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 , H _ { 2 } CO , 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 ( H _ { 2 } CO 3 _ { 03 } –2 _ { 02 } , H _ { 2 } CO 3 _ { 22 } –2 _ { 21 } , and H _ { 2 } CO 3 _ { 21 } –2 _ { 20 } ) in the protoplanetary disk around the Herbig Ae star HD 163296 with ALMA at \sim 0.5 \arcsec ( 60 AU ) spatial resolution .
Different parameterizations of the H _ { 2 } CO abundance were compared to the observed visibilities , using either a characteristic temperature , a characteristic radius or a radial power law index to describe the H _ { 2 } CO chemistry .
Similar models were applied to ALMA Science Verification data of C ^ { 18 } O . \chi ^ { 2 } minimization on the visibilities was used to determine the best-fit model in each scenario .

Results : H _ { 2 } CO 3 _ { 03 } –2 _ { 02 } was readily detected via imaging , while the weaker H _ { 2 } CO 3 _ { 22 } –2 _ { 21 } and H _ { 2 } CO 3 _ { 21 } –2 _ { 20 } lines required matched filter analysis to detect .
H _ { 2 } CO is present throughout most of the gaseous disk , extending out to \sim 550 AU .
An apparent 50 AU inner radius of the H _ { 2 } CO emission is likely caused by an optically thick dust continuum .
The H _ { 2 } CO radial intensity profile shows a peak at \sim 100 AU and a secondary bump at \sim 300 AU , suggesting increased production in the outer disk .
In all modeling scenarios , fits to the H _ { 2 } CO data show an increased abundance in the outer disk .
The overall best-fit H _ { 2 } CO model shows a factor of two enhancement beyond a radius of 270 \pm 20 AU , with an inner abundance ( relative to H _ { 2 } ) of 2 - 5 \times 10 ^ { -12 } .
The H _ { 2 } CO emitting region has a lower limit on the kinetic temperature of T > 20 K. The C ^ { 18 } O modeling suggests an order of magnitude depletion of C ^ { 18 } O in the outer disk and an abundance of 4 - 12 \times 10 ^ { -8 } in the inner disk .

Conclusions : There is a desorption front seen in the H _ { 2 } CO emission that roughly coincides with the outer edge of the 1.3 millimeter continuum .
The increase in H _ { 2 } CO 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 , or more efficient gas-phase production beyond \sim 300 AU if CO is photodisocciated in this region .