Dense Gas in M33 (HerM33es)
  We aim to better understand the emission of molecular tracers of the diffuse
and dense gas in giant molecular clouds and the influence that metallicity,
optical extinction, density, far-UV field, and star formation rate have on
these tracers. Using the IRAM 30m telescope, we detected HCN, HCO+, 12CO, and
13CO in six GMCs along the major axis of M33 at a resolution of ~ 114pc and out
to a radial distance of 3.4kpc. Optical, far-infrared, and submillimeter data
from Herschel and other observatories complement these observations. To
interpret the observed molecular line emission, we created two grids of models
of photon-dominated regions, one for solar and one for M33-type subsolar
metallicity. The observed HCO+/HCN line ratios range between 1.1 and 2.5.
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 12CO/13CO
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 C2H 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 4 and 10 mag 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.
