2026ApJ...996..128H

The destruction of giant molecular clouds (GMCs) is a key component in galaxy evolution. We theoretically model the destruction of GMCs by H II regions, which evaporate ionized gas and eject neutral gas during their expansion. H II regions follow one of three tracks, depending on the extreme-UV luminosity, S, of the ionizing OB association: the expansion can stall inside the cloud; it can break out, forming a blister (champagne) flow; or, for S > S_com, it can result in the formation of a cometary cloud. We present results for the accumulated mass loss, M_loss(t), and the final mass loss, M_loss,f, by evaporation and ejection for a range of cloud masses (10⁴ < M < 10⁷M_⊙), cloud surface densities (50 < Σ < 1000 M_⊙ pc^–2), OB association luminosities (10⁴⁴ < S < 10⁵² s^–1), and off-center position of the association. We do not consider starbursts; our neglect of radiation pressure restricts our treatment to $S\lt 1{0}^{52}[{(M/1{0}^{6}\,{M}_{\odot })}^{0.3}/({\rm{\Sigma }}/100\,{M}_{\odot }\,{\mathrm{pc}}^{-2})]$s^–1, and our neglect of gravity restricts (M/10⁶M_⊙)(Σ/100 M_⊙ pc^–2) ≲ 10. We find that M_loss,f for the range 0.1 ≲ M_loss,f/M ≲ 0.7, is proportional to S^p^, where p ∼ 0.45-0.75 depends on M, Σ, and association position. We find analytic fits to S_com as a function of Σ, M, and association position. S > S_com associations destroy at least 70% of the initial cloud. We find a critical cloud mass M_survive above which clouds never become cometary and lose ≲70% of their mass via a single association. Low-mass clouds mostly lose mass via ejection of neutral gas.