Boundary-Supported Persistence: Structural Conditions for Long-Lived Gas Without Star Formation

We develop a structural framework for understanding long-lived, non-star-forming gas systems that persist without self-gravity dominating their dynamics. Motivated by recent observational analyses of starless gas clouds, we identify a class of “threshold objects” occupying a boundary regime: coherence is sufficient for persistence and morphology, yet insufficient for irreversible mass lock-in and star formation. We distinguish mechanical closure—achieved through pressure balance and boundary-mediated confinement—from thermodynamic closure, which requires independent interface diagnostics to establish balanced energy routing. Within this framework, persistence does not imply gravitational binding or entropy minimization; instead, it reflects sustained circulation enabled by external supply and boundary work. This paper formalizes the conceptual distinctions required to interpret such systems without invoking dark halos or premature thermodynamic claims.