2 Observations and physical conditions

SWS has observed the AIB spectrum along a number of interstellar sightlines covering a wide range of excitation conditions. We discuss here three spectra which sample well the radiation field sequence covered by the SWS observations. The lines of sight selected all correspond to interface conditions, i.e., regions where fresh molecular material is directly exposed to the stellar light. At interfaces, the AIB emission is usually strong (probably because of an enhanced PAH abundance, Bernard et al. 1993): this is why we selected such regions to carry out the present study.

At the low excitation end, we have the reflection nebula NGC 2023 ( ${\rm TDT}=65602309$, SWS01 speed 3). This spectrum has already been presented in Moutou et al. (1999). SWS looked at a filament, 60'' south of the central star (central star: ${\rm RA}= 5^{\rm h}$41$^{\rm m}$38.3$^{\rm s}$, ${\rm DEC}= {-2}$$^{\circ}$16'32.6''), which is bright in fluorescent H₂ emission (Field et al. 1998). At the high excitation end, we present here a spectrum of the M17-SW photodissociation interface ( ${\rm RA}= 18^{\rm h}$20$^{\rm m}$22.1$^{\rm s}$, ${\rm DEC}= {-16}$$^{\circ}$12'41.3''; ${\rm TDT}=32900866$, SWS01 speed 4): this is position number 6 of the data presented in Verstraete et al. (1996). Finally, we also have the Orion Bar ( ${\rm RA}= 5^{\rm h}$35$^{\rm m}$20.3$^{\rm s}$, ${\rm DEC} = {-5}$$^{\circ}$25'20''; ${\rm TDT}=69501806$, SWS01 speed 4) at the position of the peak of fluorescent H₂ emission (van der Werf et al. 1996). All positions given above are in J2000.

The data reduction was undertaken with the SWS-IA3 environment running at the Institut d'Astrophysique Spatiale, Orsay. The spectrum of NGC 2023 lacks a small range around 4 $\mu$m because of bad dark current measurements. The flux calibration files CAL-G version 030 have been used. For the beam sizes, we took the values recently determined by Salama (2000). This assumes that the source completely fills the beam. To check this assumption, we compared our SWS spectra of M17-SW and of the Orion Bar to CAM-CVF data (Cesarsky et al. 1996a,b; Cesarsky et al. 2000b): the continuum fluxes (per solid angle) of the two instruments were found to agree within 20%. In the case of NGC 2023, the emission seen in the ISOCAM-map of Abergel et al. (2000, in preparation) looks homogeneous at the position of our SWS spectrum. The 6 arcsecond pixels of ISOCAM are much smaller than the SWS field of view, and since the ISOCAM image of each of our sources is smooth in the region observed by SWS, we can safely say that our sources uniformly fill the SWS beam. This statement only holds over the 5-16 $\mu$m wavelength range. In fact, to assure continuity in our spectra, we had to deviate from the Salama SWS beam sizes above 27 $\mu$m (the spectral bands 3E and 4 of the SWS, see de Graauw et al. 1996).

Figure 1: The SWS 2.4-25 $\mu$m spectra of the AIBs towards various interstellar sources (see text). The spectra have been normalized to the 6.2 $\mu$m-feature of NGC 2023; the scaling factors are given in the figure, e.g., M17-SW/3 means that the flux of M17-SW has been divided by 3. A positive constant has been added to all spectra except that of NGC 2023 to ease comparison. The zero flux level of each spectrum is indicated by the dashed line. The resolving power ( $\lambda /\Delta \lambda$) is 200 for NGC 2023 and 500 for M17-SW and the Orion Bar. The narrow, unresolved lines are due to Br$\beta$ 2.63 $\mu$m, Br$\alpha$ 4.06 $\mu$m, forbidden ionic lines ([ArII] 6.98 $\mu$m, [ArIII] 8.99 $\mu$m, [SIV] 10.5 $\mu$m, [NeII] 12.8 $\mu$m, [NeIII] 15.5 $\mu$m and [SIII] 18.7 $\mu$m) and H2 pure rotational lines (0-0 S(5) 6.91 $\mu$m, 0-0 S(4) 8.02 $\mu$m, 0-0 S(3) 9.66 $\mu$m, 0-0 S(2) 12.3 $\mu$m and 0-0 S(1) 17.0 $\mu$m).

The resulting 2.4-25 $\mu$m spectra are shown in Fig. 1. As we will see below, this reduced spectral range amply suffices in our discussion of the AIB profiles. The NGC 2023 spectrum has a resolving power $\lambda/\Delta\lambda = 200$ and those of M17-SW and of the Orion Bar have $\lambda/\Delta\lambda = 500$. The physical conditions prevailing in each interstellar region are summarized in Table 1. We assumed that the radiation field from the exciting star is a blackbody characterized by an effective temperature. In this table, we give the effective temperature of the exciting star ( $T_{\rm eff}$), the dilution factor for the blackbody ( $W_{\rm dil}$), the flux scaling factor ($\chi$) in units of the Habing field (at $\lambda = 1000$ Å) and $d_{\star}$ the distance of our line of sight from the central star (we assumed $d_{\star}$ to be the projected distance on the sky in all cases). In the case of M17-SW, several stars excite the regions we observed: in Table 1 we give effective values ( $T_{\rm eff}$, $R_{\star}$) that reproduce well the sum of all radiation fields. Our excitation sequence thus goes from NGC 2023 (cool star) to M17-SW (hot star).

   

Table 1: Physical conditions for the AIB spectra - see text.

Line of sight	NGC 2023	Orion Bar	M17-SW
$T_{\rm eff}$ (104 K)	2.3a	3.7c	4.5e
$R_{\star}$ ( $R_{\scriptsize\odot}$)	3.2a,b	8.5b	12
$d_{\star}$ (pc)	0.14	0.24	1.10
$W_{\rm dil}/10^{-13}$	2.66	6.38	0.96
$\chi/10^3$	1.2	42	12.5

The distances to NGC 2023, M17-SW and the Orion Bar were taken to be 480 pc^a, 2.2 kpc^e and 460 pc^d respectively.

References: (a) Buss et al. (1994), (b) Lang (1991), (c) Rubin et al. (1991), (d) van der Werf et al. (1996), (e) Felli et al. (1984)