Astron. Astrophys. 338, 223-242 (1998)

1. Introduction

The study of star formation in giant molecular clouds and large dark clouds is complicated by their complex structure. Relatively isolated and compact globules come closer to the theoreticians' view of a dense and isolated cloud core undergoing spherical collapse motion. Recent studies of northern Bok globules clearly demonstrated that they are active sites of low-mass star formation (e.g. Reipurth 1983; Neckel et al. 1985; Clemens & Barvainis 1988 - hereafter C&B88; Clemens et al. 1991 - hereafter CYH91; Yun & Clemens 1992; Zhou et al. 1993; Launhardt & Henning 1997 - hereafter Paper I; Bourke et al. 1997). Many globule cores are associated with very cold far-infrared sources with colour temperatures between 20 and 40 K (C&B88, Persi et al. 1990; Bourke et al. 1995a - hereafter BHR).

In a systematic survey of 59 northern globules for their 1.3 mm dust continuum emission, we detected 21 cold condensations (Paper I). A clear correlation between the mm continuum flux densities, the infrared (IR) spectral indices, and the presence of molecular outflows was found. In general, globule cores with mm continuum emission have spectral energy distributions (SEDs) typical of dense molecular cloud cores with deeply embedded young stellar objects (YSOs) ("self-embedded" protostars). Therefore, it was not surprising that most of the outflow sources were detected at 1.3 mm continuum. In contrast to this, objects with SEDs typical of pre-main sequence (PMS) stars were in general not detected. It was shown that 5 - 7% of all globules contain protostellar cores and that the majority of the globules forms stars with a typical mass of 0.5 at some time of their evolution.

Nearly all of the previous investigations were based on the globule catalogue of Clemens & Barvainis (C&B88) which is restricted to declinations north of . A gold-mine for a systematic study of globules in the southern sky is the catalogue of 1100 dark clouds and globules compiled from the ESO/SERC J survey plates by Hartley et al. (1986). BHR presented recently a comprehensive list of small isolated Bok globules from this survey including optical and IRAS properties. For these globules, Bourke et al. (1995b) searched for emission in the (1,1) and (2,2) inversion transitions of ammonia. Half of the globules were detected, but only 6% of the sources turned out to be strong ( 0.35 K). Persi et al. (1990) surveyed 482 small southern dark clouds selected from the Hartley catalogue for evidence of YSOs and found 6% of the clouds to be associated with IRAS point sources with colour temperatures typical of YSOs and PMS stars.

In this paper we present the results of the first systematic search for mm dust continuum emission and molecular outflows from 35 southern globules with cold IRAS sources. The goal of this study was to identify globules with ongoing star formation and to derive their physical properties, such as mass, luminosity, outflow activity, and evolutionary stage. In Sect. 2, we describe the selection criteria for our target list and determine the individual distances towards the globules. The observations and data reduction are described in Sect. 3. In Sect. 4, the results are presented and physical parameters are derived from the data. In Sect. 5, we discuss the properties and the evolutionary stage of the globule cores. The main results are summarized in Sect. 6.

© European Southern Observatory (ESO) 1998

Online publication: September 8, 1998
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