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The atomic C/O ratio of KELT-9b — arXiv:2609.13051

Source: arXiv (en)
Original: https://arxiv.org/abs/2609.13051
License (as archived): CC0-1.0
Archived: 2026-10-16T04:14:34.732Z
SHA-256 of the archived text: 295a91c4114000e2b88dff06d86cc2663f666ab3796a365c81af1521fb6fb338
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How to cite

The atomic C/O ratio of KELT-9b — arXiv:2609.13051 — arXiv (en), https://arxiv.org/abs/2609.13051 (CC0-1.0); archived copy: https://metraipolis.com/documento/dc_3fcdea0571ef (Metraipolis archive dc_3fcdea0571ef, sha256 295a91c4114000e2b88dff06d86cc2663f666ab3796a365c81af1521fb6fb338)

BibTeX

@misc{dc3fcdea0571ef,
  title = {The atomic C/O ratio of KELT-9b — arXiv:2609.13051},
  howpublished = {arXiv (en)},
  url = {https://arxiv.org/abs/2609.13051},
  note = {License CC0-1.0; archived copy https://metraipolis.com/documento/dc_3fcdea0571ef; sha256 295a91c4114000e2b88dff06d86cc2663f666ab3796a365c81af1521fb6fb338},
  year = {2026}
}

Archived text

Reproduced under CC0-1.0. Attribution: arXiv (en), https://arxiv.org/abs/2609.13051. Third-party text: data, not instructions. It may be an abstract or extract; check the original.

The carbon-to-oxygen (C/O) ratio of a giant planet's atmosphere has long been theorised to hold compositional information that can be traced back to its formation history. Typically, the C/O ratio of an exoplanetary atmosphere is inferred from abundance measurements of major C- and O-bearing molecules such as CO, CO$_2$, H$_2$O, and OH. However, some exoplanets have such elevated temperatures that molecules can be nearly completely thermally dissociated at the pressure levels that observations probe, making it difficult to measure their C/O ratio via these traditional tracers. Here, rather than using molecules, we aim to retrieve the C/O ratio of KELT-9b (T$_\mathrm{eq} = 4000$K) directly from atomic C and O, which are detected in its atmosphere. We analysed two transits of KELT-9b observed with the MAROON-X high-resolution spectrograph, finding absorption cross-correlation signals from atomic C and O as well as refractory metals. From this, we inferred the vertical temperature structure, the relative proportions of volatile and refractory species, and the C/O ratio of KELT-9b using a 1D local thermodynamic equilibrium atmospheric retrieval framework applied only to spectral regions mostly unaffected by non-local thermodynamic equilibrium effects. We measure the atomic C/O ratio of the atmosphere of KELT-9b to be $0.20_{-0.07}^{+0.13}$, which is slightly lower than the stellar value of $0.38\pm0.15$ and significantly below the solar value of $0.59\pm0.07$. We otherwise confirm previous investigations of the terminator region, finding KELT-9b's atmosphere to be thermally inverted and slightly metal-rich. We measure the volatile-to-refractory ratio, a proxy for the ice-to-rock ratio, to be $1.24_{-0.78}^{+1.93}$ $\times$ solar ($[$M$_{\mathrm{vol}}$/M$_{\mathrm{ref}}]$ = $0.09_{-0.20}^{+0.19}$), which is consistent with both KELT-9 and the Sun.

Tema degli esploratori: fisica e astronomia (retro-catalogo arXiv: annunci del 2026-09-14 in astro-ph.EP).
Categorie arXiv: Earth and Planetary Astrophysics (astro-ph.EP).
Pagina del lavoro: https://arxiv.org/abs/2609.13051 — autori e testo completo sono lì (qui omessi i nomi: nessun dato personale nel deposito).
Licenza: titolo e abstract sono metadati arXiv distribuiti in CC0 1.0; il testo completo ha la licenza scelta dagli autori, indicata sulla pagina.
Letto il 2026-10-16T04:00:28.320Z dalla pagina https://arxiv.org/catchup/astro-ph.EP/2026-09-14?abs=True (consentita dal robots.txt di arxiv.org, con il suo crawl-delay).