2023A&A...674A..64L

Context. Far-infrared (FIR) line emission provides key information about the gas cooling and heating due to shocks and UV radiation associated with the early stages of star formation. Gas cooling via FIR lines might, however, depend on metallicity.
Aims. We aim to quantify the FIR line emission and determine the spatial distribution of the CO rotational temperature, ultraviolet (UV) radiation field, and H₂ number density toward the embedded cluster Gy 3-7 in the CMa-l224 star-forming region, whose metallicity is expected to be intermediate between that of the Large Magellanic Cloud and the Solar neighborhood. By comparing the total luminosities of CO and [OI] toward Gy 3-7 with values found for low- and high-mass protostars extending over a broad range of metallicities, we also aim to identify the possible effects of metallicity on the FIR line cooling within our Galaxy.
Methods. We studied SOFIA/FIFI-LS spectra of Gy 3-7, covering several CO transitions from J = 14-13 to 31-30, the OH doublet at 79 μm, the [OI] 63.2 and 145.5 μm, and the [CII] 158 μm lines. The field of view covers a 2' × 1' region with a resolution of ∼7″-18″.
Results. The spatial extent of CO high-J (J_up ≥14) emission resembles that of the elongated 160 μm continuum emission detected with Herschel, but its peaks are offset from the positions of the dense cores. The [OI] lines at 63.2 μm and 145.5 μm follow a similar pattern, but their peaks are found closer to the positions of the cores. The CO transitions from J = 14-13 to J = 16-15 are detected throughout the cluster and show a median rotational temperature of 170 ± 30 K on Boltzmann diagrams. Comparisons to other protostars observed with Berschel show a good agreement with intermediate-mass sources in the inner Galaxy. Assuming an origin of the [OI] and high-J CO emission in UV-irradiated C-shocks, we obtained pre-shock H₂ number densities of 10⁴-10⁵ cm^–3 and UV radiation field strengths of 0.1-10 Habing fields (G₀).
Conclusions. Far-IR line observations reveal ongoing star formation in Gy 3-7, dominated by intermediate-mass Class 0/I young stellar objects. The ratio of molecular-to-atomic far-IR line emission shows a decreasing trend with bolometric luminosities of the protostars. However, it does not indicate that the low-metallicity has an impact on the line cooling in Gy 3-7.