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

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3. Results

3.1. Dust colour temperature

We can differentiate between regions with different temperatures by their appearance in the ISOSS/IRAS scatter plot (Fig. 1) i.e. by the distance of the corresponding data points to the general correlation line. Data points with [FORMULA] [FORMULA] 5 [FORMULA] above or below this line, (i.e. outside the strip defined by the foreground/background galactic emission) belong to regions with colder or warmer dust respectively. Individual linefits to groups of these outlier points gave slopes of 1.6, 2.6 and 4.5, corresponding to dust colour temperatures of 19.3 K, 16.3 K, and 13.9 K respectively. The lack of a continuous temperature range in the strong FIR emission regime is remarkable. For [FORMULA] [FORMULA] 20 [FORMULA] or [FORMULA] [FORMULA] 50 [FORMULA] there is no indication for colour temperatures in between 16.3 K and 19.3 K. This is an effect of the geometry of the field: Physically separated regions of warm and cold dust are seldom projected onto each other. The bright, compact sources with warm dust discussed below are too small to influence this global picture (see Fig. 1b). The cold regions are the Chamaeleon main clouds Cha I, Cha II, Cha III. They are projected on the somewhat warmer galactic background. However, for [FORMULA] [FORMULA] 50 [FORMULA] the cold Chamaeleon clouds are dominating the FIR emission. The regions with warm dust ([FORMULA] K) on the other hand belong predominantly to the near-galactic plane region ([FORMULA]). Here the line of sight penetrates deeply into the galactic plane, causing intensities [FORMULA] [FORMULA] 20 [FORMULA] and a dominance of the emission from regions with warm dust over the galactic foreground. If the Chamaeleon clouds were projected onto the galactic plane, Fig. 1a would look different.

The colour temperature map of the central 11oee [FORMULA] 8oee, covering the main clouds Cha I, Cha II and Cha III, is shown in Fig. 3. The whole map shows temperatures below 17 K except for some small warm sources in Cha I (see below). Extended warmer regions are found only in the north east of the large 20oee [FORMULA] 20oee field (see Fig. . 1), i.e. close to the galactic plane.

[FIGURE] Fig. 3. Dust colour temperature map of the central part of the investigated field, derived from an ISOSS 170 µm slew image and the 100 µm ISSA maps. A FIR background/foreground of 20 [FORMULA] and 8 [FORMULA] respectively was subtracted according to Fig. . 1. The slews are represented by 15´ wide stripes, their colors show the colour temperature variation (see bar on top). Overlayed are the contours of the 13CO (J=1[FORMULA]0) line intensity at 1.0 K km s-1 and 3.5 K km s-1 from the survey of Mizuno et al. (1998). Dust colder than 14 K is found only inside the 1.0 K km s-1 contour, i.e. is associated with molecular clouds. The small white ellipses indicate the positions of the very cold cores (Sect. 3.2). They are all associa ted with dense molecular gas (13CO line intensities [FORMULA] 3.5 K km s-1).

Cold dust [FORMULA] 15 K appears over the whole Cha I cloud (DCld 297.2-15.6), and for the most part of Cha II (DCld 303.0-14.3) and Cha III (DCld 303.0-17.1). The dust of the latter clouds is not quite as cold as in Cha I. While this could be an artefact of a higher galactic background, the cold source search (Sect. 3.2) shows a similar trend, an indication that the subtle temperature differences are real. Cold dust is as well found in G298-13, ([FORMULA]) G295-17 ([FORMULA]), (Boulanger et al. (1998)), and in G295-13 ([FORMULA]), the Musca filament, the latter one outside the region of Fig. 3. Altogether, about 3 % of the area measured shows colour temperatures below 15 K.

An [FORMULA] sized warm source is seen in Cha I North at [FORMULA], with mean brightnesses of [FORMULA] and [FORMULA]. The object is extended perpendicularly to the CO outflow lobes detected by Mattila et al. (1989). It is associated with with an [FORMULA] [FORMULA] 15 mag opaque spot (Jones et al. (1985)) and with the bright nebula CED 112 (Cederblad (1946)) Another prominent warm source, at [FORMULA], is associated with IRAS 11072-7727 also cited as Chamaeleon Infrared Nebula (Cha IRN) (see Schwartz & Heinze (1983)).

