The mass fractions of crystalline material compared to the effective temperature of the central stars (which correlates closely with the bolometric luminosity, see Fig. 3). The crystallinities are derived from the 7–17 μm part of the IRS spectra. b) similar to a), except that the crystallinities are derived from in 17–37 μm part of the IRS spectra. The symbols are identical to those in Fig. 14.