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

TL;DR

This study uses ALMA to search for five molecules in the 49 Ceti debris disk, finding very low abundances compared to protoplanetary disks, suggesting CO may be shielded by CI rather than originating from ices.

Contribution

First deep molecular line search in 49 Ceti debris disk, providing constraints on molecular abundances and implications for CO origin and shielding mechanisms.

Findings

Molecular abundances are much lower than in protoplanetary disks.

No detection of the five molecules within sensitivity limits.

Results support CI shielding hypothesis for CO survival.

Abstract

Surprisingly strong CO emission has been observed from more than a dozen debris disks around nearby main-sequence stars. The origin of this CO is unclear, in particular whether it is left over from the protoplanetary disk phase or is second-generation material released from collisions between icy bodies like debris dust. The primary unexplored avenue for distinguishing the origin of the material is understanding its molecular composition. Here we present a deep search for five molecules (CN, HCN, HCO+, SiO, and CH3OH) in the debris disk around 49 Ceti. We take advantage of the high sensitivity of the Atacama Large Millimeter/submillimeter Array (ALMA) at Band 7 to integrate for 3.2 hours at modest spatial (1") and spectral (0.8 km/s) resolution. Our search yields stringent upper limits on the flux of all surveyed molecular lines, which imply abundances relative to CO that are orders of magnitude lower than those observed in protoplanetary disks and Solar System comets, and also those predicted in outgassing models of second-generation material. However, if CI shielding is responsible for extending the lifetime of any CO produced in second-generation collisions, as proposed by Kral et al. (2018), then the line ratios do not reflect true ice phase chemical abundances, but rather imply that CO is shielded by its own photodissociation product, CI, but other molecules are rapidly photodissociated by the stellar and interstellar radiation field.