SIMBAD references

We present SWS and LWS ISO observations towards a strip across the photodissociation region (PDR) in the reflection nebula NGC 7023. SWS02 and LWS01 spectra have been taken towards the star and the brightest infrared filaments located NW and SW from the star (hereafter NW and SW PDRs). In addition, SWS02 spectra have been taken towards two intermediate positions (NW1 and SW1). This has provided important information about the extent and spatial distribution of the warm H₂ and of the atomic species in this prototypical reflection nebula. Strong emission of the [SiII] 34.8µm line is detected towards the star. While all the PDR tracers (the [CII] 157.7µm, [OI] 63.2 and 145.6µm, [HI] 21cm and the H₂ rotational lines) present a ring-like morphology with the peaks toward the NW and SW PDRs and a minimum around the star, the SiII emission is filling the hole of this ring with the peak towards the star. This morphology can only be explained if the SiII emission arise in the lowest extinction layers of the PDR (A_v<2mag) and the HII region. At least 20%-30% of the Si must be in gas phase in these layers. For A_v≥2mag, the Si is mainly in solid form (δSi=-1.3). The NW and SW PDRs have very similar excitation conditions, high density filaments (n∼10⁶cm^–3) immersed in a more diffuse interfilament medium (n∼10⁴cm^–3). In both, the NW and SW PDRs, the intensities of the H₂ rotational lines can only be fitted by assuming an ortho-to-para-H₂ ratio lower than 3 in gas with rotation temperatures from 400 to 700K. Therefore, there is a non-equilibrium OTP ratio in the region. Furthermore, the comparison between the OTP ratio derived from H₂ vibrational lines and the pure H₂ rotational lines, shows that the OTP ratio increases from ∼1.5 to 3 across the photodissociation region with larger values in the less shielded gas (A_v<0.7mag). This behavior is interpreted as a consequence of an advancing photodissociation front. We have not detected the OH, CH and CH₂ lines towards the observed positions. This is consistent with the weakness of these lines in other sources and can be explained as a consequence of the small beam filling factor of the dense gas in the LWS aperture. The CO J=17->16 line has been tentatively detected towards the star.