  Context: Asteroids with a diameter of up to a few dozen meters may spin very
fast and complete an entire rotation within a few minutes. These small and
fast-rotating bodies are thought to be monolithic objects because the
gravitational force due to their small size is not strong enough to counteract
the strong centripetal force caused by the fast rotation. Additionally, it is
not clear whether the fast spin prevents dust and small particles (regolith)
from being kept on their surface. Aims: We develop a model for constraining the
thermal conductivity of the surface of the small, fast-rotating near-Earth
asteroids. This model may suggest whether regolith is likely present on these
objects. Methods: Our approach is based on the comparison of the measured
Yarkovsky drift and a predicted value using a theoretical model that 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 this information, we performed Monte Carlo simulations and
produced a probability density distribution for the thermal conductivity.
Results: Applying our model to the superfast rotator asteroid (499998) 2011 PT,
we find that the measured Yarkovsky drift can only be achieved when 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 that $K$ is lower than 0.1 W m$^{-1}$ K$^{-1}$ is at least 95\%.
This low thermal conductivity might indicate that the surface of 2011 PT is
covered with a thermal insulating layer, composed of a regolith-like material
similar to lunar dust.
