First Detection of c-C3H2 in a Circumstellar Disk

We report the first detection of c-C3H2 in a circumstellar disk. The c-C3H2 J=6− 5 line (217.882 GHz) is detected and imaged through Atacama Large Millimeter Array (ALMA) Science Verification observations toward the disk around the Herbig Ae star HD 163296 at 0.8 resolution. The emission is consistent with that arising from a Keplerian rotating disk. Two additional c-C3H2 transitions are also tentatively detected, bolstering the identification of this species, but with insufficient signal-to-noise ratio to constrain the spatial distribution. Using a previously developed model for the physical structure of this disk, we fit a radial power-law distribution model to the c-C3H2 6− 5 emission and find that c-C3H2 is present in a ring structure from an inner radius of about 30 AU to an outer radius of about 165 AU. The column density is estimated to be 10–10 cm. The clear detection and intriguing ring structure suggest that c-C3H2 has the potential to become a useful probe of radiation penetration in disks. Subject headings: protoplanetary disks; astrochemistry; stars: formation; ISM: molecules; techniques: high angular resolution; radio lines: ISM