Footprints of the YORP effect in asteroid families

The YORP effect affects not only the rotation rate, but also the spin orientation of asteroids. For this reason, due to Yarkovsky effect, it influences also the evolution of the semimajor axis with time. In asteroid dynamical families, the combined outcome can be a depletion of objects in the central part of the family, shown by the absolute magnitude vs. semimajor axis so-called V-plot. A reclassification of asteroid families, extended to a very large database of proper elements, has recently been performed by Milani et al. [2014] (Icarus, 239, 46-73). Only some of the related V-plots, used to estimate the age of several families, exhibit the depletion predicted above. In this paper we discuss the problem, introducing the concept of the YORP-eye and a general method of analysis. We show that the effect may sometimes be located in the low H tail, and thus difficult to detect. Moreover, it may be hindered by several anomalous physical properties of the family (asymmetry, cratering origin, multiple collisions history and so on). With a new method of analysis, we identify the footprints of the effect for most of the analysed families, obtaining also an independent estimate of the age of the family. In spite of the uncertainties, we obtain a very good agreement between these ages and those estimated on the basis of the slope of the V-plot: a result which supports both methods and the underlying physics.