Structure and evolution of molecular clouds near H II regions. I. CO observations of an expanding molecular shell surrounding the Pelican Nebula.

This paper reports ¹²CO observations of a fragmented, expanding cloud network surrounding an old H II region, W80. Systematic velocity gradients and line splitting indicate the presence of a 3-6 × 10⁴ M_sun molecular shell expanding with a velocity V = 5 km s^-1 from a point centered near the peak of the radio f-f emission. We interpret this feature as the remnant of the ionization shock front system from the evolving H II region which was formed off-center in the original molecular cloud. A simple model is constructed to follow the evolution of the shocked gas layer through three phases: (1) the Strömgren phase in which a weak D-type front accumulates material; (2) a decompression phase during which the ionized gas pressure drops after the H II region bursts through its nearest cloud boundary; and (3) the rocket phase. The last stage, in which we identify the Pelican nebula, occurs several million years after the birth of the Strömgren sphere, when the shock has reached the back side of the molecular cloud and the neutral layer is accelerated by ionization and evaporation of H₂ into the H II region. The molecular gas shell appears extensively fragmented with numerous holes and several hot spots or cores with T_A^*(CO) ≈ 25 K. Extensive star formation has occurred here; there are many T Tauri and young emission-line stars embedded in the peripheral clouds and an infrared source is associated with one of the CO hot spots seen at the periphery of the H II region. Some of these stars presumably formed in the expanding H₂ shell.

These observations address the issue of destruction of molecular clouds when H II regions form within them. W80 is located at the extremity of a larger giant molecular cloud. In this case only a small part of the original giant cloud is being dissociated. Much of the gas has survived as accelerated molecular fragments.