  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.
