CO in HI self-absorbed clouds in Perseus

We have observed (CO)-C-12 J = 2 -> 1 and J = 1 -> 0 and (CO)-C-13 J = 1 -> 0 emission in two regions of H I self-absorption (HISA) in Perseus: a small, isolated HISA feature called the globule'' and a more extended HISA cloud called the complex.'' Using both large velocity gradient and Monte Carlo radiative transfer codes, we found that in the globule N((CO)-C-12) < 6.0x10(15) cm(-2), which, using photodissociation region (PDR) models, implies that N(H-2) < 9.9x10(20) cm(-2). In the complex we found that the H-2 column densities were in the range ( 1: 2 2: 2); 1021 cm(-2). By comparing the HISA and CO observations we were able to constrain the physical conditions and atomic gas fraction (f). In the globule, 8 K < T-spin < 22 K and 0.02 < f < 0.2, depending on whether the ( unknown) gas density was 102, 103, or 104 cm(-3). In the complex, 12 K < Tspin < 24 K and 0: 02 < f < 0: 05, and we were also able to constrain the gas density ( 100 cm(-3) < n < 1200 cm(-3)). These results imply that the gas in the HISA clouds is colder and denser than that usually associated with the atomic ISM and, indeed, is similar to that seen in molecular clouds. The small atomic gas fractions also imply that there is a significant molecular component in these HISA clouds, even when little or no 12CO is detected. The level of (CO)-C-12 detected and the visual extinction due to dust is consistent with the idea that these HISA clouds are undergoing a transition from the atomic to molecular phase.