The rotational transitions of carbon monoxide ( CO ) are the primary means of investigating the density and velocity structure of the molecular interstellar medium .
Here we study the lowest four rotational transitions of CO towards high–latitude translucent molecular clouds ( HLCs ) .
We report new observations of the J = ( 4–3 ) , ( 2–1 ) , and ( 1–0 ) transitions of CO towards eight high–latitude clouds .
The new observations are combined with data from the literature to show that the emission from all observed CO transitions is linearly correlated .
This implies that the excitation conditions which lead to emission in these transitions are uniform throughout the clouds .
Observed ^ { 13 } { C } O/ ^ { 12 } { C } O ( 1–0 ) integrated intensity ratios are generally much greater than the expected abundance ratio of the two species , indicating that the regions which emit ^ { 12 } { C } O ( 1–0 ) radiation are optically thick .
We develop a statistical method to compare the observed line ratios with models of CO excitation and radiative transfer .
This enables us to determine the most likely portion of the physical parameter space which is compatible with the observations .
The model enables us to rule out CO gas temperatures greater than \sim 30 { { K } } , since the most likely high-temperature configurations are 1 { pc } \mkern - 5.0 mu -sized structures aligned along the line of sight .
The most probable solution is a high density and low temperature ( HDLT ) solution , with volume density , n = { 10 ^ { 4.5 \pm 0.5 } } { { cm } ^ { -3 } } , kinetic temperature , { { T _ { k } } } \approx 8 { { K } } , and CO column density per velocity interval { { { N } _ { \scriptscriptstyle CO } / { \Delta { V } } } } = { 10 ^ { 16.6 \pm 0.3 } } { { % cm } ^ { -2 } } / ( { { { km } } { { s } } ^ { -1 } } ) .
The CO cell size is L \sim 0.01 { { pc } } ( \sim 2000 AU ) .
These cells are thus tiny fragments within the \sim 100 times larger CO-emitting extent of a typical high–latitude cloud .
We discuss the physical implications of HDLT cells , and we suggest ways to test for their existence .