NGC7023

Observations

We observed the bright reflection nebula NGC7023, at position 27" W 34" N of its illuminating star HD200775 (T$\star =$ 17000K). The beam size of the spectrometer is 14"$\times$20" or 14"$\times$27" so that it includes the brightest part of the photodissociation region (PDR). The full ISO-SWS spectrum of this object is described by Sellgren et al. (1998a) (Fig. ). We used SWS AOT01 during revolution 207. We applied a data reduction using SWS-Interactive Analysis software (last update: october 1997), giving a special care to defringing, dark current and cosmic ray impacts removal. Redundancy of the detection allows to differentiate signal and noise features: the spectrometer provides two independent scans, acquired separately with increasing and decreasing wavelengths. Both scans are not affected in the same way by glitches and so a direct comparison helps in determining real features from noise features.

General view of the spectrum

We observe several pure rotational H₂ lines in our spectrum of NGC 7023. We derive an excitation temperature of 570 $\pm$ 90 K for the warm molecular gas (Sellgren et al. 1998b). Models with gas densities of $\sim 10 ^ 6$ cm^-3 and an ultraviolet field 10 4 times the interstellar value provide the best match to the H₂ observations.

  

Figure: SWS spectrum of the reflection nebula NGC7023 in the range 2.5-25 $\mu$m.

If we look at the energy distribution in the spectrum over the entire spectral range (2.5-45 $\mu$m), we see that 60% of the emission lie in the bands and 40% in the continuum. The continuum is detected all over the mid-IR range (2.5-20$\mu$m) and increases a lot longwards of 20$\mu$m for the big grain emission. A comparison with the energy distribution of the diffuse medium as modeled by Désert et al. (1990) shows a significant dip in the continuum emission between 10 and 20 $\mu$m. This could indicate that the size class of "very small grains" are depleted compared to what would predict a constant size distribution.

Concerning the emission bands, several remarks can be done, owing to the high resolution, good quality and extended range of the spectrum:

• We do not usually observe any substructures in the main bands at 3.3-6.2-7.7-8.6-11.3-12.7 $\mu$m. This band smoothness may be due to temperature effects and/or confusion of transitions from a large number of molecular species. Temperature effects are both the intrinsic broadening of the absorption band due to the high vibrational level population (Joblin et al. 1995), and the additive contributions of emission bands with decreasing central wavelength during the cooling.
• A significant asymmetry of 6.2 and 11.3 $\mu$m is observed; it is possibly explained by anharmonic effects.
• The structure of the 7.7 $\mu$m feature complex seems very sensitive to the radiation flux, especially the ratio of the two main components 7.6 and 7.8 $\mu$m (see Boulanger et al., this conference). A third band component at 7.45$\mu$m is detected for the first time in this object.

Beyond 12 $\mu$m

Due to the atmospheric absorption, the spectral range above 12 $\mu$m was poorly known before ISO launch. In particular, it was difficult to see if the carriers of the Infrared Emission Features (IEFs) between 3 and 12 $\mu$m also possess some spectral signature at higher wavelength. The only observational investigations were of low spectral resolution and on bright and compact sources (McCarthy et al. 1978, Justtanont et al. 1996, Cox 1990). ISO made the spectroscopical exploration of this domain possible.

Also in the laboratory, the spectroscopy of interstellar analogs in this spectral range had been less studied, except for a few examples (Léger et al. 1989b, Karcher et al. 1985, Allamandola et al. 1989). Recently a more extensive study was done in the lab, in order to prepare the ISO gathering of data. Moutou et al. (1996a) showed over a sample of 40 carbonaceous aromatic species that we could expect some emission features to appear at specific positions, namely near 16.2, 18.2, 21.3 and 23.2 $\mu$m, because they are present in numerous spectra of various molecules. That means that the low-frequency modes of PAHs are not entirely characteristic of their molecular shape, but rather to a common vibrational bending of the carbonaceous skeleton, up to a certain level independent of the structure.

The spectrum displayed on Fig. contains new features, situated at 13.3, 13.6, 15.8 and 16.4 $\mu$m. The bands at 13.3 and 13.6 $\mu$m could be due to C-H vibrations for fully hydrogenated aromatic cycles, as observed in experimental spectra of PAHs (Moutou et al. 1996a). We won't discuss them more in this paper and will focus more on low-frequency modes: the next features at 15.8 and 16.4 $\mu$m are better seen in Fig. . They are both present in the independent scans at a level higher than 10 times the standard deviation over this spectral range. They are slightly asymmetrical. Their widths (approximately 0.15 $\mu$m) are comparable to the 11.3 $\mu$m feature, which agrees with a carrier of similar size and excitation mechanism.

The question is, of course, how these features observed in NGC7023 compare to the experimental spectra of PAHs. It has to be emphasized that we compare experimental absorption spectra obtained in solid phase, to emission spectra of gaseous species at high temperature, so that we cannot expect a perfect match in band profile and even in the band position (see Joblin et al. 1995).

As shown by Moutou et al. (1996a), many vibrational modes are observed in the range 600-640 cm^-1 (15.6-16.6 $\mu$m) in the 40 spectra sample of neutral PAHs. Twenty molecules show a mode in this frequency range, with varying absorption cross section and central wavelength. It seems to correspond to a specific vibrational motion of the carbonaceous skeleton of polycylic molecules. In some cases, the laboratory spectra contain additional modes between 700 and 500 cm^-1 (14.3-20 $\mu$m), than just the 16$\mu$m absorption feature. But we do not observe any related feature above the detection level in the spectrum of NGC7023, where the flux (and signal-to-noise ratio) is low up to 20$\mu$m. It is still possible that these other vibrational modes could be detected in ISO-SWS spectra, especially in sources where the mid-IR flux is higher.

It is very interesting to note that the mode near 16$\mu$m is absent in some well-known compact molecules, namely coronene (C₂₄H₁₂), pyrene (C₁₆H₁₀), peropyrene (C₂₆H₁₄) and perylene (C₂₀H₁₂). But it is strong in ovalene (C₃₂H₁₄), fluoranthene (C₁₆H₁₀), decacyclene (C₃₆H₁₈) and circumbiphenyl (C₃₈H₁₆) and exists (although weak) in hexabenzocoronene (C₄₂H₁₈) and many other more "exotic" species. From this we could see an indication that the most favourable species which may possess the signature at 15.8-16.4$\mu$m are compact molecules with more than 30 carbon atoms, and molecules containing pentagonal rings. But this observation is obviously not enough to make even a tentative identification.

The absorption spectra of some molecules are given in Fig. . The spectra are taken from Moutou et al. (1996a). In most cases shown here, the 15.8-16.4 $\mu$m band is the strongest low-frequency mode, that means it usually represents the main skeleton vibration for these species.

  

Figure: We show a comparison between laboratory absorption spectra of 9 PAH molecules, obtained in CsI pellets, and the observed spectrum of NGC7023, rebinned at a resolution power = 150. The lack of data at 16.6 $\mu$m corresponds to an instrumental artifact. Two gaussian curves are superimposed to the observed spectrum.

Except the 16$\mu$m mode, we do not detect other signatures of skeleton vibration at low frequency. It may be of interest to search for such modes in ISO spectra of sources where the mid-IR flux level is higher. It could be possible there to detect signatures of individual PAHs, as known from laboratory spectroscopy.