Work

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

Year: 2026

Type: article

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

Source: Astronomy and Astrophysics

DOI: 10.1051/0004-6361/202662567