IRAM spectra toward M33 Giant Molecular Clouds

Similarly high ratios have been observed in the Large Magellanic Cloud. The HCN/CO ratio varies between 0.4% and 2.9% in the disk of M33. The ^12^CO/^13^CO line ratio varies between 9 and 15 similar to variations found in the diffuse gas and the centers of GMCs of the Milky Way.

Stacking of all spectra allowed HNC and C_2_H to be detected. The resulting HCO^+^/HNC and HCN/HNC ratios of ~8 and 6, respectively, lie at the high end of ratios observed in a large set of (ultra-)luminous infrared galaxies. HCN abundances are lower in the subsolar metallicity PDR models, while HCO^+^ abundances are enhanced.

For HCN this effect is more pronounced at low optical extinctions. The observed HCO^+^/HCN and HCN/CO line ratios are naturally explained by subsolar PDR models of low optical extinctions between 3 and 10mag and of moderate densities of n=3x10^3^-3x10^4^cm^-3^, while the FUV field strength only has a small effect on the modeled line ratios. The line ratios are almost equally well reproduced by the solar-metallicity models, indicating that variations in metallicity only play a minor role in influencing these line ratios.