ALMA Band 9 CO(6–5) Reveals a Warm Ring Structure Associated with the Embedded Protostar in the Cold Dense Core MC 27/L1521F

Infall and outflows, coupled with magnetic fields, rapidly structure the gas around newborn protostars. Shocks from interacting components encode the temperature and density distribution, offering a direct probe of the earliest evolution history. However, interferometric observations characterizing warm envelopes using high-excitation lines remain scarce. We present Atacama Large Millimeter/submillimeter Array Band 9 observations of the Taurus dense core MC 27/L1521F, which hosts a Class 0 protostar, targeting the CO(J = 6–5) line at an angular resolution of ∼2″ (≈300 au). We detect an off-centered ringlike structure with a diameter of ∼1000 au that was not identifiable in previous low-J CO data, where emission close to the systemic velocity is strongly affected by optical depth. The ring shows a typical peak brightness temperature of ∼3 K at our resolution. Excitation considerations indicate that the detected CO(J = 6–5) emission likely arises from relatively warm (T ≳ 20 K) and dense (n(H2) ≳ 105 cm−3) gas embedded within the surrounding cold, dense core. The morphology and kinematics suggest an energetic and localized shock-heating event, potentially linked to dynamical gas–magnetic-field interactions in the earliest protostellar phase. Our results demonstrate that high-J CO observations provide a powerful new window on warm and dense gas components, enabling a more direct view of the physical processes operating at the onset of star formation.