Expanding the Volatile Abundance Inventory for High-Priority K-Dwarf JWST Exoplanet Systems

David R. Coria; Neda Hejazi; Ian J. M. Crossfield; Ismael Mireles; Casey L. Brinkman; Jared R. Kolecki; Emilio Marfil; Thomas Nordlander; Lauren M. Weiss; Diana Dragomir

AI summary · arXiv: 2611.08676v1 · astro-ph.SR

Expanding the Volatile Abundance Inventory for High-Priority K-Dwarf JWST Exoplanet Systems

David R. Coria, Neda Hejazi, Ian J. M. Crossfield, Ismael Mireles, Casey L. Brinkman, Jared R. Kolecki, Emilio Marfil, Thomas Nordlander,

and 2 more authors

Lauren M. Weiss, Diana Dragomir

Original paper Submitted to arXiv on 05 Nov. 2026 32 pages, 6 figures, 5 tables. Accepted for publication in ApJ (11/3/2026) License: CC BY 4.0 Also in: fr

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The essentials

Near-infrared spectra yielded carbon and oxygen abundances for six K-dwarf exoplanet hosts and sub-solar carbon isotope ratios for five, but not reliable oxygen isotope ratios.

Question

Can high-resolution near-infrared spectroscopy provide elemental carbon and oxygen abundances for cool K-dwarf exoplanet hosts, and can the same spectra constrain their carbon and oxygen isotope ratios? The study aims to establish host-star composition baselines for interpreting observations of their planets.

Method

The authors analyzed IGRINS H- and K-band spectra at resolving power about 45,000 for six stars, using MARCS model atmospheres, TurboSpectrum, and OH and CO lines to derive [O/H] and [C/H]. They fitted synthetic spectra to the observations and tested isotope-sensitive CO lines; the spectra had mean signal-to-noise ratios of 84.2–275.3 in K band, and they also derived gyrochronological ages using 500 Monte Carlo realizations per target.

Findings

Result The study reports [C/H] and [O/H] measurements for all six stars, including novel abundance constraints for NGTS-10, WASP-80, and HATS-72.
See the passage in the paper
This work demonstrates the viability of using near-infrared CO and OH lines to derive elemental carbon and oxygen abundances in K dwarf exoplanet host stars, revealing sub-solar to super-solar C/O ratios across the sample.
Result The derived C/O ratios range from 0.48 to 0.87 across the six host stars.
See the passage in the paper
C/O $0.53\pm 0.13$ $0.51\pm 0.14$ $0.79\pm 0.20$ $0.48\pm 0.13$ $0.87\pm 0.23$ $0.56\pm 0.14$
Result Five stars have measured sub-solar 12C/13C ratios, while WASP-80 did not yield a reliable measurement.
See the passage in the paper
Although the noise in WASP-80’s spectrum prevents a reliable ${}^{12}\mathrm{C}/^{13}\mathrm{C}$ measurement, we are able to derive ${}^{12}\mathrm{C}/^{13}\mathrm{C}$ ratios for the remaining five targets using at least three ${}^{13}\mathrm{C}$ O lines.
Authors’ interpretation The spectra were insufficient to measure reliable 16O/18O ratios, and the authors identify M-band CO features as a more promising route.
See the passage in the paper
This means that perhaps deriving ${}^{12}\mathrm{C}/^{13}\mathrm{C}$ and ${}^{16}\mathrm{O}/^{18}\mathrm{O}$ ratios is best done in the M band using fundamental rovibrational CO isotopologue lines as in Crossfield et al. [58], Coria et al. [53], and González Picos et al. [95].
Authors’ interpretation The K-dwarf carbon isotope results tentatively indicate age-related trends, but the authors caution that the sample does not cover middle-aged stars adequately.
See the passage in the paper
our K dwarf sample is quite small and does not probe the middle-aged (4-10 Gyr) parameter space, meaning the tentative trend could change or disappear with further observations

Limitations

The weak isotopologue features, spectral noise, and continuum normalization limit isotope measurements; the oxygen isotope ratio could not be reliably measured.
See the passage in the paper
While initial efforts to extract isotopic ratios (${}^{12}\mathrm{C}/^{13}\mathrm{C}$, ${}^{16}\mathrm{O}/^{18}\mathrm{O}$) from 13C16O and 12C18O lines in the K band showed promise, the weak signal-to-noise and challenging normalization of these features limit their immediate utility and are perhaps best measured from M-band CO isotopologue features.
The sample of six stars is too small and incompletely samples middle-aged K dwarfs to establish robust abundance-age trends.
See the passage in the paper
our K dwarf sample is quite small and does not probe the middle-aged (4-10 Gyr) parameter space, meaning the tentative trend could change or disappear with further observations
The ages of most targets are poorly constrained by isochrone fitting, which returned ages near the adopted upper limit.
See the passage in the paper
We obtain ages consistent with our upper bound of 14 Gyr for 5 of our 6 targets, with NGTS-10 being the exception with an age of $10.1^{+1.7}_{-1.8}$ Gyr.

