{
  "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 $\\mathrm{H_2O}$ absorption signal and a tentative $\\mathrm{CO}$ 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 $\\mathrm{H_2O}$ abundance, while the $\\mathrm{CO}$ abundance remains weakly constrained. The inferred planetary equatorial rotational velocity, $v_{\\rm eq}=4.9^{+4.4}_{-3.0}~\\mathrm{km\\,s^{-1}}$, is consistent within $1σ$ with the synchronous rotation velocity expected for a tidally locked planet ($\\sim2.8~\\mathrm{km\\,s^{-1}}$). 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.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": null,
  "arxiv": "2610.07202",
  "author_total": 18,
  "authors": [
    {
      "name": "S. Liu"
    },
    {
      "name": "F. Yan"
    },
    {
      "name": "L. Nortmann"
    },
    {
      "name": "B. Guo"
    },
    {
      "name": "D. Cont"
    },
    {
      "name": "F. Lesjak"
    },
    {
      "name": "D. Shulyak"
    },
    {
      "name": "A. Reiners"
    },
    {
      "name": "N. Piskunov"
    },
    {
      "name": "A. P. Hatzes"
    },
    {
      "name": "U. Seemann"
    },
    {
      "name": "A. Lavail"
    },
    {
      "name": "A. D. Rains"
    },
    {
      "name": "H. L. Ruh"
    },
    {
      "name": "L. Boldt-Christmas"
    },
    {
      "name": "M. Rengel"
    },
    {
      "name": "O. Kochukhov"
    },
    {
      "name": "T. Marquart"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/fgj696enyk30m.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2610.07202",
  "fulltext_url": null,
  "id": "fgj696enyk30m",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2610.07202",
  "language": null,
  "licence": "LicenseRef-arXiv-nonexclusive-1.0",
  "licence_label": "arXiv licence",
  "licence_source": "article",
  "licence_url": "http://arxiv.org/licenses/nonexclusive-distrib/1.0/",
  "metadata_licence": "CC0-1.0",
  "native_subjects": [
    [
      "arxiv",
      "astro-ph.EP"
    ]
  ],
  "openalex": null,
  "pdf_url": "https://arxiv.org/pdf/2610.07202",
  "pmcid": null,
  "pmid": null,
  "published": "2026-10-05",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/fgj696enyk30m.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:27:22Z",
      "native_id": "2610.07202",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2610.07202"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.EP",
      "subfield": "planetary"
    }
  ],
  "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",
  "title_text": "Exploring the ultrahot Neptune LTT 9779 b with CRIRES+. Non-detection of helium transmission spectra and detection of H_2O on the dayside",
  "type": "preprint",
  "updated_at": "2026-10-30T12:27:22Z",
  "url": "https://research.draconitic.com/paper/fgj696enyk30m",
  "venue": "arXiv",
  "versions": []
}