Extinction of NGC 7027.

Abstract

Emission intensities of recombination lines in hydrogenic spectra are known accurately relative to intensities in the free-free radio continuum. For NGC 7027 intensities have been measured for the radio continuum and for H I and He II lines in the wavelength range from 2.17 microns to 1640 A; comparison with the calculated emission intensities gives the wavelength-dependent extinction. Determinations of the standard interstellar extinction function are critically discussed. The extinction deduced for the total radiation from NGC 7027 has a dependence on wavelength for wavelengths between 6563 A and 1640 A which is in excellent agreement with the adopted standard results, but there are some anomalies for longer wavelengths and for the ratio of total to selective extinction. These can be explained by using a model that allows for a local contribution to the extinction which is variable over the surface of the nebula.