Discovery of OH in circumstellar disks around young intermediate-mass stars

We detect emission from multiple low-excitation rovibrational transitions of OH from the two Herbig Ae stars AB Aurigae and MWC 758 in the 3.0-3.7 mu m wavelength range (L band), using the NIRSPEC instrument on Keck II. The inner radius for the emitting region in both stars is close to 1 AU. We compare an optically thin LTE model and a thin-wedge fluorescence model, finding rotational temperatures of 650-800 K and OH abundances of 10(42)-10(45) molecules for the two stars. Comparisons with current chemical models support the fluorescence excitation model for AB Aurigae and possibly MWC 758, but further observations and detailed modeling are necessary to improve constraints on OH emission in different disk environments.