CO destruction in protoplanetary disk midplanes: Inside versus outside the CO snow surface

Context. The total gas mass is one of the most fundamental properties of disks around young stars, because it controls their evolution and their potential to form planets. To measure disk gas masses, CO has long been thought to be the best tracer as it is readily detected at (sub)mm wavelengths in many disks. However, inferred gas masses from CO in recent ALMA observations of large samples of disks in the 1-5 Myr age range seem inconsistent with their inferred dust masses. The derived gas-to-dust mass ratios from CO are between one and two orders of magnitude lower than the ISM value of similar to 100 even if photodissociation and freeze-out are included. In contrast, Herschel measurements of hydrogen deuteride line emission of a few disks imply gas masses in line with gas-to-dust mass ratios of 100. This suggests that at least one additional mechanism is removing CO from the gas phase. Aims. Here we test the suggestion that the bulk of the CO is chemically processed and that the carbon is sequestered into less volatile species such as CO2, CH3OH, and CH4 in the dense, shielded midplane regions of the disk. This study therefore also addresses the carbon reservoir of the material which ultimately becomes incorporated into planetesimals. Methods. Using our gas-grain chemical code, we performed a parameter exploration and follow the CO abundance evolution over a range of conditions representative of shielded disk midplanes. Results. Consistent with previous studies, we find that no chemical processing of CO takes place on 1-3 Myr timescales for low cosmic-ray ionisation rates, <5 x 10(-18) s(-1). Assuming an ionisation rate of 10(-17) s(-1) more than 90% of the CO is converted into other species, but only in the cold parts of the disk below 30 K. This order of magnitude destruction of CO is robust against the choice of grain-surface reaction rate parameters, such as the tunnelling efficiency and diffusion barrier height, for temperatures between 20 and 30 K. Below 20 K there is a strong dependence on the assumed efficiency of H tunnelling. Conclusions. The low temperatures needed for CO chemical processing indicate that the exact disk temperature structure is important, with warm disks around luminous Herbig stars expected to have little to no CO conversion. In contrast, for cold disks around sun-like T Tauri stars, a large fraction of the emitting CO layer is affected unless the disks are young (<1 Myr). This can lead to inferred gas masses that are up to two orders of magnitude lower. Moreover, unless CO is locked up early in large grains, the volatile carbon composition of the icy pebbles and planetesimals forming in the midplane and drifting to the inner disk will be dominated by CH3OH, CO2 and/or hydrocarbons.