**Table 1:** Observed sources.
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$^{{\rm a}}$ Continuum flux at $\sim$ 6.5 $\mu$ m.
$^{{\rm b}}$ The first reference refers to the luminosity, the second to the distance.
References: 1. Van der Tak et al. (2000b); 2. Lahuis & van Dishoeck (2000); 3. Henning et al. (1990); 4. Henning et al. (1992);
5. Giannakopoulou et al. (1997); 6. Thompson et al. (1981); 7. Anthony-Twarog (1982); 8. Persi et al. (1987).

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