2013ApJ...765L..14Q

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-C₃H₂6-5 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 10¹²-10¹³/cm2. The clear detection and intriguing ring structure suggest that c-C₃H₂ has the potential to become a useful probe of radiation penetration in disks.