Abstract The earliest stages of star formation are highlighted by complex interactions between accretion, outflow, and radiative processes, which shape the chemical and physical environment of the emerging protostar. James Webb Space Telescope observations of the low-mass, low-luminosity Class 0 protostar IRAS 16253–2429 (I16253) reveal a central compact source. This object exhibits a rich mid-IR emission spectrum of OH pure rotational lines and CO 2 rovibrational lines. Unusually for a young stellar object, it has no mid-IR line emission from H 2 O to match the other molecules. We demonstrate that the emitting OH molecules arise from UV photodissociation of H 2 O in its second absorption band at λ = 114–145 nm, and that the OH emission is a fluorescent cascade starting with highest-excitation rotational states. This situation offers the opportunity of using the IR OH spectrum to measure the UV flux from the central protostar. Thereby, we determine the disk-to-star accretion rate to be 3 × 10 −10 M ⊙ yr −1 , and demonstrate that the system luminosity arises mostly from the protostar’s photosphere rather than from accretion luminosity. The result is in accord with the measured outflow rate of I16253 and lies within the outflow/accretion-flow rate trend often inferred for protostars, and with episodic accretion as the dominant mechanism by which this protostar has grown.
Watson et al. (2026) studied this question.