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The rings are gas rich but seemingly H2 poor. Since HI rapidly converts into H2, the low fmol cannot signal the consumption of the molecular gas reservoir. Nor can metallicity effects by themselves be responsible, at least in L-S. The gas phase pressure (PISM) might be a factor, as it directly affects the HI to H2 conversion rate. Elmegreen (1993) finds fmol approx (PISM / Podot)2.2 (chi / chiodot)-1, where chi is the ambient UV-field. For L-S's ring we estimated chi with Spitzer and GALEX images, and PISM approx (pi G/2)(Sigmagas2 + (sigmagas / sigma*) Sigmagas Sigma*), with the ring's stellar surface mass density derived with IRAC 4.5 µm data. We find very high gas phase pressures, with PISM / PISM,local approx 30-400, leading us to expect fmol approx 1 everywhere. L-S's ring should be dominated by molecular gas. Why isn't it?

We used photo-dissociation models in Allen et al. (2004) to estimate average gas volume densities (n) in L-S's ring given its SigmaHI and UV-field. In the northern half of the ring, n = 100-300 cm-3, implying an ISM dominated by the Cold Neutral Medium (CNM, T = 50-100 K), i.e., the precursor of cold molecular clouds. In the southwest, where SigmaSFR and SigmaHI are both much higher, the models give n approx 2 cm-3, which taken at face value, points to an ISM dominated by the Warm Neutral Medium (WNM, T approx 7000 K). How can you form stars out of this?

We believe the answer lies in fundamental differences in the environments of rings and spiral arms. Consider that molecular clouds spend approx 20 Myrs in the arms of grand design spirals like M 51, whereas the ISM is confined in rings, equally as dense and actively forming stars, for approx 200 Myr. While molecular cloud growth is enhanced in the high Sigmagas rings, the destructive effects of SNe and OB stars are also amplified. A dominant WNM might be expected as the molecular clouds become fragmented and "over-cooked" by shocks and sustained UV-fields. CO might in this case retreat to the inner-most cloud cores resulting in weak ICO and underestimates of SigmaH2. This might explain the peculiar Schmidt Laws and enhanced SFE. At the same time, higher cloud collision rates might favor the formation of unusually large molecular cloud complexes and more efficient star formation. More work remains though results from Cartwheel and L-S are intriguing.