¹²CO over ¹³CO intensity ratio plotted vs. the IRAS flux ratio f₆₀/f₁₀₀, used as a proxy for the dust temperature. For the line ratios, the uncertainties were derived using a Monte Carlo simulation using 10 000 samples and following a similar prescription to Crocker et al. (2012). The sources are from different samples: this work (black circles), Aalto et al. (1995, blue squares), and Costagliola et al. (2011, green diamonds). Errors are plotted at 1σ level, although x-axis errors are not appreciable for many sources. The two labeled sources are at odds with the rest of the sample for their observed f₆₀/f₁₀₀ ratio.