We investigate the physical conditions of the CO gas , based on the submillimeter imaging spectroscopy from a 2 ^ { \prime } \times 7 ^ { \prime } ( 1.5 \times 5 \mathrm { pc ^ { 2 } } ) area near the young star cluster , Trumpler 14 of the Carina Nebula .
The observations presented in this work are taken with the Fourier Transform Spectrometer ( FTS ) of the Spectral and Photometric Imaging REceiver ( SPIRE ) onboard the Herschel Space Observatory .
The newly observed spectral lines include \mathrm { [ C \textsc { i } ] 370 } \mathrm { \mu m } , \mathrm { [ C \textsc { i } ] 609 } \mathrm { \mu m } , and CO transitions from \mathrm { J } = 4 - 3 to \mathrm { J } = 13 - 12 .
Our field of view covers the edge of a cavity carved by Trumpler 14 about 1 \mathrm { Myr } ago and marks the transition from H ii regions to photo-dissociation regions .
The observed CO intensities are the most prominent at the northwest region , Car I-E. With the state-of-the-art Meudon PDR code , we successfully derive the physical conditions , which include the thermal pressure ( P ) and the scaling factor of radiation fields ( G _ { \mathrm { UV } } ) , from the observed CO spectral line energy distributions ( SLEDs ) in the observed region .
The derived G _ { \mathrm { UV } } values generally show an excellent agreement with the UV radiation fields created by nearby OB-stars and thus confirm that the main excitation source of the observed CO emission are the UV-photons provided by the massive stars .
The derived thermal pressure is between 0.5 - 3 \times 10 ^ { 8 } \mathrm { K cm ^ { -3 } } with the highest values found along the ionization front in Car I-E region facing Trumpler 14 , hinting that the cloud structure is similar to the recent observations of the Orion Bar .
We also note a discrepancy at a local position ( < 0.17 \times 0.17 \mathrm { pc ^ { 2 } } ) between the PDR modeling result and the UV radiation fields estimated from nearby massive stars , which requires further investigation on nearby objects that could contribute to local heating , including outflow .
Comparing the derived thermal pressure with the radiation fields , we report the first observationally-derived and spatially-resolved P \sim 2 \times 10 ^ { 4 } G _ { \mathrm { UV } } relationship .
As direct comparisons of the modeling results to the observed ^ { 13 } \mathrm { CO } , [ O i ] 63 \mathrm { \mu m } , and [ C ii ] 158 \mathrm { \mu m } intensities are not straightforward , we urge the readers to be cautious when constraining the physical conditions of PDRs with combinations of ^ { 12 } \mathrm { CO } , ^ { 13 } \mathrm { CO } , [ C i ] , [ O i ] 63 \mathrm { \mu m } , and [ C ii ] 158 \mathrm { \mu m } observations .