2017MNRAS.469.1403K

We consider the evolution of an isentropic thermal instability in the atomic zone of a photodissociation region (PDR). In this zone, gas heating and cooling are associated mainly with photoelectric emission from dust grains and fine-structure lines ([C II] 158, [O I] 63 and [O I] 146 µm), respectively. The instability criterion has a multi-parametric dependence on the conditions of the interstellar medium. We found that instability occurs when the intensity of the incident far-ultraviolet field G₀ and gas density n are high. For example, we have 3 x 10³ < G₀ < 10⁶ and 4.5 x 10⁴ < n < 10⁶ cm^–3 at temperatures 360 < T < 10⁴ K for optical thin fine-structure lines and for typical carbon and oxygen abundances ξ_C = 1.4 x 10^–4 and ξ_O = 3.2 x 10^–4. If we take line opacities into account, these ranges of G₀, n and T are expanded. Moreover, the instability criterion depends significantly on the relation between ξ_C and ξ_O abundances. We also give examples of observed PDRs where instability could occur. For these PDRs, the characteristic perturbation growth time is t_inst ∼ 10³-10⁴ yr and the distance characterizing the formation of secondary waves is L ∼ 10^–3-5 x 10^–2 pc. For objects that are older than t_inst and have sizes of the atomic zone larger than L, we expect that instability influences the PDR structure significantly. The presence of multiple shock waves, turbulent velocities of several kilometres per second and inhomogeneities with higher density and temperature than the surrounding medium can characterize isentropic thermal instability in PDRs.