  Simple molecules like H2CO and CH3OH in protoplanetary disks are the starting
point for the production of more complex organic molecules. So far, the
observed chemical complexity in disks has been limited due to freeze out of
molecules onto grains in the bulk of the cold outer disk. Complex molecules can
be studied more directly in transitional disks with large inner holes, as these
have a higher potential of detection, through UV heating of the outer disk and
the directly exposed midplane at the wall. We use Atacama Large
Millimeter/submillimeter Array (ALMA) Band 9 (~680 GHz) line data of the
transitional disk Oph IRS 48, previously shown to have a large dust trap, to
search for complex molecules in regions where planetesimals are forming. We
report the detection of the H2CO 9(1,8)-8(1,7) line at 674 GHz, which is
spatially resolved as a semi-ring at ~60 AU radius centered south from the
star. The inferred H2CO abundance is ~10^{-8} derived by combining a physical
disk model of the source with a non-LTE excitation calculation. Upper limits
for CH3OH lines in the same disk give an abundance ratio H2CO/CH3OH>0.3, which
points to both ice formation and gas-phase routes playing a role in the H2CO
production. Upper limits on the abundances of H13CO+, CN and several other
molecules in the disk are also derived and found to be consistent with full
chemical models. The detection of the H2CO line demonstrates the start of
complex organic molecules in a planet-forming disk. Future ALMA observations
should be able to push down the abundance detection limits of other molecules
by 1-2 orders of magnitude and test chemical models of organic molecules in
(transitional) disks.
