Molecular reconnaissance of the $\beta$ Pictoris gas disk with the SMA: a low HCN/(CO+CO$_2$) outgassing ratio and predictions for future surveys

TL;DR

This study uses SMA and archival ALMA data to analyze the molecular composition of the $eta$ Pictoris gas disk, revealing a low HCN/(CO+CO$_2$) outgassing ratio and predicting future detectability of molecules like CN and HCN.

Contribution

It provides the first molecular survey of $eta$ Pictoris' gas disk, updates excitation models with UV fluorescence, and constrains the HCN/(CO+CO$_2$) ratio in exocometary outgassing.

Findings

Upper limits for several molecules including CN, HCN, HCO$^+$, N$_2$H$^+$, H$_2$CO, H$_2$S, CH$_3$OH, SiO, DCN.

CN is the most detectable molecule after CO, with an upper limit of <2.5% for HCN/(CO+CO$_2$) ratio.

Predictions that CN and HCN will be detectable with ALMA around $eta$ Pictoris.

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 $\beta$ 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 A-star'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 $\beta$ Pictoris for Solar-System-like exocometary compositions.