Ultracool dwarfs like TRAPPIST-1 are the most numerous stars in the Milky Way, but their spectra have long been considered too complex for precise chemical analysis because of overlapping molecular bands. Neighboring studies in the archive have already shown how to measure metallicity and individual elements in such stars, as well as how to use isotopes for dating and tracing nucleosynthesis. The present study follows in this vein but is the first to apply a combination of K- and M-band observations to a star with a fully convective zone, allowing the contributions of different CO isotopologues to be disentangled.
Observations were carried out with two instruments: Keck/NIRSPEC in the M band (4.6–5.0 μm) and archival spectra from CFHT/SPIRou in the K band (2.1–2.4 μm). For the analysis, a grid of atmospheric models was built with varying temperature profiles, metallicity, C/O ratio, and CO isotopologue abundances. A Bayesian retrieval with 19 free parameters fit the model to the data, and the significance of the 13CO and C18O detections was assessed by comparing models with and without these molecules.
The first values for TRAPPIST-1 were obtained: metallicity [M/H] = 0.00 (+0.06/−0.06), C/O = 0.60 (+0.02/−0.02), 12C/13C = 154 (+17/−16), and 16O/18O = 490 (+78/−64). The carbon-to-oxygen ratio turned out to be solar, while the 12C/13C ratio is higher than the solar value (89), which is typical for old stars. The oxygen ratio 16O/18O agrees with the solar value and with other M dwarfs. The detections of 13CO and C18O are confirmed with high significance according to the Bayesian factor.
This work provides the first reference chemical composition for the TRAPPIST-1 system, which is critical for interpreting future measurements of its planets' atmospheres. Unlike neighboring studies that focused either on metallicity or on individual isotopes, this one combines both approaches and yields the first C/O ratio for such a cool star. This expands the sample of chemically characterized ultracool dwarfs and allows tests of Galactic chemical evolution models on a new class of objects.
Future observations with ELT/METIS and other next-generation instruments will allow isotopic ratios to be measured for hundreds of cool dwarfs and directly for exoplanets. For TRAPPIST-1, it will be especially interesting to compare the stellar isotopic ratios with those that might be extracted from the atmospheres of its planets, if they are detected. More accurate line lists for water and CO at temperatures of 2000–3000 K will also be needed to remove systematic discrepancies between wavelength ranges.
The results will affect studies of planet formation, Galactic chemical evolution, and stellar atmospheres. They also provide a reference point for interpreting JWST data on the TRAPPIST-1 system.
The immediate next step is a homogeneous analysis of isotopic ratios for a large sample of M dwarfs with known planets. It is also important to carry out observations at higher spectral resolution (CRIRES+, PEPSI) to refine the rotation velocity and test atmospheric models.
For the field of Galactic chemical evolution, a key unresolved problem is the scatter in isotopic ratios among stars of the same age and metallicity, which is linked to uncertainties in nucleosynthesis yields from novae and massive stars. This work adds a data point for an old, solar-metallicity dwarf, but to identify systematic trends, dozens of similar measurements are needed, especially at low metallicity, where models diverge more strongly.
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
This work is the first to determine the chemical composition of TRAPPIST-1: metallicity [M/H] = 0.00, C/O ratio = 0.60, and isotopic ratios 12C/13C = 154 and 16O/18O = 490. This is achieved using high-resolution spectra in the K and M bands with Bayesian retrieval and accounting for CO isotopologues.
Where its strength is
For the first time, a reference chemical composition for the TRAPPIST-1 system has been obtained, including the C/O ratio and isotopic ratios, which is critical for interpreting the atmospheres of its planets. The combination of K and M bands allowed the contribution of different CO isotopologues to be disentangled for a star with a fully convective zone.
Where it could go next
Worth trying
Why this may matter
The reference chemical composition of TRAPPIST-1 serves as a key to interpreting future observations of its planets' atmospheres and allows testing models of planet formation and evolution around the most common class of stars.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.60, against a usual 0.64. It looks like few have drilled nearby.
TRAPPIST-1 is so faint that in the M band its brightness is comparable to that of the exoplanet β Pic b, which was observed in just 2.4 hours with the VLT. Yet TRAPPIST-1 has seven planets, while β Pic b has only one.