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Context. The main accretion phase of protostars is characterized by the ejection of material in the form of bipolar jets and outflows. In addition, external UV irradiation can potentially have a significant impact on the excitation conditions within these outflows. High-resolution observations in the mid-infrared (mid-IR) allow us to investigate the details of those energetic processes through the emission of shock-excited H₂.
Aims. Our aim is to spatially resolve H₂, ionic, and atomic emission within the outflows of low-mass protostars, and investigate its origin in connection to shocks influenced by external ultraviolet irradiation.
Methods. We analyze spectral maps of 5 Class I protostars in the Ophiuchus molecular cloud from the James Webb Space Telescope (JWST) Medium Resolution Spectrometer (MIRI/MRS). The MIRI/MRS field of view covers an area between ∼3.2″ × 3.7″ at 6 μm and 6.6″ × 7.7″ at 25 μm and with a resolution of ∼0.3 to 1″, corresponding to spatial scales of a few hundred astronomical units.
Results. Four out of five protostars in our sample show strong H₂, [Ne II], and [Fe II] emission associated with outflows and jets. Pure rotational H₂ transitions from S(1) to S(8) are found and show two distinct temperature components on Boltzmann diagrams with rotational temperatures of ∼500-600 K and ∼1000-3000 K, respectively. Both C-type shocks propagating at high pre-shock densities (n_H ≥ 10⁴ cm^–3) and J-type shocks at low pre-shock densities (n_H ≤ 10³ cm^–3) reproduce the observed line ratios. However, only C-type shocks produce sufficiently high column densities of H₂, whereas predictions from a single J-type shock reproduce the observed rotational temperatures of the gas better. A combination of various types of shocks could play a role in protostellar outflows as long as UV irradiation is included in the models. The origin of this radiation is likely internal, since no significant differences in the excitation conditions of outflows are seen at various locations in the cloud.
Conclusions. Observations with MIRI offer an unprecedented view of protostellar outflows, allowing us to determine the properties of outflowing gas even at very close distances to the driving source. Further constraints on the physical conditions within outflows can be placed thanks to the possibility of direct comparisons of such observations with state-of-the-art shock models.