Q1 parameter versus K_s −  [8.0] . Symbols are as in Fig. 1. In the top panel the GLIMPSE 8 μm measurements are plotted, while in the bottom panel the MSX 8 μm measurements (A-band) are plotted. The arrow represents the reddening vector following the extinction ratios given by Messineo et al. (2005) and Indebetouw et al. (2005). The dashed lines are a linear fit to the Q1 versus the dereddened (K_s −  [8.0] ) values of SiO masing stars (Messineo 2004) for A_K = 0, 1, 2 mag (from bottom to top). Typical errors are 0.05 mag in the x-axis and 0.14 mag in the y-axis.