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Millimetre study of star formation in southern globules *
Th. Henning 1 and
R. Launhardt 1, 2
Received 28 May 1997 / Accepted 7 July 1998
The paper presents the results of a dedicated millimetre continuum and molecular line (CO and CS) search for cold and dense (protostellar) cores and molecular outflows in 35 southern Bok globules (Dec. ). Only globules which are associated with cold IRAS point sources (FIR colour temperatures below 35 K) were selected for this study.
We could demonstrate that globules are often loosely associated with molecular cloud complexes from which they probably formed. Based on such associations, we determined reliable distances for most of the globules of our sample. It turned out that half of the selected globules are located in the local spiral arm at distances between 170 and 400 pc. The most prominent features in the spatial distribution of these globules are the Lindblad ring and the Vela-Gum complex. A group of 14 globules is located at larger distances (0.7 - 4 kpc) in the Carina arm. The objects in the far Carina arm (d 2 kpc) are clearly different from "classical" Bok globules being more massive and more luminous.
Out of the 35 globules observed, all globules were detected in the 12CO(2-1) line (detection rate 100%, 3 detection limit = 0.3 K), 24 globules were detected in the CS(2-1) line (69%, detection limit 0.2 K), and 18 globules were detected in the 1.3 mm continuum emission (51%, detection limit 40 mJy/beam). In 12 globules (34%), CO line wings indicating the presence of molecular outflows have been found, of which 8 outflows were previously unkown. The colours of the embedded IRAS point sources, the strength of the millimetre dust continuum emission, the CS line temperatures as well as the presence of molecular outflows are all well correlated with each other. Based on these results and on the IRAS colour-colour diagram, we can clearly distinguish two groups of sources: Objects with active protostellar cores ("Class 0" and "I") and globules with less dense and less centrally peaked cores (pre-protostellar cores and globules which may not form stars at all).
The objects with active protostellar cores are characterized by centrally condensed cores (typical beam-averaged density of 106cm-3) and molecular outflows. The mass spectrum of the circum-protostellar envelopes can be fitted with a single slope of -1.8 between 0.15 and 2 and a mean mass of 0.6 . Two objects in our sample clearly resemble the properties of "Class 0" protostars while the majority of the star-forming cores probably already passed the main accretion phase. Five of the colder and less-condensed objects are proposed to be pre-protostellar cores.
Key words: circumstellar matter stars: formation ISM: clouds dust, extinction ISM: jets and outflows radio continuum: ISM
Send offprint requests to: R. Launhardt
Online publication: September 8, 1998