First Astronomical Detection of the Cumulene Carbon Chain Molecule H₂C₆ in TMC1

The cumulene carbenes are important components of hydrocarbon chemistry in low mass star forming cores. Here we report the first astronomical detection of the long chain cumulene carbene H_2C_6 in the interstellar cloud TMC1, from observations of two of its rotational transitions: $J_{K,K'} = 7_{1,7} \rightarrow 6_{1,6}$ at 18.8 GHz and $8_{1,8} \rightarrow 7_{1,7}$ at 21.5 GHz, using NASA's Deep Space Network 70 m antenna at Goldstone, California. In addition we also observed the shorter cumulene carbene, H_2C_4 at the same position. The fractional abundance of H_2C_6 relative to H_2 is about $4.7 \times 10^{-11}$ and H_2C_4 is about $1.1 \times 10^{-9}$. The abundance of H_2C_6 is in fairly good agreement with gas phase chemical models for young molecular cloud cores, but the abundance of H_2C_4 is significantly larger than predicted.