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

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7. The future evolution of the fingertip cores

We may now ask what the eventual fate of the dense clumps in the fingers is likely to be. Using the virial theorem, assuming a constant density profile for the clumps, and assuming them to be immersed in a background medium which exerts a surface pressure P s, we obtain for the virial mass:

[EQUATION]

where r is the clump radius, and G is the gravitational constant (Bowers & Deeming 1984). If the clumps are immersed in background material of number density n (H 2) = 2 104 cm-3, the line widths inside and outside the clump are [FORMULA] 2 km s-1, and the clumps have a radius of r = 0.083 pc, then the estimated virial mass is M v = 21.6 [FORMULA], The clump mass consistent with the submm dust emission is M [FORMULA] 20 [FORMULA], and that estimated from the CO column density is M [FORMULA] 30 [FORMULA], indicating that the core of [FORMULA] is either marginally stable against collapse, or in the earliest stages of Class 0 development. The similarity of the clump and virial masses adds weight to the argument that all of the clumps pre-existed the exposure of the fingers to the ionising radiation of the stars. It is likely that they are gravitationally bound objects, and not density enhancements resulting from a radiation driven implosion.

When the IS-front propagates into the clumps, the effective external pressure acting on the clumps will rise from P / k = 3.5 106-1.2 108 cm-3 K, resulting in the cloud being strongly compressed. The question of whether this shock-induced implosion will lead to star formation in the clumps, or to their disruption, remains unclear, since the crossing time of the shock and the gravitational free-fall time of the clump are comparable (i.e. [FORMULA] [FORMULA] [FORMULA] [FORMULA] 105 years). It is likely that numerical simulations will be required to provide a definitive answer to both this question, and to develop an understanding of the relationship of the prodigious star formation rate in the [FORMULA] to that of the fingers.

Although we have concluded that the dense clumps observed in the fingertips have not been formed by the action of RDI, we note that the clumps have an elongated appearance as seen in Fig. 4. This is probably due to the pressure of the IS-front beginning to compress the clumps as the front propagates into the fingers.

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© European Southern Observatory (ESO) 1999

Online publication: December 22, 1998
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