Origin of the thermal-pressure–FUV-field correlation in PDRs

Determine whether the observed relationship between thermal pressure and incident FUV radiation-field strength in photodissociation regions is caused by photo-evaporation-driven compression of molecular-cloud edges or by pressure equilibrium with the adjacent ionized region.

Background

The paper derives a power-law relationship between the thermal pressure, P_th, and the incident FUV-field strength, G_0, for five photodissociation regions. This relationship is broadly consistent with prior studies and with both photo-evaporation models and models invoking pressure equilibrium between the PDR and a neighboring H II region.

Because the sample contains only five PDRs and exhibits substantial scatter in the P_th–G_0 plane, the observations do not distinguish between these physical explanations. Resolving the origin of the correlation requires a larger sample and more discriminating observational constraints.

References

Our fit lies between the theoretical behaviors predicted by photo-evaporation models and by pressure equilibrium with the adjacent ionized region. Because of the low number of PDRs studied in the present work and the significant scatter of the estimates in the $P_{\text{th}$--$G_0$ plane, the current data does not clearly favor one explanation over the other.

— Physical conditions in PDRs revealed by IGRINS ro-vibrational H2 observations  (2609.24630 - Piluso et al., 21 Sep 2026) in Section 5.3, “Correlations between physical parameters”