Detection of steam in the circumstellar disk around a massive Young Stellar Object

We report on the observation of hot water vapor ( steam) in the inner AU of a young massive star located in the star-forming region IRAS 08576-4334. The water lines are detected in a medium resolution ( R similar to 10 000) K-band spectrum taken by the infrared spectrometer ISAAC mounted on the VLT-ANTU. The water vapor is at a mean temperature of 1565 +/- 510 K, cooler than the hot CO found in the same object, which is at similar to 1660 K; and the column density is N( H2O) = ( 2.5 +/- 0.4) x 10(18) cm(- 2). The profile of both H2O and CO lines is best reproduced by the emission from a Keplerian disk. To interpret the data, we also investigate the formation of molecules and especially CO and water vapor in the inner hot and dense part of disks around young high mass stars using a pseudo time-dependent gas-phase chemical model. Molecules are rapidly photodissociated but this destruction is compensated for by an efficient formation due to fast neutral-neutral reactions. The ability of CO molecules to self-shield significantly enhances its abundance. Water molecules are sufficiently abundant to be detectable. The observed H2O/CO ratio is reproduced by gas at 1600 K and by an enhanced UV field over gas density ratio I-UV/n(H) = 10(-4)- 10(- 6). The simulations support the presence of CO and H2O molecules in the inner disks around young massive stars despite the strong UV radiation and show that the OH radical plays an essential role in hot gas chemistry.