2019A&A...628A.113L

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.In our FTS observations, important far-infrared (FIR) cooling lines in the interstellar medium, including CO J=4-3 to J=13-12, [CI] 370µm, and [NII] 205µm, 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 ∼10pc scales over a ∼60pc*60pc area and found that the shape of the observed CO SLEDs considerably changes across 30 Doradus. For example, 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 α ranges from ∼0.4 to ∼1.8. To examine the source(s) of these variations in CO transitions, we analyzed the CO observations, along with [CII] 158µm, [CI] 370µm, [OI] 145µm, H₂ 0-0 S(3), and FIR luminosity data, using state-of-the-art models of photodissociation regions and shocks. Our detailed modeling showed that the observed CO emission likely originates from highly compressed (thermal pressure P/k_B∼10⁷-10⁹K/cm³) clumps on ∼0.7-2pc scales, which could be produced by either ultraviolet (UV) photons (UV radiation field G_UV∼10³-10⁵ Mathis fields) or low-velocity C-type shocks (pre-shock medium density n_pre∼10⁴-10⁶cm^–3 and shock velocity v_s∼5-10km/s). 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 kiloparsec-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∼100-500K) and surrounded by a UV-regulated low-pressure component (P/k_B ∼ a few (10⁴-10⁵)K/cm³) that is bright in [CII] 158µm, [CI] 370µm, [OI] 145µm, and FIR dust continuum emission.