Thermal emission by particles in NGC 7027

Abstract

It is shown that the strong infrared continuum in the planetary nebula NGC 7027 can be quantitatively understood in terms of emission from particles within the nebula. Calculations for graphite flakes match the observations very well, although this argument does not preclude other particles having infrared absorptivities similar to graphite. The infrared energy arises from the absorption of nebular Lyman-a photons, the intensity of which is calculated, with rather good agreement for both the frequency distribution and the expected level of the infrared emission. Other effects due to the presence of particles in the planetary nebulae are discussed.