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 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+, to probe atmospheric escape and dayside molecular emission. No significant absorption is detected in the metastable He I λ1083 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 H2O absorption signal and a tentative CO 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 H2O abundance, while the CO abundance remains weakly constrained. The inferred planetary equatorial rotational velocity, veq=4.9+4.4−3.0 km s−1, is consistent within 1σ with the synchronous rotation velocity expected for a tidally locked planet (∼2.8 km s−1). 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.
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BibTeX
@misc{liu2026exploring,
title = {{Exploring the ultrahot Neptune LTT 9779 b with CRIRES+. Non-detection of helium transmission spectra and detection of H$_2$O on the dayside}},
author = {S. Liu and F. Yan and L. Nortmann and B. Guo and D. Cont and F. Lesjak and D. Shulyak and A. Reiners and N. Piskunov and A. P. Hatzes and U. Seemann and A. Lavail and A. D. Rains and H. L. Ruh and L. Boldt-Christmas and M. Rengel and O. Kochukhov and T. Marquart},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.07202},
eprint = {2610.07202},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.07202},
}
RIS
TY - UNPB
TI - Exploring the ultrahot Neptune LTT 9779 b with CRIRES+. Non-detection of helium transmission spectra and detection of H_2O on the dayside
AU - S. Liu
AU - F. Yan
AU - L. Nortmann
AU - B. Guo
AU - D. Cont
AU - F. Lesjak
AU - D. Shulyak
AU - A. Reiners
AU - N. Piskunov
AU - A. P. Hatzes
AU - U. Seemann
AU - A. Lavail
AU - A. D. Rains
AU - H. L. Ruh
AU - L. Boldt-Christmas
AU - M. Rengel
AU - O. Kochukhov
AU - T. Marquart
PY - 2026
DA - 2026/10/05
PB - arXiv
DO - 10.48550/arxiv.2610.07202
UR - https://arxiv.org/abs/2610.07202
AB - Ultrahot Neptunes reside in the most extreme irradiation regime of the hot Neptune desert and remain challenging targets for atmospheric characterization because of their 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^+, to probe atmospheric escape and dayside molecular emission. No significant absorption is detected in the metastable He I λ1083 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 H_2O absorption signal and a tentative CO 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 H_2O abundance, while the CO abundance remains weakly constrained. The inferred planetary equatorial rotational velocity, v_eq=4.9^+4.4_−3.0 km s^−1, is consistent within 1σ with the synchronous rotation velocity expected for a tidally locked planet (∼2.8 km s^−1). 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.
ER -