A&A 370, 1030-1043 (2001)
S. Hony1 - C. Van Kerckhoven2 - E. Peeters3,4 - A. G. G. M. Tielens3,4 - D. M. Hudgins5 - L. J. Allamandola5
1 - Astronomical Institute `Anton Pannekoek',
Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
2 - Instituut voor Sterrenkunde, K. U. Leuven, Celestijnenlaan 200B, 3001 Heverlee, Belgium
3 - SRON Laboratory for Space Research Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
4 - Kapteyn Astronomical Institute PO Box 800, 9700 AV Groningen, The Netherlands
5 - NASA/Ames Research Center, MS:245-6, Moffett Field, CA 94035-1000, USA
Received 2 August 2000 / Accepted 12 February 2001
We present 10-15 m spectra of a sample of H II regions, YSOs and evolved stars that show strong unidentified infrared emission features, obtained with the ISO/SWS spectrograph on-board ISO. These spectra reveal a plethora of emission features with bands at 11.0, 11.2, 12.0, 12.7, 13.5 and 14.2 m. These features are observed to vary considerably in relative strength to each-other from source to source. In particular, the 10-15 m spectra of the evolved stars are dominated by the 11.2 m band while for H II regions the 12.7 is typically as strong as the 11.2 m band. Analysing the ISO data we find a good correlation between the 11.2 m band and the 3.3 m band, and between the 12.7 m and the 6.2 m band. There is also a correlation between the ratio of the UIR bands to the total dust emission and the 12.7 over 11.2 m ratio. Bands in the 10-15 m spectral region are due to CH out-of-plane (OOP) bending modes of polycyclic aromatic hydrocarbons (PAHs). We summarise existing laboratory data and theoretical quantum chemical calculations of these modes for neutral and cationic PAHs. Due to mode coupling, the exact peak position of these bands depends on the number of adjacent CH groups and hence the observed interstellar 10-15 m spectra can be used to determine the molecular structure of the interstellar PAHs emitting in the different regions. We conclude that evolved stars predominantly inject compact 100-200 C-atom PAHs into the ISM where they are subsequently processed, resulting in more open and uneven PAH structures.
Key words: circumstellar matter - stars: pre-main sequence - H II regions - ISM: molecules; - planetary nebulae: general - infrared: ISM: lines and bands
Laboratory spectroscopy of PAHs shows that, besides the well known UIR bands, PAHs exhibit many weaker emission bands. In particular, the region from 10 to 15 m has a rich spectrum due to the out-of-plane bending vibrations (OOP) of aromatic H-atoms. The peak wavelength of these modes depends on the structure of the molecule; in particular on the number of neighbouring H-atoms per ring (e.g. Bellamy 1958; Hudgins & Allamandola 1999). Here we present data in this region obtained with the Short Wavelength Spectrometer (SWS) (de Graauw et al. 1996) on-board the Infrared Space Observatory (ISO) (Kessler et al. 1996). The sensitivity and medium resolving power of the instrument allows us to detect and resolve several weak features predicted by the PAH hypothesis and to determine the molecular structure of the emitting PAHs and their evolution in space.
In Sect. 2, we present the observations of our sample of H II regions, YSOs, reflection nebulae (RNe) and evolved objects. The 10-15 m regions of these sources are analysed in Sect. 3. The spectral characteristics of PAHs in this wavelength range as measured in the laboratory and calculated by quantum chemical theories are summarised in Sect. 4.1. In Sect. 4.2 the laboratory spectra are compared to the observed spectra. The molecular structures implied by the observed spectra are discussed in Sect. 5 while in Sect. 6 the origin and evolution of these molecular structures are examined. Finally in Sect. 7 our main results are summarised.