3.2. Cold FIR sources

We located 376 ISOSS peaks to be called hereafter ISOSS source s and derived their CP colour parameter in the region RA=[[FORMULA], [FORMULA]] Dec=[[FORMULA], [FORMULA]] with the following criteria:
(i) [FORMULA] (i.e. [FORMULA] rms noise),
(ii) [FORMULA] and
(iii) [FORMULA]slope[FORMULA]slope[FORMULA] (i.e. a reliable bisector slope).

The size and colour distributions of these 376 ISOSS sources are shown in Fig. 4. The sizes of the ones with [FORMULA] are similar to our angular resolution, indicating that most of these objects have a linear size of [FORMULA] 0.1 pc. The colour parameter has an asymmetric distribution. This is partly a result of the method being designed to find cold sources. The distribution has a median of [FORMULA] and a prominent tail up to [FORMULA] corresponding to dust temperatures of 14.5 K and 11.6 K, 86 % of the findings have colour parameters in the range [FORMULA], and 7 % of the 376 ISOSS sources have [FORMULA]. We excluded all the [FORMULA] ISOSS sources which were associated by cosmics or faulty readouts. Also the source position was refined for these very cold ones merging ISOSS sources which were multiple detections of the same object. The ISOSS sources with [FORMULA] have not been checked by eye systematically, but only in individual regions of interest (e.g. all the C18O cores discussed in Sect. 4).

[FIGURE] Fig. 4. Full width at half maximum (FWHM) and colour parameter (CP) distributions of the 376 ISOSS sources in Chamaeleon. The FWHM histogram of CP [FORMULA] sources is overlaid with dashed line and shaded. The CP distribution has a median of CP = 3.6 and a prominent tail up to CP = 9, corresponding to dust temperatures of 14.8 K and 11.6 K.

The [FORMULA] ISOSS sources show a random distribution, while the colder ones cluster at the main clouds Cha I, Cha II and Cha III. There were 28 very cold ISOSS source with [FORMULA]. Searching for the coldest objects of Chamaeleon we required that [FORMULA] to ensure that the ISOSS source was detected in 100 µm as well and thus CP reflects a colour temperature. This way we located 6, 2, 1 very cold objects inside the Cha I, Cha II, Cha III molecular clouds respectively. For reasons discussed below, we call these very cold objects very cold cores (VCCs). The parameters of the VCCs are listed in Table 1, where the columns are: (1) Core number, for easier identification throughout this paper, (2) ISOSS name, (3-4) equatorial coordinates, (5-7) major and minor diameters and position angle (PA) of the fitted ellipses, (8) colour parameter where the "c" index stands for "confirmed", (9-10) galactic coordinates, (11) optical association (Hartley et al. (1986)). Six of the nine VCCs are double [FORMULA] detections, i.e. merged pairs of cold ISOSS sources. The criteria of merging were that the ISOSS sources are closer than [FORMULA], appearing as crossings of the same IRAS intensity peak. To be classified as a VCC they should have an averaged CP larger than 7. One of the VCCs with "confirmed" CP value is ISOSS 11102-7648 i.e. our VCC no. 7, which was crossed by two slews both showing a [FORMULA] source there. These 6 pairs are considered to have a confirmed colour parameter, which we indicate with a "c" index in column (8). The CP values of these cores marked with "c" are more reliable than the rest of the list.


Table 1. Positions and FIR parameters of the very cold cores in Chamaeleon. [FORMULA] PA is the position angle of the major axis, measured anti-clockwise from north through east, "c" marks the CP values based on more than 1 slew measurement.

The shape of the cores was determined from the 170 µm profiles and eyeball checks of the 100 µm ISSA maps. These bias effected the values in columns (5 - 7) mostly for the single detection VCCs: no. 2, no. 6, no. 9. Fig. 5 shows the positions of the very cold objects of Cha I, overlaid on an ISOSS 170 µm slew composite map.

[FIGURE] Fig. 5. ISOSS 170 µm slew composit map of Cha I, smoothed to the lower 5´ IRAS resolution. The intensities are showed for 15´ wide stripes along slews. Intensities for many interslew positions were interpolated. We note that the actual coverage is about 3 times less than suggested by this smoothed image. Contours are drawn at [FORMULA] = 50, 75, 100, 125 [FORMULA]. Overlaid are the positions of the very cold cores, as given in Table 1.

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001