2000A&A...358.1035V

We present the results of ISO SWS and LWS grating scans towards the embedded Young Stellar Objects (YSOs) S106 IR and Cep A East. Emission from the pure rotational lines of H₂ and the infrared fine structure lines of [CII], [OI], [SI], [SiII] and [FeII], as well as absorption bands due to H₂O, CO and CO₂ ice were detected toward Cep A. In S106 we detected emission lines of H₂, CO, Hi, and a large number of ionized species including Fe, O, N, C, Si, S, Ne and Ar. S106 also shows many of the infrared PAH bands in emission. Excitation temperatures and molecular hydrogen masses were derived from the low-lying pure rotational levels of H₂ and are 500 and 730K and 8 and 3x10^–3M_☉ for S106 and Cep A, respectively. Since both objects are expected to have several solar masses of H₂ in their environment, we conclude that in both cases the bulk of the H₂ is cooler than a few hundred Kelvins. Excitation temperatures and line ratios were compared with those predicted by theoretical models for PDRs and dissociative and non-dissociative shocks. The [SI] 25.2 µm/[SiII] 34.8µm ratio is a particularly useful shock versus PDR discriminant and we conclude that S106 IR is dominated by PDR emission while Cep A East has a large shock component. From an analysis of the ionic lines in S106 we conclude that the central star must have a temperature around 37,000K, corresponding to a spectral type of O8. From its luminosity it is concluded that the driving source of Cep A must also be a massive early-type star. The absence of strong high-ionization ionic lines in its ISO spectrum shows that Cep A has not yet created a significant HII region and must be younger than S106, illustrating the process of the clearing of the surroundings of a massive young star.