2012ApJ...753L..11A

To clarify the authenticity of a recently proposed identification of H₂ CCC (linear-C₃H₂) as a diffuse interstellar band (DIB) carrier, we searched for the rotational transition of H₂ CCC at a frequency of 103 GHz toward HD 183143 using the 45 m telescope at the Nobeyama Radio Observatory. Although rms noise levels of 32 mK in the antenna temperature were achieved, detection of H₂ CCC was unsuccessful, producing a 3σ upper limit corresponding to a column density of 2.0x10¹³/cm2. The upper limit indicates that the contribution of H₂ CCC to the DIB at 5450 Å is less than 1/25; thus, it is unlikely that the laboratory bands of the B ¹ B₁-X ¹ A₁transition of H₂ CCC and the DIBs at 5450 Å (and also 4881 Å) toward HD 183143 are related.