The relationship between linear carbon chains such as carbenes (H2Cx), carbynes (HCx), and cyanopolyynes (HCxN) and the polyaromatic hydrocarbons (sheet-like molecules based on benzene rings) is likely to be the key to understanding the origin of complex organics in interstellar clouds. Extensive observations of the abundances of complex organics at various stages of pre-protostellar evolution are required to identify the specific astrochemical processes which determine the composition of the material from which protostars and their accompanying nebulae form.
It is possible that current observations are at a significant sensitivity threshold. Laboratory measurements of the production of cyanopolyynes show that the relative abundance flattens when there are more than nine carbons, reaching at asymptotic value only factor of ten less for more than 15-17 carbons (McCarthy et al. 1998). However, because the energy is distributed over many more transitions, the intensity per line still decreases, requiring greater telescope sensitivity. Since the moment of inertia increases with more carbons, all transitions with any significant population will be at centimeter wavelengths.
The origin of the diffuse interstellar bands (DIBs) has been a
mystery since their discovery many decades ago. It has been suggested
that "cumulenes" are the carriers of the DIBs. Cumulenes are
carbon chains with double bonds, instead of the alternating
single/triple bonds of other interstellar carbon chains.
The longest cumulene currently known to occur in pre-protostellar clouds is
H2C6, in which it has an abundance
of
(Langer et al. 1997).
As shown in Fig. 4.2,
H2C8
will probably have an abundance of
and a brightness
temperature of 
mK, requiring a very sensitive telescope.
At the low temperatures of these clouds, such molecules radiate most of their energy at centimeter wavelengths. It is important therefore that the SKA be very sensitive to brightness temperature. The SKA offers significant advantages. Because of its higher angular resolution it will be better coupled to the small source regions where the emission originates, assuming that the SKA is not a dilute array. Also, we have no a priori knowledge of where these emissions peak. Observations have shown that because the regions are evolving physically and chemically on comparable timescales, distributions can vary significantly between species. The mapping capability of the SKA will dramatically reduce the time to find the positions of strongest emission and allow the different evolutionary stages of closely adjacent regions to be studied.
For searches for new spectral lines whose frequencies may be uncertain, a
resolution of 
km/s is optimal.
