ISO Spectroscopy of the young bipolar nebulae S106 IR and Cep A East
Tielens, A. G. G. M.; van den Ancker, M. E.; Wesselius, P. R.
Netherlands, United States
Abstract
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 H_2 and the infrared fine structure lines of [C ii], [O i], [S i], [Si ii] and [Fe ii], as well as absorption bands due to H_2O, CO and CO_2 ice were detected toward Cep A. In S106 we detected emission lines of H_2, CO, H i, 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 H_2 and are 500 and 730 K and 8 and 3 x 10-3 M_sun for S106 and Cep A, respectively. Since both objects are expected to have several solar masses of H_2 in their environment, we conclude that in both cases the bulk of the H_2 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 [S i] 25.2 mu m/[Si ii] 34.8 mu m 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 37,000 K, 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 H ii region and must be younger than S106, illustrating the process of the clearing of the surroundings of a massive young star. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA.