We use the Sternberg et al .
( 21 ) theory for interstellar atomic to molecular hydrogen ( H I -to-H _ { 2 } ) conversion to analyze H I -to-H _ { 2 } transitions in five ( low-mass ) star-forming and dark regions in the Perseus molecular cloud , B1 , B1E , B5 , IC348 , and NGC1333 .
The observed H I mass surface densities of 6.3 to 9.2 M _ { \odot } pc ^ { -2 } are consistent with H I -to-H _ { 2 } transitions dominated by H I -dust shielding in predominantly atomic envelopes .
For each source , we constrain the dimensionless parameter \alpha G , and the ratio I _ { UV } / n , of the FUV intensity to hydrogen gas density .
We find \alpha G values from 5.0 to 26.1 , implying characteristic atomic hydrogen densities 11.8 to 1.8 cm ^ { -3 } , for I _ { UV } \approx 1 appropriate for Perseus .
Our analysis implies that the dusty H I shielding layers are probably multiphased , with thermally unstable UNM gas in addition to cold CNM within the 21 cm kinematic radius .