An optical investigation of the dumbbell planetary nebula (M27, NGC 6583)

Abstract

Imaging Fourier transform spectroscopy is used to obtain multiwavelength observations of the M27 planetary nebula's optical gas complex. The data set allows a complete cartography of the nebula's morphological properties, of its density and temperature structures in its lowexcitation zone, and of its gas excitation. Radial profiles in peak intensity indicate that the nebula is radiation-bounded along its highly inhomogeneous and clumpy minor axis. The diffuse major axis show a monotonic decrease with increasing distance from the central star and appears to be matter-bounded. Clumps, said to be formed of numerous small-scale features referred as 'knots' in the literature, are mostly located in the low-excitation outer shell. The standard low-excitation [S II] and [N II] diagnostics reveal mean density and temperature, respectively, estimated at 80 cm-3 and 10 115 K in the nebula's outer layers. Temperature fluctuations, on the plane of the sky, show a decreasing trend with increasing angular scale. A lower limit of 0.023 is obtained for the amplitude t2V of the volumetric temperature fluctuations. This indicates that temperature inhomogeneities could play a role in the solution of the abundance discrepancy problem.