Radio Search for H2CCC toward HD 183143 as a Candidate for a Diffuse Interstellar Band Carrier

To clarify the authenticity of a recently proposed identification of H2CCC (linear-C3H2) as a diffuse interstellar band carrier, we searched for the rotational transition of H2CCC at a frequency of 103 GHz toward HD 183143 using a 45-m telescope at the Nobeyama Radio Observatory. Although rms noise levels of 32 mK in the antenna temperature were achieved, detection of H2CCC was unsuccessful, producing a 3 sigma upper limit corresponding to a column density of 2.0 \times 1013 cm-2. The upper limit indicates that the contribution of H2CCC to the diffuse interstellar band at 5450 Å is less than 1/25; thus, it is unlikely that the laboratory bands of the B1B1-X1A1 transition of H2CCC and the diffuse interstellar bands at 5450 Å (and also 4881 Å) toward HD 183143 are related.