Minimal HCN emission from Molecular Clouds in M33
  Since HCN emission has been shown to be a linear tracer of ongoing star
formation activity, we have searched for HCN (J = 1->0) emission from known
GMCs in the nearby galaxy M33. No significant HCN emission has been found along
any of the lines of sight. We find two lines of sight where CO-to-HCN
integrated intensity ratios up to 280, nearly a factor of 6 above what is found
in comparable regions of the Milky Way. Star formation tracers suggest that the
HCN-to-star formation rate ratio (L_HCN/M_SFR) is a factor of six lower than
what is observed in the Milky Way (on average) and local extragalactic systems.
Simple chemical models accounting for the sub-solar N/O ratio suggest that
depletion cannot account for the high CO-to-HCN ratios. Given HCN formation
requires high extinction (A_V > 4), low metallicity may yield reduced dust
shielding and thus a high CO/HCN ratio. The turbulence and structure of GMCs in
M33 are comparable to those found in other systems, so the differences are
unlikely to result from different GMC properties. Since lower CO-to-HCN ratios
are associated with the highest rates of star formation, we attribute the
deficits in part to evolutionary effects within GMCs.
