Isentropic instability and dynamic substructures in the Orion Bar photodissociation region: Analytical and numerical insights

Context . The Orion Bar photodissociation region (PDR), which is shaped by far-ultraviolet (FUV) radiation, displays filamentary substructures and non-thermal gas motions across its ionized, atomic, and molecular zones, as observed by ALMA, JWST, and other telescopes, suggesting dynamic processes beyond static equilibrium. Aims . Our goal is to test isentropic instability as a mechanism for these features, focusing on the atomic zone’s shockwave structures and their alignment with observational data. Methods . We adapted gasdynamic equations to derive analytical solutions for shockwave pulse properties and complemented them with numerical simulations to track their evolution under varying initial conditions while incorporating a heat-loss function reflecting Orion Bar conditions. Results . Our analytical and numerical results yield pulse widths (∼0.001 pc) and periods (∼0.02 pc) that closely match the observed substructure sizes (0.002–0.004 pc) and separations (0.01 pc), while the velocity amplitudes (0.72 c S ) agree with non-thermal estimates. The leading pulse amplitude reaches a stationary value within 5000–40 000 years, which is well within the estimated ∼10 5 −year lifetime of the Orion Bar PDR. Conclusions . This study demonstrates that isentropic instability can reproduce the Orion Bar’s dynamic features, bridging theoretical predictions and high-resolution observations from ALMA and JWST.