Compact H II regions have densities [??]5 x [10.sup.3] [cm.sup.3], sizes of [??]0.5 pc, ionized gas mass of ~1 [M.sub.[dot encircle]], and EM [??] [10.sup.7] pc [cm.sup.-6] while ultracompact H II regions have sizes of [??] 0.1 pc, and thermal electron densities of [??][10.sup.4] [cm.sup.3], mass of ionized gas of ~[10.sup.-2] [M.sub.[dot encircle]], and EM [??] [10.sup.7] pc [cm.sup.-6] [2, 3].
Israel, "Compact H II regions and OB star formation," Annual Review of Astronomy and Astrophysics, vol.
Habing, "Compact H II regions near the OH source ON-2," Astronomy & Astrophysics, vol.