Astron. Astrophys. 342, 233-256 (1999)

1. Introduction

The Eagle Nebula, M16, is a prominent HII region lying 2 0.1 kpc from the Earth (Hillenbrand et al.1993). Optical images show it to be crossed by several opaque `Elephant Trunks', which appear as `fingers', or columns of dense obscuring material projected against the diffuse background nebular emission. The Eagle Nebula lies adjacent to the star cluster NGC6611, which contains the largest population of intermediate-mass pre-main-sequence stars known in any young cluster in the Galaxy (Hillenbrand et al. 1993). It is made up of several hundred optically visible stars whose masses lie in the range 3 M 8 , and ages range from 0.25 to 3 10⁶ years. While most of the stars in this cluster seem to have formed in a mini-starburst 2 10⁶ years ago, high-mass star-formation has been occurring over at least the last 6 10⁶ years (Hillenbrand et al. 1993). A recent study with the Hubble Space Telescope (HST - Hester et al. 1996) discovered a population of seventy-three `evaporating gaseous globules' (EGG's). About 7% of these have associated young stars, although it was speculated that a more complete survey capable of tracing a lower mass embedded population would find that as many as 20% of the EGG's might contain young stars (McCaughrean 1997).

The radiation from several of the nearby O-stars has photoevaporated much of the material in the original molecular cloud, and the fingers may mark the location of dense clumps which existed in the primordial molecular cloud from which NGC6611 was formed. These dense clumps would then have shielded the material lying behind them from the photoevaporative effects of the UV radiation, leading to the formation of the fingers (similar to the way that `capstones' can effect the geological erosion of material on the earth). The surface layers of these fingers are ionised by the UV radiation emitted by several O5 stars lying about 2 pc to the NW, which provide a total flux of 2 10⁵⁰ photons s^-1 (Hester et al. 1996). The material inside the surface layers must therefore be subjected to the pressure of the surrounding hot ionised gas, although the state of dynamical equilibrium will also depend on the internal pressure, magnetic field strengths and the gravitational stability of the cores.

The optical appearance of the fingers therefore suggests that the dense, opaque material near the tips of the fingers may have shielded the material lying behind from the effects of the radiation, and that this shielding may have contributed to the formation of the fingers. This paper will examine a) the structure and properties of the gas and dust in the fingers, b) the thermal and pressure equilibrium of the cores and fingers, and c) the prospects for future star formation and the subsequent evolution of the fingers. For brevity, we adopt a naming convention designating the three fingers, running from north to south as , , and , respectively.

© European Southern Observatory (ESO) 1999

Online publication: December 22, 1998
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