Astron. Astrophys. 350, 659-671 (1999)

1. Introduction

Bok globules are isolated dark clouds where star formation can take place (e.g. Bok & Reilly 1947, Clemens & Barvainis 1988, Clemens et al. 1991, Henning & Launhardt 1998). They can be described as the simplest molecular clouds and, for this reason, they can be considered ideal objects in which to study the formation and the first evolutionary stages of the star forming process. In particular, Bok globules are thought to host low-mass star formation as confirmed by the quite low estimated luminosities, 1-10 (e.g. Clemens et al. 1991, Launhardt & Henning 1997).

In this scenario, the globule CB3 (Clemens & Barvainis 1988; LBN594), associated with the IRAS source 00259+5625, stands out for its distance (2500 pc according Launhardt & Henning 1997, while the average distance for Bok globules is around 600 pc) and for its high luminosity: 930 taking into account the emission from 1 and 1300 µm (Launhardt et al. 1997). Thus, CB3 appears a different kind of object, because, despite classified as Bok globule, seems to be associated with a process of intermediate- or high-mass star formation. Actually, a luminosity of 930 would correspond, considering a single object, to a B3-B5 star, with a mass of about 5-6 . For this reason, while on the one hand CB3 is usually excluded when the global properties of Bok globules are summarized, on the other hand it can be considered as a precious opportunity: its investigation and its comparison with the other globules would allow to study the different star forming process leading to (i) low-mass young stellar objects (YSOs), like in the case of the majority of this kind of objects, or (ii) higher mass YSOs, like suspected for CB3.

CB3 has been recently investigated at different wavelengths, obtaining information which has allowed to draw a first draft about its nature. Observations at near-infrared (NIR; Yun & Clemens 1995, Launhardt et al. 1998), millimetre (Launhardt & Henning 1997), ISO (Infrared Space Observatory; Launhardt et al. 1998) and submillimetre wavelengths (Launhardt et al. 1997) have revealed continuum emission at slightly different positions suggesting the occurrence of several sites of star formation. Wang et al. (1995) have investigated this globule through C¹⁸O and H₂CO observations, concluding that CB3 is a candidate globule for collapse. Moreover, Yun & Clemens (1992, 1994) discovered and mapped a CO J = 1-0 outflow associated with the CB3 globule, while Launhardt et al. (1999) reported a CS J = 2-1 map. Codella & Muders (1997) obtained an high resolution SO = 1₀-0₁ map, showing that CB3 is a strong SO source and a good laboratory where to study the sulphur chemistry, usually associated with the star forming process.

With this project we have further investigated the CB3 globule through a multiline survey at mm-wavelengths, which allow to probe a wide range of physical conditions. The main aims of this work are: (i) to carefully investigate the structure of the molecular outflow through high spatial and spectral resolution CO observations, (ii) to trace the high-density ambient gas (through CS and H¹³CO⁺) and the highest excitation conditions of the outflow motion (SiO and CH₃OH), (iii) to carry out a systematic study of the distribution of the molecular species thought to be associated with shock-chemistry, taking a detailed look at the sulphuretted molecules whose chemistry is far to be completely understood (SO, SO₂, H₂S, OCS and H₂CO). We aimed to find a relationship of spectral and/or positional feature of the observed molecules with the evolutionary stage of a star forming region (SFR). In addition, the indication of the production of sulphur molecules at the bow shocks located at the end points of young molecular outflow (e.g. Martín-Pintado et al. 1992, Chernin et al. 1994, Bachiller & Peréz Gutiérrez 1997a) will be verified. Finally, we will try to use as chemical clocks some abundance ratio such as SO/H₂S, SO₂/H₂S and OCS/H₂S in order to study the evolutionary stage of bipolar outflows, as predicted by the models of Charnley (1997).

© European Southern Observatory (ESO) 1999

Online publication: October 4, 1999
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