Fig. 1.
From: The 21-μm and 30-μm circumstellar dust features in evolved C-rich objects
Color-color diagram of the sources with good quality measurements in both the IRAS 12-μm and 25-μm bands, and in the MSX C (12.13 μm) and E (21.34 μm) bands, with the dashed line being a quadratic fit to these data. The selected candidates are denoted by diamond-surrounded dots. Asterisk: known strong 21-μm sources; cross: AGB stars with 10- and 20-μm silicate emission; square: O-rich PPNe with silicate emission; triangle: C-rich PPN without 21-μm emission.