IR signature in NGC7023

The C₆₀⁺ species would re-emit any energy absorbed from a neighboring star through its four IR modes. Two of these modes have been measured in the laboratory to fall at 7.11 and 7.51 $\mu$m (Fulara et al. 1993). The low frequency modes have not been measured so far for the cation. Moutou et al. (1996b) find that approximately 35% of the emitted power emerges at 7.11 $\mu$m, and another 25% is emitted at 7.51 $\mu$m. If the intensity ratio of the low-frequency modes in C₆₀⁺ is similar to that of C₆₀, the low-frequency modes should represent less than 40% of the total energy.

>From complementary observations of NGC 7023, conducted during revolution 455 with the observing mode AOT SWS06, we got a higher signal-to-noise ratio (up to 20) in the reduced wavelength range 7.0-8.7 $\mu$m. The spectrum has been rebinned at a resolution R = $\lambda / \Delta \lambda$ = 1150 (Fig. ).

  

Figure: The SWS06 spectrum of the reflection nebula NGC 7023 in the wavelength range 7.0-7.8 $\mu$m. The expected locations of the C₆₀⁺ emission bands (Fulara et al. 1993) are marked by vertical lines. The spectrum is the mean value of upwards and downwards scans, rebinned at R = $\lambda / \Delta \lambda$ = 1200. One-half the difference between upwards and downwards scans reduced independently is also plotted to provide an estimate of the noise.

We expect any C₆₀⁺ features to be broadened and shifted to longer wavelength by 0-20 cm^-1 (less than 0.1$\mu$) compared to the Neon matrix measurement, if observed in emission from warm interstellar material (Joblin et al. 1995). The amount of the wavelength shift may vary a lot. The tentative detection of these features in these wavelength ranges would lead to an estimate of C₆₀⁺ abundance in NGC7023, or to an upper limit.

The derived upper limit for N_{C<<104>>60⁺}/N_H is equal to 2 $\times$ 10^-7 for a 3$\sigma$ detection. If we use the total carbon abundance as measured by Cardelli et al. (1996) in the local interstellar medium (2.6 $\times$ 10^-4 relative to hydrogen), we find an upper limit of less than 0.1% of carbon contained in C₆₀⁺. This value is much lower than the abundance deduced from NIR observations by Foing and Ehrenfreund. The reason for this discrepancy can be either a overestimated oscillator strength of the 9600 Å bands, or a destruction of C₆₀⁺ in NGC7023 (double ionisation process). An answer could be found by searching for the 9577 and 9632 Å bands towards the star HD200775, which illuminates the reflection nebula NGC7023. As mentioned above (section 2.3), the DIBs are not strong relative to the reddening towards NGC7023, and we may expect that the 9577 and 9632 Å DIBs would suffer the same weakeness. That would be consistent with the IR observations.