Introduction

There is an increasing body of evidence for a population of large molecules in the interstellar medium. Large molecules have been suggested as the source of near infrared continuum radiation and the near infrared emission bands observed in reflection nebula, planetary nebula, HII regions and active galaxies (Duley and Williams 1981, Léger and Puget 1984, Allamandola, Tielens and Barker 1985, d'Hendecourt et al. 1986, Barker, Allamandola and Tielens 1987). Large molecules have also been proposed as the carriers of the diffuse interstellar bands (vander Zwet and Allamandola 1985, Léger and d'Hendecourt 1985, Crawford, Tielens and Allamandola 1985). The large molecules proposed have between 30 and 100 atoms with suggested abundances in the range 10−7−10−6 relative to hydrogen.

Mathis, Rumpl and Nordsieck (1977; MRN) determined a size distribution for grains that would fit the extinction measured to many sources. However, existing extinction measurements do not extend far enough into the ultraviolet to infer the population of the smallest grains. An extrapolation of the MRN distribution to very small grains having between 30 and 100 atoms gives an abundance roughly the same as that proposed for the large molecules, suggesting that the large molecules may be an extension of the interstellar grain distribution.

It appears that a population of small grains or large molecules exists in much of the interstellar medium. We will use the terms large molecules and small grains interchangeably to refer to clusters of between 30 and 100 atoms.