GMCs and Star Formation in Galaxies. I. Mass Loss from a GMC Induced by an H II Region

Abstract 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 (10 4 < M < 10 7 M ⊙ ), cloud surface densities (50 < Σ < 1000 M ⊙ pc −2 ), OB association luminosities (10 44 < S < 10 52 s −1 ), and off-center position of the association. We do not consider starbursts; our neglect of radiation pressure restricts our treatment to S < 1 0 52 [ ( M / 1 0 6 M ⊙ ) 0.3 / ( Σ / 100 M ⊙ pc − 2 ) ] s -1 , and our neglect of gravity restricts ( M /10 6 M ⊙ )(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.