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

¹ Department of Theoretical Physics, Lebedev Physical Institute, Samara, Russia
² Department of Physics, Samara National Research University, Samara, Russia

^★ Corresponding authors.

Received: 11 March 2025
Accepted: 15 July 2025

Abstract

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 ∼10⁵−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.