Definition of the various zones for the two models, for R4 (54 pc). Each panel shows the binned trends of the right panels of Fig. 13 (grey circles with error bars). The fitted trend (black symbols with error bars) is the grey trend corrected for the offset (Table 8). We define the three zones depending on their starlight intensities. Each zone corresponds to a different regime of the observed gas-to-dust mass ratio. The blue-green-red stripes are the envelopes of the fits of Eq. (25) to the trend. The yellow and purple dashed error bars, with the star symbol, represent the 90% confidence uncertainities on and , respectively (Table 5). This complex figure is explained in details in Sect. 4.3.6.