Correlations between modeled I_CO (left two plots) and I_HCN (right two plots) and their respective excitation temperatures and optical depths determined using Eqs. (13) and (14). The formatting is the same as in Fig. 3. We find that CO optical depth decreases with increasing I_CO and CO excitation, in general agreement with the findings of previous studies (e.g., Bolatto et al. 2013; Narayanan et al. 2012; Narayanan & Krumholz 2014). In our models, HCN appears subthermally excited and moderately optically thick, also in agreement with the findings of previous studies (e.g., Dame & Lada 2023; Jiménez-Donaire et al. 2017).