Molecular Reconnaissance of the β Pictoris Gas Disk with the SMA: A Low HCN/(CO+CO₂) Outgassing Ratio and Predictions for Future Surveys

Abstract

The exocometary origin of CO gas has been confirmed in several extrasolar Kuiper belts, with CO ice abundances consistent with Solar System comets. We here present a molecular survey of the β Pictoris belt with the Submillimeter Array (SMA), reporting upper limits for CN, HCN, HCO + , N 2 H + and H 2 CO, as well as for H 2 S, CH 3 OH, SiO and DCN from archival ALMA data. Non-detections can be attributed to rapid molecular photodissociation due to the Astar's strong UV flux. CN is the longest-lasting and most easily detectable molecule after CO in this environment. We update our NLTE excitation model to include UV fluorescence, finding it plays a key role in CO and CN excitation, and use it to turn the SMA CN/CO flux ratio constraint into an upper limit of < 2.5% on the HCN/(CO+CO 2 ) ratio of outgassing rates. This value is consistent with, but at the low end of, the broad range observed in Solar System comets. If sublimation dominates outgassing, then this low value may be caused by decreased outgassing for the less volatile molecule HCN compared to CO. If instead UV photodesorption or collisional vaporization of unbound grains dominates outgassing, then this low ratio of rates would imply a low ice abundance ratio, which would in turn indicate a variation in cometary cyanide abundances across planetary systems. To conclude, we make predictions for future molecular surveys and show that CN and HCN should be readily detectable with ALMA around β Pictoris for Solar-System-like exocometary compositions.

Extrasolar Kuiper belts

“…rich planetesimals, and so that all have associated secondary gas disks, but that only a fraction of these have CO at detectable levels. While no other gas molecules have been detected in debris disks, this is to be expected given the lower abundance relative to CO and moreover the even shorter lifetime of most molecules (Matrà et al 2018b).…”

New Millimeter CO Observations of the Gas-rich Debris Disks 49 Cet and HD 32297

“…Fig. 2a, except for β Pic where it was taken from Matrà et al (2018a). As the plot shows, while the CO content of β Pic can be explained within an unshielded or weakly shielded model, all CO-rich disks require substantial shielding with CO > 100, i.e.…”

Upper limits on the water vapour content of the β Pictoris debris disk

“…Matrà et al (2017a,b) and Marino et al (2016) found that the CO+CO 2 mass abundances of the comets in the β Pic, Fomalhaut and HD 181327 disks are consistent with abundances measured in solar system comets. Using data from the Submillimeter Array (SMA), Matrà et al (2018) showed that the comets in the β Pic disk have a HCN/(CO+CO 2 ) ratio of outgassing rates that is consistent with, but at the low end of, what is observed for solar system comets. This might be caused by the outgassing mechanism, or it might indicate a true depletion of HCN with respect to solar system comets.…”