Why it matters

The authors argue that near-infrared H- and K-band spectroscopy offers an alternative for measuring carbon and oxygen in cool dwarfs whose optical spectra complicate traditional abundance analyses. Extending this work to more K-dwarf planet hosts would provide needed stellar-composition baselines for interpreting exoplanet atmospheres and testing formation and chemical-evolution models.

Written by AI from the paper. Passages are checked word for word against the text we analysed; the rest may contain errors.

AI-generated keywords: K-dwarf stars; stellar abundances; carbon-to-oxygen ratio; carbon isotopes; near-infrared spectroscopy

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Comprehensive Summary
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Original abstract

The paper addresses a gap in chemical abundance measurements for cool K-dwarf stars. Their crowded, molecule-rich spectra make traditional optical analyses challenging. These measurements are important for exoplanet research: a host star’s composition provides a baseline for interpreting its planet’s atmosphere and can help test theories of planetary formation and evolution. The authors studied six K-dwarf exoplanet hosts targeted for planetary characterization: K2-141, NGTS-10, WASP-43, WASP-69, WASP-80, and HATS-72. They measured stellar carbon and oxygen abundances and investigated whether carbon and oxygen isotope ratios could be recovered from near-infrared spectra. The team analyzed high-resolution IGRINS spectra covering the H and K bands at a resolving power of about 45,000. Observations came from Gemini South and Lowell Observatory. The H-band signal-to-noise ratios ranged from about 172 to 408, while the K-band ratios ranged from about 84 to 275. The researchers compared the observations with synthetic spectra generated using MARCS stellar-atmosphere models and the TurboSpectrum synthesis code. They inferred oxygen abundances from H-band OH lines and carbon abundances from K-band CO lines. For each usable line, they fitted synthetic spectra across a grid of abundances and combined the line-by-line results. They then held the measured elemental abundances fixed while testing models with different carbon isotope ratios. Stellar ages were estimated using both isochrone fitting and gyrochronology, which uses stellar rotation. The resulting carbon abundances, expressed relative to the Sun, range from −0.03 to +0.26 dex, while oxygen abundances range from −0.12 to +0.35 dex. The carbon-to-oxygen ratios are subsolar for five stars and near-solar for WASP-80: 0.53 ± 0.13 for K2-141, 0.51 ± 0.14 for NGTS-10, 0.79 ± 0.20 for WASP-43, 0.48 ± 0.13 for WASP-69, 0.87 ± 0.23 for WASP-80, and 0.56 ± 0.14 for HATS-72. These measurements provide new carbon and oxygen constraints for NGTS-10, WASP-80, and HATS-72, as well as additional constraints for K2-141 and WASP-43. The authors measured the carbon isotope ratio, ¹²C/¹³C, for five stars. Values range from 38 to 52: 51 ± 12 for K2-141, 48 ± 11 for NGTS-10, 50 ± 12 for WASP-43, 38 ± 10 for WASP-69, and 52 ± 16 for HATS-72. All are below the solar ratio of about 91, suggesting enrichment in carbon-13. The WASP-80 spectrum did not support a reliable measurement. The authors could not determine reliable oxygen isotope ratios because the relevant isotopologue lines are too weak at the available signal-to-noise levels. The study demonstrates that near-infrared OH and CO lines can provide useful elemental abundances for cool K dwarfs, complementing optical analyses and informing JWST observations of their planets. Isotope measurements remain more challenging: the carbon results are sensitive to continuum placement, spectral noise, and adopted stellar parameters, while the oxygen isotope features are even weaker. The sample is small, so the reported trends between isotope ratios, age, and metallicity are tentative. Age estimates also have limitations.

Reading the Chemistry of K-Dwarf Exoplanet Hosts

Cool K-dwarf stars are appealing targets for exoplanet studies, but their spectra are difficult to read. At the temperatures of these stars, many spectral features overlap, while molecules add further complexity. As a result, measuring the elements in their atmospheres can be challenging with traditional optical methods.

