Structure of a thermally subcritical core. The figure shows the CO abundance, mean extinction, and gas density in the same format as Fig. 3. Here the model has the parameters of a stable, starless, thermally subcritical core. The asymmetries are perturbations from an internal oscillation. The oscillations are further illustrated in Figs. 6 and 7 of Broderick et al. (2007). Parameters are listed in Table 1.