We report on the detection with the Infrared Space Observatory ( ISO ) of a molecular band at 57.5 \mu m ( 174 cm ^ { -1 } ) in carbon-rich evolved stars and in Sgr B2 .
Taking into account the chemistry of these objects the most likelihood carrier is a carbon chain .
We tentatively assign the band to the \nu _ { 5 } bending mode of C _ { 4 } for which a wavenumber of 170-172.4 cm ^ { -1 } has been derived in matrix experiments ( Withey et al .
1991 ) .
An alternate carrier might be C _ { 4 } H , although the frequency of its lowest energy vibrational bending mode , \nu _ { 7 } , is poorly known ( 130-226 cm ^ { -1 } ) .
If the carrier is C _ { 4 } , the derived maximum abundance is nearly similar to that found for C _ { 3 } in the interstellar and circumstellar media by Cernicharo et al .
( 2000 ) .
Hence , tetra-atomic carbon could be one of the most abundant carbon chain molecules in these media .