|Figure 1: Spectra of 3 sources that show features in the region of interest. The dashed lines are the continua mentioned in the text|
|Mode||(J2000)||(J2000)||(1.6 10-6 W/m2)|
|AFGL 437||01(2)||03 07 23.68||+58 30 50.62||86300810||O8.5||1E5||Star forming region|
|IRAS 03260+3111||01(3)||03 29 10.37||+31 21 58.28||65902719||B9||2E4||Herbig AeBe|
|NGC 2023||01(3)||05 41 38.30||-02 16 32.59||65602309||B1.5V||3E2||Refl. Nebula|
|HD 44179||01(4)||06 19 58.20||-10 38 15.22||70201801||B8V||5E6||post-AGB|
|IRAS 07027-7934||01(2)||06 59 26.30||-79 38 48.01||73501035||WC10||2E7||PN|
|HD 97048||01(4)||11 08 04.61||-77 39 16.88||61801318||A0||2E4||Herbig AeBe|
|IRAS 12405-6238||01(3)||12 43 31.93||-62 55 11.39||29400410||O9.5||3E5||H II|
|-||01(1)||14 59 53.49||-54 18 07.70||07903307|
|-||01(2)||14 59 53.49||-54 18 07.70||43400768|
|IRAS 15384-5348||01(2)||15 42 17.16||-53 58 31.51||29900661||1E4||H II|
|CD-42 11721(off)||01(2)||16 59 05.82||-42 42 14.80||28900461||B0||-||Herbig AeBe (off pointing)|
|CD-42 11721||B0||-||Herbig AeBe|
|-||01(2)||16 59 06.82||-42 42 07.60||08402527|
|-||01(2)||16 59 06.80||-42 42 07.99||64701904|
|-||01(3)||17 09 00.91||-56 54 47.20||13602083|
|-||01(1)||17 09 00.91||-56 54 48.10||27301339|
|HB 5||01(3)||17 47 56.11||-29 59 39.70||49400104||120 000||-||PN|
|NGC 6537||01(3)||18 05 13.14||-19 50 34.51||70300475||A0||-||PN|
|GGD 27-ILL||B1||3E6||Star forming region|
|-||01(2)||18 19 12.04||-20 47 30.98||14802136|
|-||01(2)||18 19 12.03||-20 47 30.59||14900323|
|IRAS 18240-0244||01(1)||18 26 40.00||-02 42 56.99||14900804||WC8||-||PN|
|IRAS 18317-0757||01(2)||18 34 24.94||-07 54 47.92||47801040||O8||-||H II|
|IRAS 18416-0420||01(2)||18 44 15.19||-04 17 56.40||13402168||O5.5||-||H II|
|IRAS 18502+0051||01(2)||18 52 50.21||+00 55 27.59||15201645||O7||-||H II|
|TY CRA||B9||6E3||Herbig AeBe|
|-||01(3)||19 01 40.71||-36 52 32.48||33400603|
|-||01(1)||19 01 40.70||-36 52 32.59||34801419|
|-||01(3)||19 01 40.71||-36 52 32.48||71502003|
|BD +30 3639||01(3)||19 34 45.20||+30 30 58.79||86500540||WC9||1E5||PN|
|IRAS 19442+2427||01(2)||19 46 20.09||+24 35 29.40||15000444||O7||6E6||H II|
|BD+40 4124||01(3)||20 20 28.31||+41 21 51.41||35500693||B2||1E4||Herbig AeBe|
|S106 IRS4||01(2)||20 27 26.68||+37 22 47.89||33504295||08||1E5||H II|
|NGC 7023||B3||5E2||Refl. Nebula|
|-||01(4)||21 01 31.90||+68 10 22.12||20700801|
|-||01(2)||21 01 30.40||+68 10 22.12||48101804|
|NGC 7027||200 000||2E5||PN|
|-||01(4)||21 07 01.71||+42 14 09.10||02401183|
|-||01(1)||21 07 01.70||+42 14 09.10||23001356|
|-||01(2)||21 07 01.70||+42 14 09.10||23001357|
|-||01(3)||21 07 01.70||+42 14 09.10||23001358|
|-||06||21 07 01.50||+42 14 10.00||33800505|
|-||01(4)||21 07 01.63||+42 14 10.28||55800537|
|IRAS 21190+5140||01(2)||21 20 44.85||+51 53 26.59||15901853||-||2E5||H II|
|-||01(2)||21 29 58.42||+51 03 59.80||05602477|
|-||01(3)||21 29 58.42||+51 03 59.80||15901777|
|-||01(2)||21 29 58.42||+51 03 59.80||36801940|
|IRAS 22308+5812||01(2)||22 32 45.95||+58 28 21.00||17701258||O7.5||3E3||H II|
|IRAS 23030+5958||01(2)||23 05 10.57||+60 14 40.60||22000961||O6.5||7E3||H II|
|IRAS 23133+6050||01(2)||23 15 31.44||+61 07 08.51||22001506||O9.5||7E5||H II|
The data were processed using SWS interactive analysis product; IA3(see de Graauw et al. 1996) using calibration files and procedures equivalent to pipeline version 7.0. If a source has been observed multiple times and these observations are of similar quality and of comparable flux-level these data are co-added after the pipeline reduction. Since the features we discuss here are fully resolved in all observing modes, we combine the data obtained in all different modes to maximise the S/N. Further data processing consisted of bad data removal, rebinning on a fixed resolution wavelength grid, removing fringes and splicing of the sub-bands to form a continuous spectrum.
