Chiron activity and thermal evolution

We discuss the results obtained by simulating the thermal evolution of 2060 Chiron, the largest of the Centaur objects. The study of Chiron is extremely important because of the presence of activity in spite of its large distance from the Sun. In this paper we study the thermal evolution of a Chiron-like body under different assumptions concerning the initial composition and taking into account its evolutionary history. The composition of Centaurs and Kuiper belt objects is almost unknown: we can guess that Chiron's "cometary behavior" is an indication of a composition dominated by different ices. We have therefore computed the thermal evolution of Chiron using a numerical code developed to study the thermal evolution of cometary nuclei. As in the case of comets, we assume that the ice is mixed with dust and that the body is highly porous. CO is present in the mixture both as ice and as gas trapped in the amorphous ice. Our conclusion is that Chiron's activity can be explained only by assuming a composition in which CO is present not far from the object's surface as an ice or as a gas trapped in the amorphous ice.