P.J. Richards 1, S.C. Russell 2, & L.B.G. Knee 3
1 CLRC, Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, UK
2 39 Ridgeland Drive, Teringie SA 5072, Australia
3 National Research Council of Canada
4 Herzberg Institute of Astrophysics
5 Dominion Radio Astrophysical Observatory, P.O.Box 248, Penticton, BC, V2A 6K3, Canada
The youngest pre-stellar objects are expected, in the inside-out scenario,
to be collapsing high density cores of dust and gas surrounded by relatively
static envelopes of matter (Shu, 1977 ApJ 214 p488; Zhou, 1992 ApJ 394 p204).
Therefore, due to their optical appearance, Bok globules are of great interest
in the study of star formation, as they are possible candidates for harboring
such objects. The low temperatures and high column densities of gas and dust
in these regions only allow detection in the FIR. Although IRAS has detected
YSOs in a number of globules (Yun & Clemens, 1990 ApJ, 367 L76), it had
insufficient sensitivity and resolution to positively identify pre-stellar
objects. ISOPHOT provides better resolution and spectral coverage in the FIR
and a survey of 21 optically selected dark clouds without any associated IRAS
point sources has been carried out as part of the ISOPHOT Guaranteed Time
programme. The results from a preliminary analysis of these data, based on
single short scans across the globules at 100 and 200 microns, has already been
presented (Russell et al, 1998, Star Formation with ISO, p214), but was
restricted to an investigation of the morphology of the FIR emission. As part
of this programme, further observations, consisting of 10'x10' maps at 100
and 200 microns with ISOPHOT using the P22 observing mode, have been made of
three globules CB202, CB220 and CB236. The initial single short scans across
these globules have yielded detections of cold dust at 200 microns, but, due
to the limited field of view, it has not been possible to determine the total
flux from these small extended sources. These additional observations over a
larger region around the globules has enabled the whole source to be
identified relative to the background. This, together with the better coverage
of the surrounding background, provides accurate dust temperatures for these
sources. The results of a study of the FIR emission from the regions
surrounding the globules will also be presented.