For all spectra the amount of shifting between sub-bands required falls well within the calibration uncertainties in the region of interest: 7 to 16 m. Any jumps between bands are due to flux calibration and dark current uncertainties. The effect of dark current is most important in low flux cases while the flux calibration uncertainties will dominate in bright sources. Below 20 Jy we apply offsets to correct for dark current uncertainties. In these low signal cases the typical noise level in the dark current measurements of 1-2 Jy introduces offset uncertainties >5-10 per cent dominating the flux calibration uncertainties. Above 20 Jy we apply scaling factors to correct for flux calibration uncertainties. The splicing introduces little uncertainty in the measured strengths since most features fall completely within one ISO/SWS sub-band. An exception to this is the band strength of the 12.7 m feature. This feature is sensitive to the way band 2C (7 to 12.5 m) and 3A (12 to 16.5 m) are combined. This introduces an extra uncertainty of the band strength of typically 20-30 per cent for the weakest features.
Some SWS data, especially in band 3A, are affected by fringes. We have corrected for fringes in those sources where they occur, using the aarfringe tool of IA3 on the rebinned spectrum. In the method we apply fringes are fitted with sine functions with periods in the range where fringes are known to occur and divided out. Note that the features we study here are much broader than any of fringe periods, therefore the intensities we measure are not directly affected by the fringes. However in some cases after fringe removal the continuum is more easily determined.
SWS spectra of many sources, including stars enshrouded in both carbon-rich and oxygen-rich dust and sources without any circumstellar material show very weak structure around 13.5 and 14.2 m at the 3 to 4 per cent level relative to the continuum possibly due to residual instrumental response. The emission features discussed here are all stronger than this with a maximum of 85 per cent of the continuum in the reflection nebula NGC 7023. Near 11.03 m there is a residual instrumental feature which coincides with the weak 11.0 m feature that we observe in our spectra. We have included the effect of this feature in the uncertainty on the intensities in Table 2.
Many sources in this sample have strong narrow emission lines in their
spectrum, in particular the strong [Ne II] line at 12.81 m
is perched on top of the 12.7 m UIR band. This line and the UIR
band are easily separated at the resolution of the SWS instrument. We
remove the contribution from this line by fitting a Gaussian profile
to the line and subtracting that profile prior to rebinning. The
spectrum of NGC 7027 has a very strong
[Ne V] emission line at 14.32
m. We have removed the part of the spectrum which contains this
We also include in Table 1 the spectral type of the
illuminating source and an estimate of the flux density at the
location where the PAH emission originates from in units of the
average interstellar UV field (Habing 1968). We have
derived these estimates from the observed IR flux (
and the angular size of the PAH emission region
(Wolfire et al. 1989). This estimate is based on the assumption
that all the UV light is absorbed in a spherical shell with the
angular size of the object and re-emitted in the IR. The flux density
at the shell is given by:
|Figure 2: An overview of the observed features. All spectra have been continuum subtracted and are scaled to have the same integrated intensity in the 12.7 m feature. The sources are ordered according to their 11.2/12.7 m band strength ratio (bottom to top). The ratio of the 11.2 m to the 12.7 m feature spans a full decade. Sources with broad solid CO absorption beyond 15 m|
An overview of the observed features near 13.5 and 14.2
m. HD 44179 and NGC 7027 have been scaled by a
factor of 0.3
In Fig. 2 we show the continuum subtracted spectra after normalising to the integrated strength of the 12.7 m feature. The sources are ordered according to the strength of the 11.2 m feature relative to the 12.7 m band. Relative to the 12.7 m band, the sources with the weakest 11.2 m feature are the H II regions (at the bottom of Fig. 2), while the evolved stars show the strongest 11.2 m band.
