Spatially resolved thermal dust emission in the L1157 outflow reveals grain-driven molecular enrichment

Protostellar outflow shocks reshape local dust properties and molecular chemistry. The L1157 outflow is an archetypal chemically rich shocked region, but the thermal dust associated with its successive shocks has remained unresolved because molecular-line contamination obscures the broadband continuum. We aim to resolve the dust emission toward L1157 B0-B1-B2 and establish observational constraints on the relationship between the dust evolution and shock-driven chemistry. We obtained new James Clerk Maxwell Telescope (825 906,μm) spectral-line observations and Submillimeter Array (1.1 1.4,mm) continuum observations toward L1157 B0-B1-B2, probing -- -- spatial scales from 0.4,pc to 1200,au. After removing molecular-line contamination on a pixel-by-pixel basis, we derived the dust temperature and density profile and managed to constrain the dust spectral index using continuum data from 70,μm to 1.3,mm. Combining with previous ̊m NH_3 observations, these data were interpreted using a newly developed physicochemical shock model. The line-corrected continuum maps reveal the dust distribution across successive shocks in the southern lobe of L1157. At ̊m 10^4,au resolution, we recovered the dust temperature from the outflow cavities to the protobinary envelope. The dust opacity index (β≈1.8 2.3) indicates that grains have not grown to millimeter sizes throughout the shocked regions. At ̊m 10^3,au resolution, the dust emission resolves into compact clumps along the precessing jet, whereas gaseous ̊m NH_3 -- peaks at the shocked fronts, reaching abundances of with respect to ̊m H_2, even where the 0.85 and 1.3,mm dust emission is detected at only 3 5σ. Our modeling shows that rm NH_3 forms predominantly on grain surfaces and is released through shock-induced sputtering, with the highest abundances occurring where -- post-shock re-adsorption remains inefficient. Spatially resolved dust continuum imaging provides a direct observational probe of grain evolution, highlighting the fundamental role of dust evolution in shaping the chemistry of protostellar shocks.