The Surprisingly Low Carbon Mass in the Debris Disk around HD 32297

Gas has been detected in a number of debris disks. It is likely secondary, i.e., produced by colliding solids. Here, we report ALMA Band 8 observations of neutral carbon in the CO-rich debris disk around the 15-30 Myr old A-type star HD 32297. We find that C⁰ is located in a ring at ∼110 au with an FWHM of ∼80 au and has a mass of (3.5 ± 0.2) × 10^-3 M _⊕. Naively, such a surprisingly small mass can be accumulated from CO photodissociation in a time as short as ∼10⁴ yr. We develop a simple model for gas production and destruction in this system, properly accounting for CO self-shielding and shielding by neutral carbon, and introducing a removal mechanism for carbon gas. We find that the most likely scenario to explain both C⁰ and CO observations is one where the carbon gas is rapidly removed on a timescale of order a thousand years and the system maintains a very high CO production rate of ∼15 M _⊕ Myr^-1, much higher than the rate of dust grind-down. We propose a possible scenario to meet these peculiar conditions: The capture of carbon onto dust grains, followed by rapid CO re-formation and rerelease. In steady state, CO would continuously be recycled, producing a CO-rich gas ring that shows no appreciable spreading over time. This picture might be extended to explain other gas-rich debris disks.