The Eagle Nebula's fingers - pointers to the earliest stages of star formation?

White, G. J.; Nelson, R. P.; Holland, W. S.; Robson, E. I.; Greaves, J. S.; McCaughrean, M. J.; Pilbratt, G. L.; Balser, D. S.; Oka, T.; Sakamoto, S.; Hasegawa, T.; McCutcheon, W. H.; Matthews, H. E.; Fridlund, C. V. M.; Tothill, N. F. H.; Huldtgren, M. and Deane, J. R. (1999). The Eagle Nebula's fingers - pointers to the earliest stages of star formation? Astronomy & Astrophysics, 342 pp. 233–256.



Molecular line, millimetre/submillimetre continuum, and mid-IR observations are reported of the opaque fingers which cross the Eagle Nebula. The fingers are surprisingly warm when viewed in the CO J= 3-2 lines, with kinetic temperatures approaching 60 K, although the lines are relatively narrow. Most of the mass in the fingers is concentrated in cores which lie at the tips of the fingers, and contain from ~ 10 to 60 M, representing 55-80% of the mass of the individual fingers. The integrated mass contained in the three fingers and the nearby extended material is ~ 200 M. The velocity fields of the gas are complex and the material is very clumpy. The best evidence for coherent velocity structure is seen running along the central finger, which has a velocity gradient ~ 1.7 km s-1 pc-1 . The fingers contain several embedded submm continuum cores, with the most intense located at the tips of the fingers. The continuum spectra of these cores shows that they are much cooler, Tdust ~ 20 K, than Tgas ~ 60 K of their respective fingers. A simple thermal and chemical model of a finger was developed to study the physical environment, which takes into account the external UV illumination ( ~ 1700 G0), and the chemical and thermal structure of a finger. The model predictions are consistent with all of the available observations. The fingers appear to have been formed after primordial dense clumps in the original cloud were irradiated by the light of its OB stars. These clumps then shielded material lying behind from the photoevaporative dispersal of the cloud, and facilitated the formation of the finger structures. The cores in the tips of the fingers appear to be at a very early stage of pre-protostellar development: there are no embedded infrared sources or molecular outflows present. The pressure inside the cores is just less than that of the surrounding gas, allowing them to be compressed by the external pressure. The cores are probably just starting the final stages of collapse, which will lead to the formation of a condensed, warm object. It is well known that such characteristics are expected from the earliest stages of objects popularly known as `protostars'. The cores in the tips of the Eagle Nebula's fingers have characteristics similar to those expected to occur in the earliest stages of protostellar formation.

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