With an aim of probing the physical conditions and excitation mechanisms of warm molecular gas in individual star-forming regions , we performed Herschel SPIRE Fourier Transform Spectrometer ( FTS ) observations of 30 Doradus in the Large Magellanic Cloud ( LMC ) .
In our FTS observations , important far-infrared ( FIR ) cooling lines in the interstellar medium ( ISM ) , including CO J =4–3 to J =13–12 , [ C i ] 370 \mu m , and [ N ii ] 205 \mu m , were clearly detected .
In combination with ground-based CO J =1–0 and J =3–2 data , we then constructed CO spectral line energy distributions ( SLEDs ) on \sim 10 pc scales over a \sim 60 pc \times 60 pc area and found that the shape of the observed CO SLEDs considerably changes across 30 Doradus , e.g. , the peak transition J _ { p } varies from J =6–5 to J =10–9 , while the slope characterized by the high-to-intermediate J ratio \alpha ranges from \sim 0.4 to \sim 1.8 .
To examine the source ( s ) of these variations in CO transitions , we analyzed the CO observations , along with [ C ii ] 158 \mu m , [ C i ] 370 \mu m , [ O i ] 145 \mu m , H _ { 2 } 0–0 S ( 3 ) , and FIR luminosity data , using state-of-the-art models of photodissociation regions ( PDRs ) and shocks .
Our detailed modeling showed that the observed CO emission likely originates from highly-compressed ( thermal pressure P / k _ { B } \sim 10 ^ { 7 } –10 ^ { 9 } K cm ^ { -3 } ) clumps on \sim 0.7–2 pc scales , which could be produced by either ultraviolet ( UV ) photons ( UV radiation field G _ { UV } \sim 10 ^ { 3 } –10 ^ { 5 } Mathis fields ) or low-velocity C-type shocks ( pre-shock medium density n _ { pre } \sim 10 ^ { 4 } –10 ^ { 6 } cm ^ { -3 } and shock velocity \varv _ { s } \sim 5–10 km s ^ { -1 } ) .
Considering the stellar content in 30 Doradus , however , we tentatively excluded the stellar origin of CO excitation and concluded that low-velocity shocks driven by \sim kpc scale processes ( e.g. , interaction between the Milky Way and the Magellanic Clouds ) are likely the dominant source of heating for CO .
The shocked CO-bright medium was then found to be warm ( temperature T \sim 100–500 K ) and surrounded by a UV-regulated low pressure component ( P / k _ { B } \sim a few ( 10 ^ { 4 } –10 ^ { 5 } ) K cm ^ { -3 } ) that is bright in [ C ii ] 158 \mu m , [ C i ] 370 \mu m , [ O i ] 145 \mu m , and FIR dust continuum emission .