We present spectrally resolved observations of triatomic carbon ( C _ { 3 } ) in several ro-vibrational transitions between the vibrational ground state and the low-energy \nu _ { 2 } bending mode at frequencies between 1654–1897 GHz along the sight-lines to the submillimeter continuum sources W31C and W49N , using Herschel ’ s HIFI instrument .
We detect C _ { 3 } in absorption arising from the warm envelope surrounding the hot core , as indicated by the velocity peak position and shape of the line profile .
The sensitivity does not allow to detect C _ { 3 } absorption due to diffuse foreground clouds .
From the column densities of the rotational levels in the vibrational ground state probed by the absorption we derive a rotation temperature ( T _ { rot } ) of \sim 50–70 K , which is a good measure of the kinetic temperature of the absorbing gas , as radiative transitions within the vibrational ground state are forbidden .
It is also in good agreement with the dust temperatures for W31C and W49N .
Applying the partition function correction based on the derived T _ { rot } , we get column densities N ( C _ { 3 } ) \sim 7–9 \times 10 ^ { 14 } cm ^ { -2 } and abundance x ( C _ { 3 } ) \sim 10 ^ { -8 } with respect to H _ { 2 } .
For W31C , using a radiative transfer model including far-infrared pumping by the dust continuum and a temperature gradient within the source along the line of sight we find that a model with x ( C _ { 3 } ) =10 ^ { -8 } , T _ { kin } =30-50 K , N ( C _ { 3 } ) =1.5 10 ^ { 15 } cm ^ { -2 } fits the observations reasonably well and provides parameters in very good agreement with the simple excitation analysis .