Increased H2CO production in the outer disk around HD 163296

Context. The gas and dust in circumstellar disks provide the raw materials to form planets. The study of organic molecules and their building blocks in such disks offers insight into the origin of the prebiotic environment of terrestrial planets. Aims. We aim to determine the distribution of formaldehyde, H2CO, in the disk around HD 163296 to assess the contribution of gas-and solid-phase formation routes of this simple organic. Methods. Three formaldehyde lines were observed (H2CO 3(03)-2(02), H2CO 3(22)-2(21), and H2CO 3(21)-2(20)) in the protoplanetary disk around the Herbig Ae star HD 163296 with ALMA at similar to 0.5 '' (60 AU) spatial resolution. Different parameterizations of the H2CO abundance were compared to the observed visibilities, using either a characteristic temperature, a characteristic radius or a radial power law index to describe the H2CO chemistry. Similar models were applied to ALMA Science Verification data of (CO)-O-18. In each scenario, chi(2) minimization on the visibilities was used to determine the best-fit model in each scenario. Results. H2CO 3(03)-2(02) was readily detected via imaging, while the weaker H-2CO 3(22)-2(21) and H2CO 3(21)-2(20) lines required matched filter analysis to detect. H2CO is present throughout most of the gaseous disk, extending out to similar to 550 AU. An apparent 50 AU inner radius of the H2CO emission is likely caused by an optically thick dust continuum. The H2CO radial intensity profile shows a peak at similar to 100 AU and a secondary bump at similar to 300 AU, suggesting increased production in the outer disk. In all modeling scenarios, fits to the H2CO data show an increased abundance in the outer disk. The overall best-fit H2CO model shows a factor of two enhancement beyond a radius of 270 +/- 20 AU, with an inner abundance (relative to H-2) of 2-5 x 10(-12). The H2CO emitting region has a lower limit on the kinetic temperature of T > 20 K. The (CO)-O-18 modeling suggests an order of magnitude depletion of (CO)-O-18 in the outer disk and an abundance of 4-12 x 10(-8) in the inner disk. Conclusions. There is a desorption front seen in the H2CO emission that roughly coincides with the outer edge of the 1.3 millimeter continuum. The increase in H2CO outer disk emission could be a result of hydrogenation of CO ices on dust grains that are then sublimated via thermal desorption or UV photodesorption. Alternatively, there could be more efficient gas-phase production of H2CO beyond similar to 300 AU if CO is photodisocciated in this region.