The [ C ii ] fine–structure line at 158 \mu m is an excellent tracer of the warm diffuse gas in the ISM and the interfaces between molecular clouds and their surrounding atomic and ionized envelopes .
Here we present the initial results from Galactic Observations of Terahertz C ^ { + } ( GOT C+ ) , a Herschel Key Project devoted to study the [ C ii ] fine structure emission in the galactic plane using the HIFI instrument .
We use the [ C ii ] emission together with observations of CO as a probe to understand the effects of newly–formed stars on their interstellar environment and characterize the physical and chemical state of the star-forming gas .
We collected data along 16 lines–of–sight passing near star forming regions in the inner Galaxy near longitudes 330°and 20° .
We identify fifty-eight [ C ii ] components that are associated with high–column density molecular clouds as traced by ^ { 13 } CO emission .
We combine [ C ii ] , ^ { 12 } CO , and ^ { 13 } CO observations to derive the physical conditions of the [ C ii ] –emitting regions in our sample of high–column density clouds based on comparison with results from a grid of Photon Dominated Region ( PDR ) models .
From this unbiased sample , our results suggest that most of [ C ii ] emission originates from clouds with H _ { 2 } volume densities between 10 ^ { 3.5 } and 10 ^ { 5.5 } cm ^ { -3 } and weak FUV strength ( \chi _ { 0 } = 1 - 10 ) .
We find two regions where our analysis suggests high densities > 10 ^ { 5 } cm ^ { -3 } and strong FUV fields ( \chi _ { 0 } = 10 ^ { 4 } -10 ^ { 6 } ) , likely associated with massive star formation .
We suggest that [ C ii ] emission in conjunction with CO isotopes is a good tool to differentiate between regions of massive star formation ( high densities/strong FUV fields ) and regions that are distant from massive stars ( lower densities/weaker FUV fields ) along the line–of–sight .