A search for naphthalene in diffuse interstellar clouds

We have obtained high resolution optical spectroscopy of 10 reddened O-type stars with UVES at VLT to search for interstellar bands of the naphthalene cation (C$_{10}$H$_{8}$$^+$) in the intervening clouds. No absorption features were detected near the laboratory strongest band of this cation at 6707 \AA except for star HD 125241 (O9 I). Additional bands in the optical spectrum of this star appear to be consistent with other transitions of this cation. Under the assumption that the bands are caused by naphthalene cations we derive a column density N$_{Np^+}$ = (1.2$\pm$ 0.3) x 10 $^{13}$ cm$^{-2}$ similar to the column density claimed in the Perseus complex star Cernis 52 (Iglesias-Groth et al. 2008). The strength ratio of the two prominent diffuse interstellar bands at 5780 and 5797 \AA suggests the presence of a $\sigma$-type cloud in the line of sight of HD 125241.