find that the transmission spectrum is mostly featureless, with a possible feature around 3.17 μ m. We assess the significance of any features in the transmission spectrum with a suite of nonparametric models, which confirm the presence of a potential feature in the NRS1 bandpass and an offset between the NRS1 and NRS2 detectors. To investigate the atmospheric composition of TOI-260 b, we run a series of PLanetary Atmospheric Tool for Observer Noobs retrievals. We do not detect any clear molecular signatures, but the combined data from the two visits are sufficient to constrain the atmospheric metallicity to greater than 200 × solar, assuming no opaque deck ≲2.5 mbar. We also investigate causes of the potential feature near 3.17 μ m; while we find some compatible gaseous species and cannot fully discard the possibility of an astrophysical origin, we suspect a systematics origin owing to the variance in strength and position of the feature. Overall, this look at TOI-260 b adds to the small sample of radius valley planets, which already seem to show a diversity in their atmospheric compositions. Determining the true nature of these enigmatic planets will require a larger telescope time investment.