Limits on chemical complexity in diffuse clouds : search for CH 3 OH and HC 5 N absorption ⋆

Context. An unexpectedly complex polyatomic chemistry exists in diffuse clouds, allowing detection of species such as C 2 H, C 3 H 2 , H 2 CO, and NH 3 , which have relative abundances that are strikingly similar to those inferred toward the dark cloud TMC-1. Aims. We probe the limits of complexity of diffuse cloud polyatomic chemistry. Methods. We used the IRAM Plateau de Bure Interferometer to search for galactic absorption from low-lying J=2-1 rotational transitions of A-and E-CH 3 OH near 96.740 GHz and used the VLA to search for the J=8-7 transition of HC 5 N at 21.3 GHz. Results. Neither CH 3 OH nor HC 5 N were detected at column densities well below those of all polyatomics known in diffuse clouds and somewhat below the levels expected from comparison with TMC-1. The HCN/HC 5 N ratio is at least 3-10 times higher in diffuse gas than toward TMC-1.