Astron. Astrophys. 344, 342-354 (1999)

1. Introduction

The eastern, dense regions of the iuchi clouds (L 1688 or the `main cloud') are currently in a highly active state of star formation. Observations of the young stars associated with this cloud provided the empirical basis (Wilking & Lada 1983) of what has become the standard theory of low-mass star formation (Shu et al. 1987). In this paper, we present far infrared observations of the , which are unhindered by telluric atmospheric absorption.

The spatial resolution of the ISO -LWS (Infrared Space Observatory, see: Kessler et al. 1996; Long Wavelength Spectrometer, see: Clegg et al. 1996 and Swinyard et al. 1996) is comparable to a typical Jeans length in the  ¹ and the instrument appears ideal to study the energy balance of the cloud by means of spectroscopy of the major cooling lines, viz. molecular and atomic fine structure lines in the FIR (far infrared). The wide spectral range (40-200 µm) of the LWS permits simultaneously the cooling by the gas and dust components to be addressed and hence the study of the gas and dust coupling on spatial scales relevant to star formation.

The presence of early type stars which provide potential sources of external heating of the cloud can be expected to produce a PDR (Photon Dominated Region; see the review by Hollenbach & Tielens 1997). The cooling of such regions is dominated by [C II] 158 µm and/or [O I] 63 µm line emission, where the relative importance of the lines will depend on the cloud density and the flux of ultraviolet radiation. For relatively weak radiation fields, 10³ times the Habing Field (Habing 1968), weak-type PDR s should develop (Hollenbach et al. 1991) and, as such, the could be expected to present a test case of models of such interface regions in the interstellar medium.

In the present contribution, we present ISO -LWS observations of the in the far infrared (Sects. 2 and 3). The analysis of these data will naturally lead the discussion toward the topic of PDR s. However, we shall first attempt to develop an empirical model (Sects. 4.1-4.3) before comparing the results with theoretical model calculations (Sect. 4.4). In Sect. 4.5, we revisit the related issues of the mass and the star formation efficiency of the main cloud. Finally, in Sect. 5 we summarise our main conclusions from the present work.

© European Southern Observatory (ESO) 1999

Online publication: March 10, 1999
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