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Issue A&A
Volume 386, Number 3, May II 2002
Page(s) 1055 - 1073
Section Diffuse matter in space
DOI 10.1051/0004-6361:20020168



A&A 386, 1055-1073 (2002)
DOI: 10.1051/0004-6361:20020168

The ISO-LWS map of the Serpens cloud core

II. The line spectra
B. Larsson1, R. Liseau1 and A. B. Men'shchikov1, 2

1  Stockholm Observatory, SCFAB, Roslagstullsbacken 21, 106 91 Stockholm, Sweden
    e-mail: bem@astro.su.se; rene@astro.su.se
2  Max-Planck-Institut für Radioastronomie, Auf dem Hügel, Bonn, Germany
    e-mail: sasha@mpifr-bonn.mpg.de

(Received 30 November 2001 / Accepted 29 January 2002 )

Abstract
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 $^{\prime}$ $\times$ 8 $^{\prime}$. The fine structure line emission in [O I] 63 $\mu$m and [C II] 157 $\mu$m is extended on the arcminute scale and can be successfully modelled to originate in a PDR with $G_0 = 15 \pm 10$ and n(H 2) in the range of  cm -3. Extended emission might also be observed in the rotational line emission of H 2O 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 SMM 9/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, H 2O 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 $M_{\odot}$). The molecular abundances, $X=N({\rm mol})/N$(H 2), are $X=(1,\,0.1,\,0.02,\,\ge$ $0.025) \times 10^$ for CO, H 2O, OH and 13CO, respectively. Our data are consistent with an ortho-to-para ratio of 3 for H 2O. OH appears highly overabundant, which we tentatively ascribe to an enhanced (X-ray) ionisation rate in the ${\rm Serpens \, cloud \, core}$ ( $\zeta \gg 10^\,{\rm s}^$). 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 $^{\prime \prime}$ pixel resolution of the CAM-CVF permits us to see that the region of rotational H 2 emission is offset from SMM 1 by 30 $^{\prime \prime}$, at position angle 340 $^{\circ}$, which is along the known jet flow from the Class 0 object. This H 2 gas is extinguished by $A_{\rm V}$ = 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.


Key words: ISM: individual objects: ${\rm Serpens \, cloud \, core}$, FIRS 1/SMM 1 -- ISM: abundances -- ISM: molecules -- ISM: clouds -- ISM: jets and outflows -- stars: formation

Offprint request: B. Larsson, bem@astro.su.se

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