The unidentified IRAS sources for which quality-3 flux density data are available from the IRAS PSC in the 12, 25, and 60 m bands and for which M spectral assignments by DJM were available were used to generate the - vs. - colour-colour diagram which is presented in Fig. 13; there are 156 sources in this category. A large number of these sources can no longer be called unidentified as they have been observed in other wavelength bands, although not always with positive detection. Thirty-eight of these have been observed in the near-IR bands by Guglielmo et al. (1993) and some of these have been classified by them as O-rich stars. Sources 06528-1405, 07231-0300, 07528-3928, 08043-3709, 09519-6007, and 10360-5633 have been identified by Kwok et al. (1997) as the variable stars GR CMa, V686 Mon, AO Pup, OY Pup, V363 Car, and CoD-56 3464, respectively. The data on these sources are presented in Table 6, in which the entries in Cols. 1-11 are: IRAS Name of the source, galactic longitude and latitude, spectral type of the source (from DJM), the IRAS magnitudes at 12, 25, and m and the (-) and where these colours are as defined by VH (refer to text), IVAR and LRS type from the IRAS PSC, respectively. The superscripts on the IRAS Name of the source in Col. 1 cite references to available literature on these sources. According to VH, the colour-colour diagram provides information on the evolutionary stage of the sources.
The IRAS sources of this study occupy the regions II, IIIa, VIa, VIb and VII (see VH). Sources in region II are classified by VH as variable stars with "young'', O-rich CSE, those in region IIIa as stars with more evolved, O-rich CSE, those in region VIa as non-variable stars with relatively cold dust shells at large distances from the star (of which a significant part are C-rich), IRAS sources in region VIb are classified as variable stars with relatively hot dust close to the star and cooler dust at large distances (some of which are known to be O-rich), and those in region VII as variable stars with more evolved, C-rich CSE. However, over 50 sources, i.e., about 1/3 of those plotted in Fig. 13, fall in region VIa and VII but have M-star photospheres as per the objective-prism classifications. As is well known due to the overlap in the colours of different spectral types in certain regions of the colour-colour diagram, it is not possible to classify the sources unambiguously as either O-rich or C-rich sources. This is in conformity with our findings based on spectral type data.
A parameter study of the spectral evolution of a typical post-AGB star with particular emphasis on the evolution of infrared colours carried out by van Hoof et al. (1997) shows that the variations of the several model parameters (considered by them) results in a variety of different paths in the IRAS colour-colour diagram. As a consequence of this, the same location in the IRAS colour-colour diagram can be occupied by objects with an entirely different evolutionary past. They therefore conclude that the location in the IRAS colour-colour diagram cannot a-priori give a unique determination of the evolutionary status of an object. This finding is in consonance with our own findings also.
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