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We present spectrally resolved observations of triatomic carbon (C3) inseveral ro-vibrational transitions between the vibrational ground state and thelow-energy nu2 bending mode at frequencies between 1654-1897 GHz along thesight-lines to the submillimeter continuum sources W31C and W49N, usingHerschel's HIFI instrument. We detect C3 in absorption arising from the warmenvelope surrounding the hot core, as indicated by the velocity peak positionand shape of the line profile. The sensitivity does not allow to detect C3absorption due to diffuse foreground clouds. From the column densities of therotational levels in the vibrational ground state probed by the absorption wederive a rotation temperature (T_rot) of ~50--70 K, which is a good measure ofthe kinetic temperature of the absorbing gas, as radiative transitions withinthe vibrational ground state are forbidden. It is also in good agreement withthe dust temperatures for W31C and W49N. Applying the partition functioncorrection based on the derived T_rot, we get column densities N(C3)~7-9x10^{14} cm^{-2} and abundance x(C3)~10^{-8} with respect to H2. For W31C,using a radiative transfer model including far-infrared pumping by the dustcontinuum and a temperature gradient within the source along the line of sightwe find that a model with x(C3)=10^{-8}, T_kin=30-50 K, N(C3)=1.5 10^{15}cm^{-2} fits the observations reasonably well and provides parameters in verygood agreement with the simple excitation analysis.

Excitation and Abundance of C3 in star forming cores:Herschel/HIFI observations of the sight-lines to W31C and W49N