We have observed ^ { 12 } CO J = 2 \rightarrow 1 and J = 1 \rightarrow 0 , and ^ { 13 } CO J = 1 \rightarrow 0 emission in two regions of HI Self-Absorption ( HI SA ) in Perseus : a small , isolated HI SA feature called the globule and a more extended HI SA cloud called the complex .
Using both Large Velocity Gradient and Monte Carlo radiative transfer codes we found that , in the globule , N ( ^ { 12 } CO ) < 6.0 \times 10 ^ { 15 } cm ^ { -2 } which , using PDR models , implies that N ( H _ { 2 } ) < 9.9 \times 10 ^ { 20 } cm ^ { -2 } .
In the complex we found that the H _ { 2 } column densities ranged from 1.2 - 2.2 \times 10 ^ { 21 } cm ^ { -2 } .
By comparing the HI SA and CO observations we are 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 is 10 ^ { 2 } , 10 ^ { 3 } , or 10 ^ { 4 } cm ^ { -3 } .
In the complex , 12 K < T _ { spin } < 24 K , 0.02 < f < 0.05 , and we were also able to constrain the gas density ( 100 < n < 1200 cm ^ { -3 } ) .
These results imply that the gas in the HI SA 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 HI SA clouds , even when little or no ^ { 12 } CO is detected .
The level of ^ { 12 } CO detected and the visual extinction due to dust is consistent with the idea that these HI SA clouds are undergoing a transition from the atomic to molecular phase .