In Fig. 4 several often-used colour-colour-diagrams are plotted. In principle they can be used to distinguish between oxygen-rich and carbon-rich objects. The lines in the and colour diagram indicate sequences for Galactic AGB stars that are based on our own work in the case of and taken from van Loon et al. (1997) for . There is at least a scatter of 1 magnitude around each of these relations, basically limiting the use of these two colour-colour diagrams as O-/C-separators.
The region where O-rich (indicated by O) and C-rich stars (indicated by C) exist in the colour-colour diagram is taken from Epchtein et al. (1987; and in magnitudes). The region occupied by C-stars in the diagram was determined by us from Galactic AGB stars. In all these four diagrams the loci and regions are valid for AGB stars with uninterrupted mass-loss. Post-AGB stars, or AGB stars with detached dust shells, can occupy different positions in these diagrams than indicated by their chemistry.
There are some obvious outliers, in particular the known carbon-rich objects S2, S12 and S14 in the and diagrams. S2 was suggested by WFMC to be a possible post-AGB star and the colours are at least consistent with that: the far-infrared dust shell is still red, but the near-infrared colours are relatively blue. For S14 something similar may be the case. Since S12 has a normal position in the diagram, its unusual colour may be due an overestimated 25 µm flux by a factor of 30. This is, admittedly, an unusually large factor.
Based on the location in the colour-colour diagrams one may hypotesize that S4, 16, 24 and 25 are carbon-rich and S27 and 28 are oxygen-rich.
© European Southern Observatory (ESO) 1998
Online publication: March 10, 1998