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We have studied two star-forming dense cores in the GMC Mon R2 using IRAS HIRES images and maps obtained with QUARRY for molecular transitions such as $CO\;1-0$, $^{13}CO\;1-0$, $CS\;2-1$, $HCO^{+} \;1-0$ and $HCN\;1-0$. Both cores are found to be located on a large expanding shell revealed by our Nyquist-sampled $CO$ maps covering $\sim 3^{\circ} \times 3^{\circ}$ region of Mon R2. Each of dense cores is previously known to contain an ultracompact HII region powered by a B0 type star and a bipolar outflow. IRAS HIRES images reveal that each of them has a relatively large fragmented ring-like dust emission feature, which is silhouetted by a number of reflection nebulae and IRAS point sources. The gas emission of these cores manifests remarkable differences in terms of their morphologies revealed by different tracers and the relative intensities of these tracers. We suggest that both dense cores are preexisting clumps before the formation of the large expanding shell, and now harbor two generations of young stellar objects with the younger generation being triggered by the event responsible for the large expanding shell. We also suggest that the different gas properties of the cores could be a result of different chemical evolution of the cores with the formation of young stars of different masses.
