Exploring the ultrahot Neptune LTT 9779 b with CRIRES+. Non-detection of helium transmission spectra and detection of H2O on the dayside

Abstract

Ultrahot Neptunes reside in the most extreme irradiation regime of the hot Neptune desert and remain challenging targets for atmospheric characterization because of their intrinsically low planet-to-star flux ratios. We present high-resolution transit and dayside thermal emission spectroscopy of the ultrahot Neptune LTT 9779 b obtained with CRIRES+$, aimed at probing both atmospheric escape and dayside molecular emission. No significant absorption is detected in the metastable łambda 1083,nm triplet during transit, yielding a 3σ upper limit of 0.45% on the excess helium absorption depth and supporting previous non-detections that suggest weak ongoing atmospheric escape. For the dayside emission spectra in the K band, our cross-correlation analysis reveals an He i absorption signal and a tentative feature, with the combined signal reaching a detection significance of 4.8σ. Both the CRIRES^+ and previous JWST data favor a non-inverted temperature structure, providing no evidence of a thermal inversion on the planetary dayside. We further perform a joint atmospheric retrieval combining the CRIRES^+ high-resolution spectra with JWST/NIRISS SOSS secondary-eclipse observations. The joint retrieval yields a low abundance, while the abundance remains weakly constrained. The inferred planetary equatorial rotational velocity, v_ ̊m eq =4.9^ +4.4 _ -3.0 -1 , is consistent within 1σ with the synchronous rotation velocity expected for a tidally locked planet (sim2.8 ). However, the large uncertainties prevent us from placing strong constraints on the planet's rotation state. Future phase-resolved CRIRES$^+ observations with broader orbital-phase coverage and higher signal-to-noise ratios would improve the abundance constraints and probe the atmospheric dynamics of this rare ultrahot Neptune. -1