3. The 15-20 µm plateaus
Many of the observed sources in our samples show a broad plateau between 15 and 20 µm. Fig. 1 shows two examples in which the plateau is clearly present.
To be able to compare the plateaus observed in different sources we locally subtract a spline continuum. The rising dust continuum and the presence of features at both shorter and longer wavelength give some freedom in drawing a continuum. This mostly affects the strength of the plateau, but has only a minor influence on the shape. We estimate that this introduces an uncertainty of, at most, 30 in the plateau strength. An exception to this is the young planetary nebula NGC 7027 where the presence of a strong 30 µm feature at the long wavelength side of the plateau makes the extent and the strength of the plateau more difficult to determine.
3.2. The shape of the plateaus
The continuum subtracted profiles are shown in Fig. 2. Overall, the profile of the plateau is very similar between the different sources (see Fig. 3). However, when examined in detail, some small, but significant differences become apparent. Three sources show a distinct feature at 16.4 µm (NGC 7027, CD 42, IRAS 03260; Fig. 4), which may be present in others as well (IRAS 15384, S 106, IRAS 23133, IRAS 18502), but is not apparent in all. This feature has also been observed by Moutou et al. (1999) in the reflection nebula NGC 7023. The latter paper also reports a feature at 15.8 µm. While this feature does not stick out immediately in our sample, comparison of the profiles in IRAS 03260, IRAS 23133 and S 106 suggests the presence of a 15.8 µm feature in the latter two. The presence of a dip in the plateau of NGC 7027 at 17.5 µm suggests the presence of distinct components around 17 µm and perhaps 18 µm. We note that around this wavelength, CD 42 and IRAS 23133 show a weak emission feature. The profile of the plateau may also differ around 19 µm but that depends to some extent on the adopted continuum, which is not always well established.
Sources which show the PAH bands in their spectra, often show a plateau of emission extending from 15 to about 20 µm. The plateau may well be present in all the sources of our PAH samples but difficult to recognize because of strong, rising dust continua in some and the presence of spectral structure due to crystalline silicates in others. However, the 15-20 µm emission plateau is not detected in sources characterized by only O-rich dust such as O-rich AGB stars (Molster et al. 1999; Sylvester et al. 1999; Voors 1999) and we conclude that the carrier of this plateau is carbonaceous in origin. Finally, from the observed spectral variations in our sample, we interpret this plateau as a collection of blended emission features.
3.3. Integrated strength of the plateaus
The H ii regions of the sample have strong [Ne iii] and [S iii] lines at 15.55 and 18.7 µm. These lines are removed prior to the integrated strength measurements. The intensities listed in Table 2 are obtained by integrating the continuum subtracted fluxes between 15.2 and 20 µm . Perusal of this table shows that the integrated strength of the plateau is highly variable relative to the C-C stretching mode at 6.2 µm ( = 0.6-4.4) and the C-H out of plane bending mode at 11.2 µm ( = 0.5-5.5; see Fig. 1).
Table 2. Integrated strength of the features
© European Southern Observatory (ESO) 2000
Online publication: June 5, 2000