1 Introduction

The Serpens dark cloud is currently forming a dense cluster of low to intermediate mass stars (Strom et al. 1976; Kaas 1999). This star forming complex is situated in the inner Galaxy, not very far from the direction toward the Galactic Centre ( $\ell^{\rm II}$ = 32$^{\circ }$). At the distance of 310 pc (de Lara et al. 1991), the ${\rm Serpens ~ cloud ~ core}$ is only 30 pc from the nominal galactic plane, i.e. well within the scale height of the molecular gas of about 80 pc (Dame et al. 1987). Not totally unexpected, IRAS-observations revealed intense and patchy far infrared (FIR) emission on a sloping background. Earlier observations had discovered discrete FIR sources of relatively low luminosity (Nordh et al. 1982; Harvey et al. 1984).

The empirical classification scheme developed by Lada & Wilking (1984), and later extended by André et al. (1993), is based on the Spectral Energy Distribution (SED) of the young stellar objects. In a previous paper (Larsson et al. 2000), we were discussing the SEDs of the submm-sources in the ${\rm Serpens ~ cloud ~ core}$ and for the dominating source, SMM 1 (also known as Serpens FIRS 1; Harvey et al. 1984), we reached the conclusion that its SED classifies it as Class 0. The existence of a circumstellar disk has been announced by Brown et al. (2000). An immediate question is then, whether SMM 1 shows any detectable or deducable spectroscopic evidence of disk accretion and/or of dynamical infall. Over a 0.2 pc (about 2$^{\prime}$) region toward the ${\rm Serpens ~ cloud ~ core}$, Williams & Myers (2000) reported signs of infall. Observations toward SMM 1 of molecular line profiles (Mardones et al. 1997 and Gregersen et al. 1997), have not so far been able to reveal a clear "collapse signature''. This is possibly because of confusion with the known mass outflow activity of the source (Rodríguez et al. 1989; Eiroa et al. 1992; McMullin et al. 1994; White et al. 1995; Davis et al. 1999; Williams & Myers 2000; Testi et al. 2000) and/or different velocity components in the cloud complex. On the other hand, Hogerheijde et al. (1999) found, from continuum interferometric observations, the density profile of the source to be consistent with the theoretical expectation of a collapsing cloud.

The earlier molecular line results were based on observations of low excitation transitions, which are not particularly (or not all) sensitive to the conditions expected to prevail in the deeper layers of the source. In this paper, we present spectral line data of the ${\rm Serpens ~ cloud ~ core}$, both for a spatial map and for pointed deep integrations, which contain lines also of very high excitation. These are potentially better suited to "penetrate'' to regions which were previously hidden from view. Our interpretation of the results will especially focus on the evolution of this star forming complex.

In Sect. 2, we reiterate the LWS observations and a summary is given for the data reductions, whereas a more detailed account is provided in Appendix A. The resulting line spectra are presented in Sect. 3. These results are discussed at some depth in Sect. 4. We first exploit relatively simple analytical methods, which result in spatially averaged properties. These should be useful to limit the parameter space for more sophisticated numerical modelling. This is done for the transfer of both continuum and line radiation, and these models lead to some valuable conclusions. In a summarising discussion, we make an attempt to bring these various pieces of information into a coherent physical picture. Finally in Sect. 5, we summarise our main conclusions from this work.