5. The size distribution of interstellar PAHs
The observed spectrum of IR emission features can be used to determine the sizes and abundances of the emitting species (ATB). Here, we are particularly interested in the observed variations from source to source in the strength of the 15-20 µm plateau relative to the PAH bands. These variations span a factor 10 (see Table 2) which is much larger than expected from laboratory or theoretical calculations on the intrinsic strength of modes in this wavelength region. Here we use a simple model to show that these variations can be attributed to changes in the size distribution.
Theoretical calculations have shown that PAHs with sizes up to C-atoms can contribute to the emission in the 15-20 µm range (Schutte et al. 1993). Typically, we expect the emission in the 15-20 µm plateau to be dominated by those PAHs/PAH-clusters whose temperature upon absorption of a UV photon is best matched to this wavelength region (ie., ; ATB). For comparison, the bands at 3.3, 6.2, 7.7 and 11.2 µm are carried by PAHs in the size range 20 to a few hundred, while the 6-9 µm plateau underneath the 6.2 and 7.7 µm features is mainly due to PAHs with 500 C-atoms (ATB). Adopting standard dust parameters and measured UV absorption cross sections for the PAHs (ATB), the abundance of carbon, , locked up in the carriers of given spectral features - ie., PAHs of different sizes - can be derived from the ratio of the flux in these features to the total dust continuum, ,
where is the UV absoption cross section per C-atom ( cm-2 per C-atom (Joblin et al. 1992)) and the abundance is given in parts per million relative to H. We have calculated these fractional abundances for the carriers of the PAH bands, 6-9 µm plateau and 15-20 µm plateau for the sources presented in this paper using our own SWS data and the LWS data obtained for these sources through parallel ISO programs. The results are presented in Fig. 7 for the two extremes in our sample, NGC 7027 and S 106. The PAH emission in NGC 7027 originates from a region of (Graham et al. 1993), completely contained within the SWS beam. S 106 is somewhat extended compared to the SWS beam but analysis of ISOCAM images in the PAH features suggests that only a small fraction of the flux in the PAH bands is missing in the SWS spectrum (Joblin et al. in preparation). The Far-IR dust emission is well contained within the LWS beam.
The abundance of carbon locked up in small (20-100) PAHs is calculated to be an order of magnitude higher in NGC 7027 than in S 106 (Fig. 7). Not only do the absolute abundances differ, the relative abundance from big to small PAHs changes even more between the two objects. It seems that, where NGC 7027 has most of its carbon locked up in small PAHs, these little ones are less important in S 106. The increased importance of large species is readily apparent when comparing the observed spectrum of S 106 with that of other sources (see Fig. 2). Although there are variations from source to source, most of the H II regions have abundances and abundance patterns similar to S 106. In contrast, the YSOs, CD 42 and IRAS 03260, have considerably higher abundances of small PAHs relative to big PAH clusters, and resemble the PN, NGC 7027, in that respect. These variations in the size distribution of small species may reflect the extent to which coagulation has progressed during the formation of these different classes of sources. In particular, the Herbig AeBe stars, CD 42 and IRAS 03260, were probably formed from molecular cloud cores which were less dense than those that formed the much more massive stars at the center of H II regions. Coagulation may also have played less of a role in the ejecta of NGC 7027. We note that this interpretation of the observed variations in plateau to PAH band strength in terms of variations in the size distribution is supported by the assignment of the 16.4 µm band - so prominent in NGC 7027, CD 42 and IRAS 03260 - to small PAHs (cf., Sect. 4).
© European Southern Observatory (ESO) 2000
Online publication: June 5, 2000