The spectral characteristics of the features are summarised in Table 2. Note that the uncertainties quoted in the table reflect the noise level and the freedom in drawing the continuum within the methodology used to measure these bands. Other ways of decomposing the broad, blended bands and the underlying continuum will give other results (e.g. Boulanger et al. 1998; Uchida et al. 2000; Verstraete et al. 2001). However these differences are systematic differences and do not affect the source-to-source variations we observe. The intensities of the 11.2 and 12.7 m features are obtained by direct integration above the chosen continuum. We measure the peak position of the 11.2 and 12.7 m bands by fitting them with template spectra of these features. The template spectrum for the 11.2(12.7) m feature is constructed by adding the continuum subtracted spectra with each 11.2(12.7) m feature normalised to have the same integrated intensity. This way each source has equal contribution to the template spectrum. We use a -minimisation routine to fit the template to the sources, allowing for both a wavelength shift and scaling in strength. The shifts that we determine for the 11.2 m band are very small except for HD 44179 and IRAS 17047 where this band is much broader than the template spectrum (cf., Table 2, see also Peeters et al. 2001, in prep.). Although there are differences between the detailed profiles of the 12.7 m band we detect no significant shift of the band as a whole. For the weak features near 13.5 and 14.2 m, the parameters have been determined through fitting of Gaussian profiles. We adopted a local linear continuum for the very weak 11.0 m feature because of the severe blending of this band with the 11.2 m band. The weak 12.0 m band is close to both the 11.2 and the 12.7 m band. For only a few sources we measure the intensity of this band, for the other sources we refrained from detailed analysis. However Table 2 does note whether we detect this band.
The profile of the 11.2 m feature is asymmetric with a sharp blue rise and a more gradual decline to longer wavelengths (Roche et al. 1989; Witteborn et al. 1989). This will be discussed in more detail for this sample by Peeters et al. (2001, in prep.). The 12.7 m band is also asymmetric but in the opposite way with a slow blue rise and a sharp red decline between 12.8 and 12.9 m. Because of their intrinsic weakness, the profiles of the 10.6, 11.0, 12.0, 13.5, and 14.2 m features in the individual sources are not well determined however in the averaged spectrum, these features appear symmetric (cf. Fig. 7).
Because here we want to study variations in the relative strength of the UIR bands to each other, not differences in absolute intensities differences due to intrinsic luminosity and distance of the source, we use 3-feature intensity ratio correlations. Although we observe variations in all ratios, we find only three that correlate and these are shown in Figs. 4-6. First, we find that the CH stretch mode at 3.3 m correlates with the 11.2 m band (cf. Fig. 4). Note that the slope of the trend is roughly 1, which means that the / is on the average constant at a value of 3-4.
Second, the 12.7 m band correlates with the CC stretch mode at 6.2 m, albeit with more scatter (cf. Fig. 5) than the 11.2/3.3 ratio. The 7-9 m complex is well correlated with the 6.2 m band and similar plots can be made with these interchanged. Inspection of Fig. 2 and Table 2 shows that there is some indication for both the / and the / m band to be higher in H II regions, however only about half the sources have such high S/N that these intensities can be reliably measured and this trend is not statistically significant.
Lastly, we show in Fig. 6 the correlation between the ratio of the flux emitted in the PAH bands relative to the total flux emitted in the IR ( ) and the changing / ratio. We measure the by integrating the SWS data and Long Wavelength Spectrometer (LWS) data if available. For those sources without LWS data we use a blackbody fitted to IRAS measurements in the wavelength region from 45-200 m. We do not apply corrections for aperture differences between the instruments. We estimate an uncertainty of 15 per cent on the . Again different classes of objects occupy different parts in this diagram.
We also checked for correlations between band strength ratios and the flux density; , however we do not detect any correlations.
