  Several studies of nearby active galaxies indicate significantly higher
HCN-to-CO intensity ratios in AGN than in starburst (SB) environments. HCN
enhancement can be caused by many different effects, such as higher gas
densities/temperatures, UV/X-ray radiation, and non-collisional excitation. As
active galaxies often exhibit intense circumnuclear SB, high angular
resolution/sensitivity observations are of paramount importance to
disentangling the influence of SB from that of nuclear activity on the
chemistry of the surrounding molecular gas. The tight relation of HCN
enhancement and nuclear activity may qualify HCN as an ideal tracer of
molecular gas close to the AGN, providing complementary and additional
information to that gained via CO. NGC6951 houses nuclear and SB activity,
making it an ideal testbed in which to study the effects of different
excitation conditions on the molecular gas. We used the new ABCD configurations
of the IRAM PdBI to observe HCN(1-0) in NGC6951 at high angular resolution
(1''). We detect very compact (<50pc) HCN emission in its nucleus, supporting
previous hints of nuclear gas structure. Our observations also reveal HCN
emission in the SB ring and resolve it into several peaks, leading to a higher
coincidence between the HCN and CO distributions than previously reported. We
find a significantly higher HCN-to-CO intensity ratio (>0.4) in the nucleus
than in the SB ring (0.02-0.05). As for NGC1068, this might result from a
higher HCN abundance in the centre due to an X-ray dominated gas chemistry, but
a higher gas density/temperature or additional non-collisional excitation of
HCN cannot be entirely ruled out, based on these observations. The compact HCN
emission is associated with rotating gas in a circumnuclear disk/torus.
