The chemical composition of CO-rich comet C/2023 H2 (Lemmon)

Aims. We report on the composition of comet C/2023 H2 (Lemmon) (hereafter C/2023 H2) as measured with CRIRES+ at VLT/ESO, under the Director Discretionary Time program: 2112.C-5015. Methods. We observed C/2023 H2 between November 24 and 27, 2023, after perihelion and during its close approach to Earth. We used three settings sampling the spectral region from ≈ 2.5 to 5 μm, to search for fluorescence emission lines of H2O, HCN, C2H2, NH3, C2H6, H2CO, CH3OH, CH4, and CO. Results. C/2023 H2 spectra are dominated by signatures from hyper-volatile species, namely CO, C2H6, and CH4, while it is particularly difficult to identify lines from less volatile species such as water or methanol. When compared to other comets, C/2023 H2 has an overall typical-to-enriched composition, with CO showing one of the highest infrared values reported so far in Solar System comets within 2 au from the Sun. In this respect, C/2023 H2 shares many similarities with C/2013 R1 (Lovejoy), C/2009 P1 (Garrad), and C/1999 T1 (McNaught-Hartley), although still being rather unique. Results from the analysis of optical spectra (≈300-650 nm) obtained on November 16, 2023, using the Intermediate Dispersion Spectrograph at the Isaac Newton Telescopes are consistent with the infrared ones. Conclusions. Assuming that the composition of C/2023 H2 is original, this comet most likely formed in a region of the disc where CO was particularly enriched, or it could have been captured from other planetary systems forming in the Suns birth cluster. Similarities with a few other comets suggest the existence of a sub-class of CO-enriched comets, which may be currently under-sampled. Alternatively, water sublimation may have been ineffective due to the presence of a thick dust mantle covering the nucleus surface.