We emphasise that, while all UIR bands show a loose correlation in the
absolute intensity (see also Cohen et al. 1986, 1989), these three are the only tight correlations
present in this sample.
|Figure 4: Bands strength ratios as derived from the SWS spectra. Hexagons are H II regions, stars intermediate mass star forming regions, squares RNe and triangles are PNe|
|Figure 5: Bands strength ratios as derived from the SWS spectra. Plotting symbols are the same as in Fig. 4|
|Figure 6: The ratio between the flux emitted in the PAH bands over the total amount of IR radiation against the / ratio. Plotting symbols are the same as in Fig. 4|
Summary of the laboratory results on CH out-of-plane bending
modes for solo, duo, trio and quartet hydrogens on matrix isolated
neutral polycyclic aromatic hydrocarbons and their cations. (Adapted from Hudgins
et al. 2000b).
is the lower limit of the region in m. is the upper limit of the region in m.
The cross-section values for the solo, trio, and quartet modes per hydrogen are the averages over the spectra in the database. However, the A values for the duo mode per hydrogen decreases rapidly with size and settles to slightly less than 4 km/mole for PAHs with more than 24 carbon atoms. This value is more appropriate to use in determining the edge structures of PAHs that dominate emission in this wavelength region.
Total average cross-sections over both neutrals and cations in the database.
|Figure 7: A comparison of the average interstellar spectrum (top) with the ranges for the out-of-plane bending modes (bottom). The average spectrum was obtained by co-adding the continuum subtracted spectra after normalisation to the 12.7 m band strength. The boxes indicate the wavelength regions associated with the out-of-plane bending vibrations for different types of adjacent hydrogen atoms determined from matrix isolated spectroscopy of neutral and cationic PAHs (see Hudgins et al. 2000b for details). In this comparison it should be kept in mind that the emission process leads to a small (0.1 m) wavelength redshift in the peak position|
There are two points that emerge from an analysis of the laboratory database that are of particular importance to the observational data presented here. The first involves the effect of ionisation on the characteristic wavelength regions of the various CH adjacency classes. The second is the intrinsic integrated absorption strengths (A values) which are derived for the various adjacency classes. Together these results provide the tools to not only qualitatively infer the sorts of PAH edge structures present, but also quantitatively determine their relative amounts. As shown below, this allows one to place stringent constraints on the emitting interstellar PAH family.
Perusal of Fig. 7 and the wavelength limits listed in Table 3 shows that, while the ranges for neutral PAHs are not modified substantially compared to Bellamy (1958), ionisation causes some important changes in region boundaries. These data expand on the initial report that the PAH cation solo hydrogen position is substantially blue shifted with respect to the wavelength for its neutral counterpart while the domains indicative of the other types of hydrogen are less affected by ionisation (Hudgins & Allamandola 1999). Considering these modified domains and taking into account the roughly 0.1 m redshift in the peak position for PAHs emitting at temperatures of 500-1000 K (Flickinger et al. 1991; Brenner & Barker 1992; Colangeli et al. 1992; Joblin et al. 1995; Cook & Saykally 1998) allows us to draw the following conclusions.
|Figure 8: Comparison between the mean interstellar spectrum (panel b)) and synthetic PAH spectra showing the distribution over peak positions of the OOP modes in the Hudgins database (Hudgins et al. 2000b). The shaded surfaces in panels a) and c) represent the contributions per mode for neutral and positively charged PAHs respectively. Each area represents the average absorption cross-section per functional group|
One should bear in mind, that the measured species are probably smaller than those that dominate the interstellar population and that, for stability reasons, the interstellar PAH family might be skewed to a few of these molecules or the edges structures they represent (cf. Sect. 5). Comparing the interstellar spectrum (Fig. 8b) with these averaged laboratory spectra (Figs. 8a,c) allows us to further refine the discussion:
It is also immediately clear from Fig. 8 that the interstellar spectrum does not reflect an equal distribution over the different functional groups but is dominated by the contribution of solo modes. This reflects the molecular structure of the emitting PAHs.
