This work is based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, The Netherlands, and the UK) with the participation of ISAS and NASA.

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Table 1 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?/A+A/372/173

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... (CIA)

CIA is a joint development by the ESA Astrophysics Division and the ISOCAM consortium led by the ISOCAM PI, C. Cesarsky, Direction des Sciences de la Matière, CEA, France.

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... total

The detection level is more directly related to the temporal noise than to the spatial noise. In terms of $\sigma_{\rm temp}$ , the effective completeness level is $\sim$ $12\,\sigma_{\rm temp}$ .

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... fields

S1 and SR3 are optically-visible stars of spectral types B3 and B7, respectively, which clearly emit enough UV photons to excite PAHs. The other two objects, WL16 and WL22, show PAH features in their mid-IR spectra (e.g. Moore et al. 1998), and may be young embedded early-type (i.e., early A or late B) stars.

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...1998)

We here adopt the tracks of D'Antona & Mazzitelli (1998) as they provide the most extensive set of PMS models available to date, including a tight grid of masses and ages. In the future, we plan to check the details of our results using other tracks such as those of Baraffe et al. (1998) or Palla & Stahler (1999).

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