JWST COMPASS: NIRSpec/G395H Transmission Observations of the Sub-Neptune HD 15337 c

Abstract We present the 3–5 μ m transmission spectrum of HD 15337 c (TOI-402.02), a sub-Neptune (2.526 R ⊕ , 6.792 M ⊕ , T eq ∼ 656 K) around a K1V star observed as part of the JWST COMPASS program. We reduce two observations with two pipelines and find consistent transmission spectra. The resulting median precisions in 30 pixel spectroscopic bins for visit 1 are ∼40 ppm and ∼70 ppm and for visit 2 are ∼30 ppm and ∼54 ppm for NRS1 and NRS2, respectively. We attribute the differing precisions to the lack of adequate pre-transit baseline in visit 1 from an early transit arrival caused by previously undetected transit timing variations (TTVs), hinting at a potential exterior companion. Our median JWST timing precision is 10 s, revealing TTVs >20 minutes when combined with previous TESS and CHEOPS data, highlighting JWST’s TTV measurement capabilities. The transmission spectrum of HD 15337 c is featureless and can best be described by a step function with an offset between the NRS1 and NRS2 detectors, likely caused by instrumental systematics. From thermochemical equilibrium retrievals we find that, to >3 σ , the data can rule out atmospheres with metallicities