The ratio of the number of solo to duo (s/d), solo to trio (s/t) and solo to quartet (s/q) groups for NGC 7027 and IRAS 18317 as deduced from their 10 to 15 m spectra.
|Figure 9: Examples of molecular structures simultaneously satisfying the structural constraints set by the observed band strength ratios of the number of solo, duo and trio modes for different interstellar regions. The number of solo, duo, trio and quartet functional groups are noted s,d,t and q respectively. Solo modes are associated with long straight molecular edges. Duos and trios, on the other hand, correspond to corners. Quartets are due to pendant rings attached to the structure. The numbers in the molecular structures indicate the number of adjacent CH groups per aromatic ring|
Examples of the types of PAHs which simultaneously satisfy these different structural constraints are shown in Fig. 9. In constructing these structures one should keep in mind that solo H's represent long straight edges, while the corners in the structures give rise to duos or trios. To match the dominance of the 11.2 m feature in NGC 7027 one is naturally driven towards rather large molecules with at least 100-200 carbon atoms and long straight edges. This is entirely in keeping with previous theoretical calculations of the molecular sizes of the PAH species which account for most of the emission in these features (Schutte et al. 1993). As illustrated in Fig. 9 structure 1 approximates the ratios listed in Table 4 for NGC 7027, these ratios requires a preponderance of solo hydrogens over duos and trios. The extreme quartet to solo group ratio observed for NGC 7027 is not well reproduced even by Fig. 9 structure 1. In order to reproduce that ratio in one single molecule one has to go to even larger molecules. Rather, we surmise the extreme ratio reflects the presence of molecules without any quartet groups. Quartet groups represent pendant rings on the molecule, which can be taken off without altering the other ratios strongly. Of course many other PAH structures that match the observed ratios are possible. However these structures are all very similar to this and the conclusion is the same: in NGC 7027 the PAH family is dominated by large compact PAHs.
In contrast, for IRAS 18317 the situation is very different. The observed ratios force one to include more corners or uneven edges. Structure 4 in Fig. 9 illustrates one way of achieving this. Of course this effect can also be achieved by going to smaller compact structures of the type shown in structure 1 or by breaking up structure 4 in two or more fragments. Structures 2 and 3 shown in Fig. 9 are intermediate between these two extremes and have solo/duo, solo/trio, and solo/quartet ratios consistent with the relative interstellar band intensities shown in Fig. 2 for the objects which lie between the extremes, NGC 7027 and the IRAS 18317. Thus we observe a structural evolution where closed, compact species dominate the emission in some regions, while open, uneven structures are more important in others. We surmise that this structural evolution as revealed by the smooth spectral evolution shown in Fig. 2 reflects the variations in chemical history and excitation environment in these regions.
The spectral identification of the 12.7 band is much less clear and it is not possible at this time to assign this band unambiguously to either neutral or cationic PAHs. In the existing database there is only one species with a strong band that matches well in position, which "happens'' to be a cation. Furthermore the strength of the interstellar 12.7 m band correlates with the strength of the 6.2 m feature (see Sect. 3.4). The strength of the modes between 6 and 9 m are greatly enhanced upon ionisation, and thus, one way to understand this correlation is to assume that the 12.7 is also predominantly carried by cations.
Thus, seemingly, the 11.2 m and 12.7 m bands represent a dichotomy of interstellar PAHs with the former carried mainly by neutral and the latter by positively charged PAHs. The origin of this interrelation between charge and spectral characteristics in unclear. There is no indication in the laboratory experiments for a causal relation between, for example, charge state and the relative strength of the solo to trio modes. Considering also the discussion on the molecular structures implied by the relative fraction of the solos to duos and trios (Sect. 5; Fig. 9) we are forced to conclude that the good correlation between the 11.2 and 3.3 m bands and between the 12.7 and the 6.2 m bands reflects a correlation of molecular structure and charge state with environment. Indeed when using PAHs containing some 50 C-atoms (Leger & Puget 1984) the correlation between the / and / is well reproduced by model calculations of Bakes et al. (2000) by only varying the degree of ionisation. Thus, those environments which favour large PAHs and the 11.2 m band (structure 1 in Fig. 9) also favour neutral PAHs. While in regions where open uneven molecular structures and the 12.7 m band (structure 4 in Fig. 9) dominate, PAHs are predominantly charged.
