High CO/H₂ ratio supports an exocometary origin for a CO-rich debris disc

¹ School of Physics, Trinity College Dublin, College Green, Dublin 2, Ireland
² Department of Astronomy, University of Wisconsin-Madison, Madison, WI 53706, USA
³ Department of Astronomy, Van Vleck Observatory, Wesleyan University, Middletown, CT 06459, USA
⁴ William H. Miller III Dept. of Physics and Astronomy, John’s Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA
⁵ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁶ European Space Agency (ESA), European Space Astronomy Centre (ESAC), Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
⁷ Center for Astrophysics, Harvard and Smithsonian, 60 Garden Street, Cambridge, MA 02138-1516, USA
⁸ NASA Goddard Space Flight Center, Greenbelt, USA
⁹ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 4 November 2025
Accepted: 20 February 2026

Abstract

Context. Over 20 exocometary belts host detectable circumstellar gas, mostly in the form of CO. Two competing theories for its origin have emerged, positing that the gas is either primordial or secondary. Primordial gas survives from the belt’s parent protoplanetary disc and is therefore H₂-rich. Secondary gas is outgassed in situ by exocomets and is relatively H₂-poor. Discriminating between these scenarios has not been possible for belts that host unexpectedly large quantities of CO.

Aims. We aim to break this gas origin dichotomy through direct measurement of H₂ column densities in two edge-on, CO-rich exocometary belts around ∼15 Myr-old A-type stars, constraining the CO/H₂ ratio and CO gas lifetimes. Observing edge-on belts enables rovibrational absorption spectroscopy against the stellar background.

Methods. We present near-IR CRIRES+ spectra of HD 110058 and HD 131488, which provide the first direct probe of H₂ gas in CO-rich exocometary belts. We targeted the H₂ (v=1-0 S(0)) line at 2223.3 nm and the ¹²CO v = 2 → 0 rovibrational lines in the range 2333.8-2335.5 nm and derived constraints on column densities along the line of sight to the stars.

Results. We detect ¹²CO strongly, but not H₂, in the CRIRES+ spectra. This allows us to place 3σ lower limits on the CO/H₂ ratios of >1.35 × 10⁻³ and >3.09 × 10⁻⁵ for HD 110058 and HD 131488, respectively. These constraints demonstrate that, at least for HD 110058, the exocometary gas is compositionally distinct and significantly H₂-poor compared to the <10⁻⁴ CO/H₂ ratios typical of protoplanetary discs. For HD 131488, we further compared the CO photodissociation timescale to the age of the system through simple shielding arguments, and find that we cannot formally rule out a primordial origin; however, we suggest that a more realistic model of CO survival likely supports a secondary origin for this system as well. Overall, a high CO/H₂ ratio for HD 110058 indicates that the gas in this CO-rich belt is most likely not primordial in composition, supporting the presence of exocometary gas.