Search for interstellar pyrrole and furan

Unsuccessful searches for the organic ring molecules pyrrole (C4H4N) and furan (C3H4O) are reported. In TMC-1, an upper limit to the pyrrole column density of 4 x 10 to the 12th/sq cm is found an order of magnitude less than the column density of HC5N in this source. In Sgr B2, the upper limit to the pyrrole column density is 6 x 10 to the 13th/sq cm, about 0.4 times the HC5N column density. These results suggest that chemical processes in these clouds favor the formation of chains over rings of comparable molecular weight. During these searches, a number of unidentified millimeter wavelength lines were also found; their frequencies are reported here.