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 NGC7027 intensities have been measured for the radio continuum and for Hi and Hen lines in the wavelength range from X = 2.17¿uni to X= 1640 Â: comparison with the calculated emission intensities gives the extinction ^4 Determinations of the standard interstellar extinction function, ^4 are critically discussed. The extinction deduced for the total radiation from NGC7027 has a dependence on wavelength for 6563 k>\> 1640Â 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 using a model which allows for a local contribution to the extinction which is variable over the surface of the nebula.