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

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

G.J. White 1, R.P. Nelson 2, W.S. Holland 3, E.I. Robson 3, J.S. Greaves 3, M.J. McCaughrean 4, G.L. Pilbratt 5, D.S. Balser 6, T. Oka 7, S. Sakamoto 8, T. Hasegawa 9, W.H. McCutcheon 10, H.E. Matthews 3, C.V.M. Fridlund 5, N.F.H. Tothill 1, M. Huldtgren 11 and J.R. Deane 1

1 Department of Physics, Queen Mary and Westfield College, University of London, Mile End Road, London E1 4NS, UK
2 Astronomy Unit, Queen Mary and Westfield College, University of London, Mile End Road, London E1 4NS, UK
3 Joint Astronomy Centre, 660 N A`hku Place, University Park, Hilo, Hawaii 96720, USA
4 Astrophysikalisches Institut Potsdam, An der Sternwarte 16, D-14482 Potsdam, Germany
5 ESA Astrophysics Division, Space Science Department, ESTEC, P.O.Box 299, 2200 AG Noordwijk, The Netherlands
6 NRAO, P.O. Box 2, Green Bank, WV 24944, USA
7 Cosmic Radiation Laboratory, RIKEN, 2-1 Hirosawa, Saitama 351-01, Japan
8 Nobeyama Radio Observatory, Nobeyama, Minamisaku, Nagano 384-13, Japan
9 Institute of Astronomy, University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181, Japan
10 University of British Columbia, Physics Department, Vancouver, BC V6T 2A6, Canada
11 Stockholm Observatory, SE-13336 Saltsjobaden, Sweden

Received 24 June 1998 / Accepted 16 October 1998

Abstract

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 [FORMULA] 10 to 60 [FORMULA], representing 55-80% of the mass of the individual fingers. The integrated mass contained in the three fingers and the nearby extended material is [FORMULA] 200 [FORMULA]. 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 [FORMULA] 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, [FORMULA] [FORMULA] 20 K, than [FORMULA] [FORMULA] 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 ([FORMULA] 1700 [FORMULA]), 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 .

Key words: infrared: ISM: continuum – ISM: molecules – ISM: individual objects: Eagle Nebula – ISM: H ii regions – ISM: general ISM: clouds

Send offprint requests to: g.j.white@qmw.ac.uk

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© European Southern Observatory (ESO) 1999

Online publication: December 22, 1998
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