The ISO-LWS map of the Serpens cloud core - II. The line spectra

We present spectrophotometric ISO imaging with the LWS and the CAM-CVF of the Serpens molecular cloud core. The LWS map is centred on the far infrared and submillimetre source FIRS 1/SMM 1 and its size is 8' x 8'. The fine structure line emission in [O I] 63 mum and [C II] 157 mum is extended on the arcminute scale and can be successfully modelled to originate in a PDR with G(0) = 15 +/- 10 and n(H-2) in the range of (10(4)-10(5)) cm(-3). Extended emission might also be observed in the rotational line emission of H2O and high-J CO. However, lack of sufficient angular resolution prevents us from excluding the possibility that the emssion regions of these lines are point like, which could be linked to the embedded objects SMM9/S 68 and SMM 4. Toward the Class 0 source SMM 1, the LWS observations reveal, in addition to fine structure line emission, a rich spectrum of molecular lines, superposed onto a strong, optically thick dust continuum (Larsson et al. 2000). The sub-thermally excited and optically thick CO, H2O and OH lines are tracing an about 10(3) AU source with temperatures higher than 300 K and densities above 10(6) cm(-3) (M = 0.01 M.). The molecular abundances, X = N(mol)=N(H-2), are X = (1, 0.1, 0.02, greater than or equal to 0.025) x 10(-4) for CO, H2O, OH and (CO)-C-13, respectively. Our data are consistent with an ortho-topara ratio of 3 for H2O. OH appears highly overabundant, which we tentatively ascribe to an enhanced (X-ray) ionisation rate in the Serpens cloud core (zeta >> 10(18) s(-1)). We show that geometry is of concern for the correct interpretation of the data and based on 2D-radiative transfer modelling of the disk/torus around SMM 1, which successfully reproduces the entire observed SED and the observed line profiles of low-to-mid-J CO isotopomers, we can exclude the disk to be the source of the LWS-molecular line emission. The same conclusion applies to models of dynamical collapse (inside-out infall). The 6 pixel resolution of the CAM-CVF permits us to see that the region of rotational H-2 emission is offset from SMM1 by 3000, at position angle 340degrees, which is along the known jet flow from the Class 0 object. This H2 gas is extinguished by A(V) = 4.5 mag and at a temperature of 10(3) K, which suggests that the heating of the gas is achieved through relatively slow shocks. Although we are not able to establish any firm conclusion regarding the detailed nature of the shock waves, our observations of the molecular line emission from SMM 1 are to a limited extent explainable in terms of an admixture of J-shocks and of C-shocks, the latter with speeds of about (15-20) km s(-1), whereas dynamical infall is not directly revealed by our data.