Low thermal conductivity of the superfast rotator (499998) 2011 PT

Abstract

Context. Asteroids up to a few tens of meters in diameter may spin very fast, completing an entire rotation in a period of few minutes. These small and fast rotating bodies are thought to be monolithic objects, since the weak gravitational force due to their small size is not strong enough to counteract the large centripetal force caused by the fast rotation. This argument makes the rubble-pile structure not feasible for such objects. Additionally, it is not clear whether the fast spin prevents dust and small particles (regolith) to be kept on their surface. Aims. We aim to develop a model to constrain the thermal conductivity of the surface of the small, fast-rotating near-Earth asteroids. This model may suggest whether the presence of regolith is likely or not. Methods. Our approach is based on the comparison between the measured Yarkovsky drift and a predicted value using a theoretical model, which depends on the orbital, physical and thermal parameters of the object. The necessary parameters are either deduced from statistical distribution derived for near-Earth asteroids population or determined from observations with associated uncertainty. With that information available, we perform Monte Carlo simulations, producing a probability density distribution for the thermal conductivity.Results. Applying our model to the super-fast rotator asteroid (499998) 2011 PT, we find that the measured Yarkovsky drift can be achieved only if the thermal conductivity K of the surface is low. The resulting probability density function for the conductivity is bimodal, with two most likely values being around 0.0001 and 0.005 W m −1 K −1 . Based on this, we find that the probability of K being smaller than 0.1 W m −1 K −1 is at least 95 per cent. This low thermal conductivity could be a clue that the surface of 2011 PT is covered with a thermal insulating layer, composed by a regolith-like material similar to lunar dust.