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Astron. Astrophys. 364, 769-779 (2000)

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1. Introduction

The ISOPHOT Serendipity Survey (ISOSS) is the first large scale survey in the 200 µm band. It has mapped a substantial fraction of the sky with 3´ wide strip scans while ISO was slewing between pointed observations (Lemke et al. (1996); Bogun et al. (1996); Kessler et al. (1996)). Besides detecting large numbers of point sources, which in most cases at higher galactic latitudes are galaxies (Stickel et al. (1998a,b)), a lot of ISOSS slew measurements show extended FIR sources such as interstellar clouds and cloud cores in the nearest star forming regions. To a large extent, these galactic sources have not yet been observed beyond the IRAS 100 µm limit. The sample we may obtain analysing the complete ISOSS database is much larger than any of the previous FIR air-born (see eg. Keene 1981), sub-mm range ground-based (see eg. Launhardt et al. 1997), or balloon (see eg. Ristorcelli et al. 1998) measurements.

For sources larger than the beamsize ( [FORMULA] 2´ for ISOSS), the measured intensity directly reflects the 170 µm surface brightness of the object, after the background emission has been determined and subtracted. For all common dust models, the 170 µm surface brightness is a measure of a combination of column density and temperature of the big grain component, as long as the source is not optically thick. Since the properties of this dust are relatively well known, one other FIR wavelength is sufficient for an estimation of the dust temperature. The best data set for a comparison with ISOSS is the IRAS Sky Survey Atlas (ISSA, Wheelock et al. (1994)) at 100 µm. A combination can provide several parameters that characterize galactic molecular clouds and cold cloud cores.

Model calculations for diffuse clouds predict silicate dust grain temperatures down to [FORMULA] 15 K (Draine & Lee (1984)). Analysis of COBE data has proved the presence of a widespread 15 K cold dust component in addition to the overall 17.5 K cirrus component in the Milky Way (Lagache et al. (1998)). Due to COBE's low resolution, a spatial distinction between the two components could not be given. It is however expected that dust in diffuse clouds is slightly colder than dust of the surrounding interstellar medium (ISM), due to the attenuation of the interstellar radiation field (ISRF). In regions totally shielded from UV radiation, e.g. in dense cores, the ISM might cool down to [FORMULA] 10 K (Kr"ugel & Walmsley (1984); Benson & Myers (1989)). Reliable measurements of dust temperatures below 13 K are very rare so far (see eg. Lada et al. 1981). With an ISOSS / IRAS comparison, we can determine dust temperatures down to [FORMULA] 11 K. Below this, 100 µm emission becomes too weak (for a typical 170 µm-brightness of [FORMULA] 10 [FORMULA]) to be detected by IRAS, and only upper limits for the temperature can be given.

We have started the FIR analysis of cold clouds and cloud cores in Chamaeleon. The Chamaeleon as a test region was chosen for three reasons: (i) because previous molecular line studies revealed dense molecular clouds with and without star formation, (ii) because its ISRF is typical for the solar neighbourhood (i.e. not enhanced localy by massive star formation activity), (iii) because it is only [FORMULA] 150 pc away (Knude & Hog (1998)). At this distance, a typical 1 pc sized molecular cloud and a 0.2 pc sized molecular cloud core have apparent angular diameters of 24´ and 5´ respectively, thus can be resolved by ISSA and ISOSS observations.

Previous C18O and ISOPHOT FIR observations of the Cha I cloud (Haikala et al. (1998)) revealed cold cores with angular diameters between 4´ and 10´, typical 150 µm surface brightnesses of [FORMULA] and colour temperatures between 13 K and 16 K. All of these cores should be comfortably brighter at 170 µm than the confusion limited detection threshold of 2 [FORMULA] found for the Chamaeleon region (see below). The completeness of our search for very cold cores is thus restricted only in terms of ISOSS sky coverage, which is between 20 % and 30 % for the three Chamaeleon main clouds (even higher in the central parts) and between 10 % and 15 % in the intercloud medium.

This paper locates the regions of lowest FIR colour temperatures in Chamaeleon-Musca. We compare our FIR results with available CO observations and show that the detected cold FIR objects can be identified with dense molecular clouds, and that the cores with lowest colour temperatures are cold in terms of gas temperature too.

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001