A star’s composition is more than a detail about the star itself. It gives researchers a reference point for interpreting the atmosphere of its planet and can help test ideas about how planets form and change over time. In

“Expanding the Volatile Abundance Inventory for High-Priority K-Dwarf JWST Exoplanet Systems,” David R. Coria, Neda Hejazi, Ian J. M. Crossfield, Ismael Mireles, Casey L. Brinkman, Jared R. Kolecki et al.

, the team develops a near-infrared approach to studying carbon and oxygen in six K-dwarf exoplanet hosts.

Looking in the Near-Infrared

The researchers observed K2-141, NGTS-10, WASP-43, WASP-69, WASP-80, and HATS-72 with IGRINS, a high-resolution spectrograph that captures light in the H and K bands. The spectra, taken at Gemini South and Lowell Observatory, had a resolving power of about 45,000. Their signal-to-noise levels varied by band and star, providing the team with a way to investigate which chemical measurements the data could support.

Instead of relying on crowded optical features, the analysis focused on molecular lines in the infrared. Oxygen abundances were inferred from OH lines in the H band, while carbon abundances came from CO lines in the K band. The team compared observed spectra with synthetic spectra made using MARCS stellar-atmosphere models and the TurboSpectrum code. For each usable line, they tested a range of abundances and combined the line-by-line estimates.

Carbon and Oxygen Across Six Stars

The measured carbon abundances, relative to the Sun, span −0.03 to +0.26 dex. Oxygen abundances range from −0.12 to +0.35 dex. Together, these measurements provide new carbon and oxygen constraints for NGTS-10, WASP-80, and HATS-72, and add to existing constraints for K2-141 and WASP-43.

The team also calculated each star’s carbon-to-oxygen ratio, or C/O. This ratio is useful for describing the balance of two important elements in a planetary system. Five of the six stars have subsolar C/O ratios, while WASP-80 is near solar:

K2-141: 0.53 ± 0.13
NGTS-10: 0.51 ± 0.14
WASP-43: 0.79 ± 0.20
WASP-69: 0.48 ± 0.13
WASP-80: 0.87 ± 0.23
HATS-72: 0.56 ± 0.14

These ratios give researchers a more complete picture of the chemical environments in which the planets formed. They can also help place future observations of the planets’ atmospheres in context, including observations with JWST.

Can the Spectra Reveal Isotopes?

Elements can occur in different isotopic forms, and their relative amounts may preserve clues about a star’s history. The researchers therefore tested whether the same spectra could reveal the carbon isotope ratio, ¹²C/¹³C. They kept the measured carbon and oxygen abundances fixed and compared models with different isotope ratios.

They obtained measurements for five stars, with results between 38 and 52. Each is below the solar ratio of about 91, indicating relative enrichment in carbon-13. The WASP-80 spectrum did not support a reliable estimate.

K2-141: 51 ± 12
NGTS-10: 48 ± 11
WASP-43: 50 ± 12
WASP-69: 38 ± 10
HATS-72: 52 ± 16

Oxygen isotopes proved harder to measure. The relevant molecular features were too weak at the available signal-to-noise levels for reliable oxygen isotope ratios. The study thus shows an important difference between detecting elemental abundances and resolving isotopic detail: the former is achievable in these data, while the latter demands stronger or cleaner spectral features.

What the Results Mean—and What Remains Uncertain

This work demonstrates that near-infrared OH and CO lines can provide useful carbon and oxygen abundances for cool K dwarfs, complementing optical analyses. Those measurements strengthen the stellar baseline needed to interpret exoplanet atmospheres and guide the study of high-priority systems.

The isotope results, however, should be treated cautiously. Carbon isotope estimates are sensitive to continuum placement, spectral noise, and the adopted stellar parameters. The sample is small, so possible relationships between isotope ratios, stellar age, and metallicity remain tentative. The authors estimated ages using both isochrone fitting and gyrochronology, a method based on stellar rotation, but age estimates also have limitations.

Overall, the study expands the chemical inventory available for six K-dwarf planet hosts and shows both the promise and the limits of near-infrared spectroscopy. It offers a practical route to measuring key elements in cool stars, while highlighting the higher data quality needed to make isotope measurements—and any trends inferred from them—more secure.

Created on 09 Nov. 2026

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Expanding the Volatile Abundance Inventory for High-Priority K-Dwarf JWST Exoplanet Systems

The essentials

Question

Method

Findings

Limitations

Why it matters

Read further

Definitions

Reading the Chemistry of K-Dwarf Exoplanet Hosts

Looking in the Near-Infrared

Carbon and Oxygen Across Six Stars

Can the Spectra Reveal Isotopes?

What the Results Mean—and What Remains Uncertain

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