The Crescent Nebula (NGC6888) Region in Cygnus

Click here to see a wide field view of NGC 6888

The Crescent Nebula (NGC 6888) around the WR 136 star is Wolf-Rayet bubble. In most Wolf-Rayet bubbles the outermost edge of the [O III] emission leads the H alpha emission. Gruendl et al suggest that these offsets, when present, are due to the shock from the Wolf-Rayet bubble expanding into the circumstellar envelope. This image enhaced to show the H alpha (red) and [O III] (green) shows a clear offset between the H and [O III] emission. The [O III] emission seems to be centered on the WR star, while the H shell is off-center.

What follows is a quote from Gruendl et al.

Physical Significance of H and [O III] Morphologies

H and [O III] line images of a nebula provide useful diagnostics of its physical structure. H is a recombination line and thus shows the overall distribution of ionized material, but because the line strength of H decreases with temperature its sensitivity drops for temperatures greater than 104 K. On the other hand, the [O III] line originates in a forbidden transition whose upper level is populated by collisional excitation; therefore its intensity increases with temperature, and [O III] line images trace high-excitation regions with temperatures 104 K. Consequently, we expect the H and [O III] morphologies to be different, especially over regions where physical conditions change rapidly.

Behind a shock front, radiative cooling takes place; as the temperature drops, the density increases. Thus, a displacement between [O III] emission and H emission occurs (Cox 1972). The magnitude of the displacement depends on the postshock temperature, which is determined by the shock velocity, and the cooling rate, which is dependent on the density and metallicity. In cases in which a shock propagates into a dense medium, the cooling rate behind the shock front is high, the cooling zone is narrow, and the offset between H and [O III] emission peaks is minimal. For a shock propagating in a tenuous medium, the cooling rate is lower, the cooling zone is wider, and thus the offset between the H and [O III] emission peaks is larger and may be observable. If the ambient density is low enough, it is possible that [O III] emission is observable behind the shock, while H emission is too faint to be detected. Finally, if a shock propagates through a high-density medium and then into a low-density medium, it is possible to see bright H emission associated with the dense medium and a leading [O III] front associated with the recently shocked low-density medium. This could produce the largest displacement between the leading edges of H and [O III] emission.

References:

GRUENDL R.L. et al. A MORPHOLOGICAL DIAGNOSTIC FOR DYNAMICAL EVOLUTION OF WOLF-RAYET BUBBLES. THE ASTRONOMICAL JOURNAL, 120:2670-2678, 2000 November

Dufour R.J. et al. New HST Observations of the Wolf-Rayet Nebula NGC 6888. AAS 200th meeting, Albuquerque, NM, June 2002

 

Technical Details:

Object(s): NGC6888 The Crescent Nebula in Cygnus
Observing Site: La Hita Observatory, Toledo, Spain
Date and Time: May 26, 2006 (July 5, 2005 90 min of H alpha data)
Camera: SBIG STL 11000M
Telescope: Takahashi FSQ @ f/5
Filters: Astrodon 6nm emission line filters, H-alpha (R), O III(G)
Exposure: H alpha 150 min (10x15min); OIII 60 min (4x15 min)
Processing Software: CCD Soft, Images Plus, Photoshop
Comments: Shown at 50%

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