Luminosity of the 69 μm feature (L₆₉/L_∗) compared with the I₂₃/I₆₉ ratio. Filled symbols indicate the objects for which the 69 μm feature has been detected. The numbers refer to the objects and are given in Table 2. The arrows indicate the upper limits of the 69 μm feature. The stars indicate band-strength ratios of temperature-dependent forsterite opacities multiplied by black bodies of corresponding temperatures. Thus the emission profile of optically thin dust emission is modelled and serves as a simple first comparison to the observed feature ratios.