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Surprisingly strong CO emission has been observed from more than a dozendebris disks around nearby main-sequence stars. The origin of this CO isunclear, in particular whether it is left over from the protoplanetary diskphase or is second-generation material released from collisions between icybodies like debris dust. The primary unexplored avenue for distinguishing theorigin of the material is understanding its molecular composition. Here wepresent a deep search for five molecules (CN, HCN, HCO+, SiO, and CH3OH) in thedebris disk around 49 Ceti. We take advantage of the high sensitivity of theAtacama Large Millimeter/submillimeter Array (ALMA) at Band 7 to integrate for3.2 hours at modest spatial (1") and spectral (0.8 km/s) resolution. Our searchyields stringent upper limits on the flux of all surveyed molecular lines,which imply abundances relative to CO that are orders of magnitude lower thanthose observed in protoplanetary disks and Solar System comets, and also thosepredicted in outgassing models of second-generation material. However, if CIshielding is responsible for extending the lifetime of any CO produced insecond-generation collisions, as proposed by Kral et al. (2018), then the lineratios do not reflect true ice phase chemical abundances, but rather imply thatCO is shielded by its own photodissociation product, CI, but other moleculesare rapidly photodissociated by the stellar and interstellar radiation field.

A Deep Search for Five Molecules in the 49 Ceti Debris Disk