All well-investigated very young and massive stars with optically thick circumstellar shells (BN-like objects) show the sillicate feature near 9.5 μm. In addition to the only BN object known in the pre-IRAS era to show the 18-μm feature in absorption our search through the IRAS LRS catalogue revealed two other objects of this type:GL 2591 and S 140-IRS. The IR point source GL 961 which is classified in the IRAS scheme as an object with a red spectrum and no 10-μm band (IRAS class 5n) was shown to be a BN object with a typical self-absorption profile in the 10-μm region and an optical depth in the band centre of about 2.5.
In the IRAS Point Source Catalogue we found nine BN-like objects with good flux quality and no confusion by extended sources. The energy distributions of all show a steep increase from 12 μm towards 100 μm. The colour indices defined by Rij = log (2iSi / iSi) (i, j = 1 : 12 μm ; i=2 : 25 μm ; i =3 : 60 μm ; i=4 : 100 μm) are in the ranges 0.0≤ R12≤1.0, 0.0≤R23≤0.5, and -0.2≤R34≤0.2. R12 and R34 are similar to the indices derived from IRAS data for IR objects with H2O masers in the Orion and Cepheus regions. For R23 the range occupied by BN-like objects seems to be narrower. Radiative transfer calculations show that the flux below 25 μm can be produced by a compact circumstellar dust shell. The IRAS observations for 1≥25 μm ca II for a second and more extended cold envelope.
From photoeIectric photometry at our 90-cm telescope we derived continuum and emlssion band fluxes within the bandpasses of the.standard IHW filters. These data were converted to give product ion rates for CN, C3, C2 , and soIids. The observations cover the range of pre-periheIion distances from'^2.1 AU to 1.1 AU and include one post-periheI ion measurement at 1.7 AU. The product ion rates of the gaseous components show a strong dependence on heliocentric distance. The result is compared with the behaviour of other comets and theoretical considerations. The dependence is less steep for the solids. This may be due to relatively pronounced backscattering of the grains. During one pre-perihe ion night (r = 1.5 AU) intensity profiles along three sections through the coma of P/Halley were measured. Compared with the Haser model the
profiles show a global anisotropy of the coma and possibly local structure.