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High CO/H 2 ratio supports an exocometary origin for a CO-rich debris disc
Kevin Smith (Trinity College Dublin), Luca Matrà (Trinity College Dublin), Ke Zhang (University of Wisconsin–Madison), Aoife Brennan (Trinity College Dublin), A. Meredith Hughes (Wesleyan University), Christine Chen (Space Telescope Science Institute, Johns Hopkins University), Isabel Rebollido (European Space Agency, European Space Astronomy Centre), David J. Wilner (Smithsonian Astrophysical Observatory, Center for Astrophysics Harvard & Smithsonian), Aki Roberge (Goddard Space Flight Center), Seth Redfield (Wesleyan University), Antonio S. Hales (National Radio Astronomy Observatory), Karin I. Öberg (Smithsonian Astrophysical Observatory, Center for Astrophysics Harvard & Smithsonian)
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 H 2 -rich. Secondary gas is outgassed in situ by exocomets and is relatively H 2 -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 H 2 column densities in two edge-on, CO-rich exocometary belts around ∼15 Myr-old A-type stars, constraining the CO/H 2 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 H 2 gas in CO-rich exocometary belts. We targeted the H 2 (v=1-0 S(0)) line at 2223.3 nm and the 12 CO 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 12 CO strongly, but not H 2 , in the CRIRES+ spectra. This allows us to place 3σ lower limits on the CO/H 2 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 H 2 -poor compared to the <10 −4 CO/H 2 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/H 2 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.