7 Conclusions

1.

A unique attempt to probe the circumstellar envelope of IRC +10$\degr$ 216, using optical absorption spectroscopy towards background stars lying beyond the envelope, has been performed.

2.

Analysis using Kurucz model atmospheres and spectral synthesis techniques has been performed for the most favourable target Star 6, located at 37$\arcsec$ from IRC +10$\degr$ 216. The analysis confirms that this target is much more distant than the carbon star envelope; the target is found to be a somewhat metal-poor G-dwarf with $T_{\rm eff} = 5650$ K.

3.

The circumstellar hydrogen column density towards this target is expected to be $\sim$ $1.8\times 10^{21}$ cm^-2, which would correspond to $A_{\rm v}=0.9$ mag in the diffuse interstellar medium, and hence the expectation of strong DIBs. Extinction maps of the field suggest also that there is little contamination of the line-of-sight by foreground or background interstellar matter, with $A_{\rm v}~{\rm (ISM)}\la 0.1$ mag.

4.

The IRC+10$\degr$ 216 CSE is definitely detected in K  I and most probably in Na  I also. Circumstellar Na  I is very probably also detected towards a second target located at 153$\arcsec$  from IRC +10$\degr$ 216.

5.

Diffuse band absorptions arising in the circumstellar envelope are not observed. The strong DIB, $\lambda$6284, is detected, but its origin is interstellar. The weakness of the DIB is commensurate with the known low reddening towards this region, and its observed velocity also negates a circumstellar origin.

6.

Overall, the data suggest that no DIB carrier exists in the envelope of IRC +10$\degr$ 216 with the abundances relative to hydrogen typical of the diffuse ISM.

Acknowledgements

We are grateful to the Leverhulme Trust for an award and for the support of one of us (TRK) and NM thanks the French Programme National of CNRS Physico-chimie du Milieu Interstellaire. We thank the WHT service observing support staff, and acknowledge with thanks use of CDS (Centre de Données de Strasbourg) and the ESO/ECF Centre for archive material. Lastly, we are grateful for the comments of an anonymous referee.