This is probably also the origin of the correlation between the / ratio and the / (cf. Fig. 6). The / measures the PAH/dust abundance ratio. The loose correlation suggests that for the ISM sources the PAH abundance is lower. We recognise that PNe inject freshly synthesised PAHs into the ISM where they are mixed and processed by FUV photons and shocks. This processing will lead to a slow destruction of the PAHs.
The dominant molecular structure reflects the integrated history of the PAH family and we note that all sources with a strong 11.2 m band are PNe, which have formed their PAHs within the last some 1000 years. Because open uneven molecular structures are kinetically more reactive to the addition of carbon atoms than compact structures, the predominance of the latter in chemically reactive regions where PAHs have recently formed can be rationalised (Frenklach & Feigelson 1989; Cherchneff et al. 1992). In contrast regions with relatively strong 12.7 m bands are all H II regions where luminous stars illuminate material which has been processed in the ISM for some 109 years. This processing irreversibly leads to a breaking down of the molecular structure because reformation is prohibited by the low temperature of the ISM.
This does not directly explain why the 11.2 m band correlates with the neutral PAH indicator while the 12.7 m emission feature correlates with bands attributed to ions. The charge state is rapidly set by the charge balance, which is dominated by local physical conditions, or more specifically the ionisation parameter, , where is the FUV radiation field, is the gas-temperature and the electron density. Thus rather than history, ionisation reflects the present. Possibly most of the destruction is occurring presently and is also driven by local physical conditions.
Observationally, our analysis also argues against dehydrogenation. First, we observe a constant ratio of the 3.3 m band (all CH oscillation) to the 11.2 m band (only solo CH oscillation). However, we would expect a non-linear behaviour since, when dehydrogenation commences the number of solo H increases as duos and trios are converted to solo's and only at high dehydrogenation does the relation between the 3.3 and the 11.2 m bands become linear (Schutte et al. 1993). Secondly, if the variation in / reflects dehydrogenation than we would expect that decreasing H coverage (i.e. decreasing / ) would correlate with increasing CC/CH mode emission (i.e. / ). The opposite is actually observed (cf. Fig. 5). We conclude therefore that dehydrogenation has little influence on the observed interstellar UIR spectrum.
We have summarised new laboratory spectroscopy results for the CH out-of-plane bending vibrations on isolated neutral and cationic PAHs. Different number of adjacent CH bonds give rise to vibrations in distinctly different wavelength regions. The modes are therefore good diagnostics of the molecular structure of the emitting species. Upon ionisation the solo CH vibrations are shifted to shorter wavelength compared to the solo modes in neutral. The cross-sections per mode are not strongly modified upon ionisation. We attribute the weak bands at 10.6 and 11.0 m to solo modes in positively charged PAHs, the strong 11.2 m band the solo modes in neutral. The weak 12.0 m band we assign to the duo modes, the 12.7 m to trio modes and the 13.5 m feature to quartet vibrations.
From the average cross-sections per mode we have constrained the relative numbers of solo, duo, trio and quartet CH groups in different sources for the PAH species that effectively emit in this wavelength region. The spectra of PNe with a dominant 11.2 m feature arises from large (100-150 C-atom) compact PAHs with long straight edges. In contrast the H II region spectra are due to smaller or more irregular PAHs.
We propose a scenario in which large compact PAHs are formed in the winds around evolved stars. These PAHs are consequently degraded in the ISM. From the correlations between charge indicators, which are set by the local physical conditions, and the 11.2/12.7 m band strength ratio, which is determined by the molecular structure, we conclude that much of this degradation happens on a short timescale in the emission objects themselves.
The authors wish to thank the referee dr. L. Verstraete whose comments have helped to improve the paper. SH acknowledges the support from an NWO program, grant 616-78-333. EP acknowledges the support from an NWO program, grant 783-70-000. CVK is a Research Assistant of the Fund for Scientific Research. DMH and LJA gratefully acknowledge support under NASA's IR Laboratory Astrophysics (344-02-06-01) and Long Term Space Astrophysics programs (399-20-01). IA3 is a joint development of the SWS consortium. Contributing institutes are SRON, MPE, KUL and the ESA Astrophysics Division. This work was supported by the Dutch ISO Data Analysis Center(DIDAC). The DIDAC is sponsored by SRON, ECAB, ASTRON and the universities of Amsterdam, Groningen, Leiden and Leuven.