Visual and infrared observations of the distant Comets P/Stephan-Oterma (1980g), Panther (1980u), and Bowell (1980b)

Abstract

Broadband observations of comets P/Stephan-Oterma (1980g), Bowell (1980b), and Panther (1980u) in the visual [0.5≾ λ(µm)≾0.9] and infrared [1.2≾λ(µm)≾20] wavelength regions are reported together with measurements in the 1.5-2.4-µm wavelength range having 5% spectral resolution. The visual data indicate the existence of solid grains in extended halos around the nuclei of the three comets. The visual photometric profiles of comets P/Stephan- Oterma and Panther are interpreted as evidence that grains around Panther and those close to the nucleus of P/Stephan-Oterma are sublimating. Broadband near-infrared and thermal infrared measurements of comet Panther suggest the presence of 2—4-µm radius particles in the coma. The particles within a 5.8X 10^6-m diameter region centered on the comet have a total cross section of 10^8 m^2 and a near-infrared geometric albedo of about 14%. Comet Bowell presents a total cross section of 3 X 10^8 m^2 within a 1.2 X 10^7-m region centered on the comet and its coma grains also have an albedo of 14%. The near-infrared spectrum of P/Stephan-Oterma is a featureless solar-reflection continuum. The near-infrared spectra of Bowell and Panther exhibit features which are similar in the two comets. The spectral features are not due to H_2O, CH_4, or CO_2 ices nor to emissions from gases released from the nuclei nor to reflection from mineral grains of known composition in the comae. The spectrum of solid ammonia provides the best match to the near infrared; it is nevertheless significantly different from the comet spectra. The synthesis of the visual data with the infrared data is attempted in terms of a model involving a mantle of volatile material on the nuclei of Bowell and Panther, but not on P/Stephan-Oterma. The composition of the mantle cannot be exactly specified from the existing data but a complex molecule incorporating the N-H bond may be present.