A New Unidentified Far-Infrared Band in NGC 7027

Abstract

We report on the detection of a molecular band centered at ~98 μm (~102 cm^-1), observed with the Infrared Space Observatory in the young planetary nebula NGC 7027. The band structure and intensity cannot be reproduced by atomic fine-structure lines, recombination lines, or the rotational emission of abundant molecules. We discuss the possible contribution of the low-energy bending modes of pure carbon chains to the unidentified far-IR bands observed in C-rich evolved objects. In particular, we speculate that the band emission could arise from the ν₉ and ν₇ bending modes of C₆ and C₅, for which wavenumbers of 90+/-50 and 107+/-5 cm^-1 have been estimated from photoelectron spectroscopy.

ID="FN1"> 1Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the P.I. countries: France, Germany, the Netherlands, and the United Kingdom) and with the participation of ISAS and NASA.