Search

Context. Simple molecules such as 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 because of freeze-out of molecules onto grains in the bulk of the cold outer disk. Aims. Complex molecules can be studied more directly in transitional disks with large inner holes because these have a higher potential of detection through the UV heating of the outer disk and the directly exposed midplane at the wall. Methods. We used Atacama Large Millimeter/submillimeter Array (ALMA) Band 9 (~680 GHz) line data of the transitional disk Oph IRS 48, which was previously shown to have a large dust trap, to search for complex molecules in regions where planetesimals are forming. Results. 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 indicates that both ice formation and gas-phase routes play a role in the H2CO production. Upper limits on the abundances of H13CO+, CN and several other molecules in the disk were also derived and found to be consistent with full chemical models. Conclusions. The detection of the H2CO line demonstrates the start of complex organic molecules in a planet-forming disk. Future ALMA observations are expected to reduce the abundance detection limits of other molecules by 1-2 orders of magnitude and test chemical models of organic molecules in (transitional) disks. [ABSTRACT FROM AUTHOR]