Photoevaporation of Circumstellar Disks Due to External Far‐Ultraviolet Radiation in Stellar Aggregates

When stars form within small groups (with N ⋆ ≈ 100 − 500 members), their circumstellar disks are exposed to relatively little EUV (hν> 13.6 eV) radiation but a great deal of FUV (6 eV < hν < 13.6 eV) radiation ( ∼ 103 times the local interstellar FUV field) from the most massive stars in the group. This paper calculates the mass loss rates and evaporation time scales for circumstellar disks exposed to external FUV radiation. Previous work treated large disks and/or intense radiation fields in which the disk radius rd exceeds the critical radius rg where the sound speed in the FUV heated surface layer exceeds the escape speed; it has often been assumed that photoevaporation occurs for rd> rg and is negligible for rd < rg. Since rg> ∼100 AU for FUV heating, this would imply little mass loss from the planet-forming regions of a disk. In this paper, we focus on systems in which photoevaporation is suppressed because rd < rg and show that significant mass loss still takes place as long as rd/rg> ∼ 0.1 − 0.2. Some of the gas extends beyond the disk edge (or above the disk surface) to larger distances where the temperature is higher, the escape speed is lower, and an outflow develops. The resulting