The spectrum of P/Gehrels 3 does not show any sign of activity but a featureless red continuum.
The spectrum presented in Fig. 1 has been obtained when the comet was at 4.5 AU from the Sun: at that distance the models without stable crust foresee an activity level of the order of mol s-1 for H2O flux, while the model with a stable crust shows a water activity of mol s-1. The volatiles flux remains quite constant along the whole orbit of P/Gehrels, depending on the original amount of volatiles defined in the models. The dust flux is, in any case, very low.
From the results described above it can be argued that a comet, without crust, on the present orbit of P/Gehrels 3 should have a level of water flux quite low, due to the large distance from the Sun (the minimum distance from the Sun is 3.5 AU) and the low surface temperatures reached by the body. Also the dust flux is very small, being of the order of few kilograms per second at the perihelion. At 4.5 AU from the Sun, the distance where the comet was at the epoch of this observation, the dust flux foreseen by our models is of the order of 0.01 Kg/s.
In the model in which a thick crust is present, the dust flux is negligible and the water flux is strongly reduced. The activity level depends on the crust thickness and porosity: in the hypothesis of a very "old" and devolatilized comet, with a large amount of material forming a cohesive crust, the comet could be quite completely inactive and could assume an asteroidal aspect.
At 4.5 AU from the Sun, all the models discussed foresee very low dust and gas fluxes, even if the most volatile species are emitted along the whole comet orbit. No sign of gas emission has been found in the P/Gehrels 3 spectrum. This observation should be consistent with the general results of the models. The obtained featureless spectrum can be considered as typical for comet nuclei and can be added to the few spectra of comet nuclei known to date. The spectrum of the comet P/Gehrels 3 is very similar to those of D-type, primitive asteroids. This implies that Trojan and dark asteroids need more investigation to better understand their composition, evolution and origin.
© European Southern Observatory (ESO) 2000
Online publication: February 25, 2000