Near infrared studies have been made of the extended emission from, and stellar clusters within, three visual reflection nebulae, NGC 7023, 2023, and 2068. The extended emission from each nebula consists of a smooth continuum from 1.25 to 4.8 μm, which can be described by a greybody with a color temperature of ~1000 K, and strong emission features at 3.3 and 3.4 μm. The spectrum is the same in all three sources, and is independent of position over regions 0.4-0.9 pc in diameter within each source. The 2.2 μm surface brightness distributions in NGC 7023 and 2023 agree well with the distributions of visual reflected light. The continuum emission cannot be explained by free-free emission, reflected light, fluorescent processes, field stars, or thermal emission from grains in equilibrium with the stellar radiation field.
A model is proposed in which the extended emission is due to thermal emission from very small grains (radius ~10 Å) which are briefly heated to ~1000 K by absorption of an ultraviolet photon. This model explains the agreement between near infrared and visual surface brightness distributions, and the constancy of the energy distribution with offset from the central stars. The numbers of 10 Å sized grains required by the observations are in agreement with the numbers expected from an extrapolation of the grain size distribution of Mathis, Rumpl, and Nordsieck (1977) to smaller grain sizes.
Clusters of young stars found associated with the reflection nebulae NGC 7023, 2023, and 2068 have also been studied at near infrared wavelengths. At least 30-60 % of the stars found at 2.2 μm are pre-main sequence objects, as indicated by their infrared excesses, hydrogen line emission, or irregular variability. The spatial distributions and observed luminosity functions of these young open clusters are derived, and the inferred mass function and star formation efficiencies are discussed.
