YORP-Yarkowski evolution of asteroid families

The orbital evolution of the asteroids within the families is entangled with their spin dynamics since the Yarkovsky effect strongly depends on the obliquity of the body. To have a full picture of this dynamics it is essential to consider, in addition to the 'normal' Yorp (NYORP), also the so-called Tangential YORP (TYORP), caused by asymmetric light emission by boulders or other structures on the surface of an asteroid, and mutual collisions which cause step changes in the spin of asteroids. Introducing these effects in a model which tracks the coupled orbital and rotational evolution of asteroid families, under the action of the Yarkowsky (diurnal and seasonal), YORP and collision effects, we are able to recover the characteristic V-shape configuration in the semimajor axis vs absolute magnitude (H) space. Adding to the complexity of the model, it is observed how the simple NYORP spin dynamic pathway hypothesized by the analytical model by Golubov and Scheeres (The Astronomical Journal, Vol. 157, article id. 105, 2019), is altered by the introduction of TYOPR and collisions. Since TYORP tends to accelerate the asteroid spin rate rather than to decelerate it in all known cases, a number of asteroids is progressively accelerated towards the breakup limit. The smooth YORP cycles coupling the spin axis obliquity and the rotation rate and driving the obliquity back and forth between the extremes of its evolution scale, is futher disturbed by the introduction of the collisions. With collisions at play, the cycles are almost uncorrelated. Even if the initial rotation rates are slow, the objects can be driven close to the rotational breakup. In fact, the collisions not only change the angular momentum vector of the body but can also alter the subsequent YORP evolution by changing the surface properties of the body. First, the time evolution of a number of synthetic families is explored with this model looking in detail at different size regimes. The specific contributions by TYORP and collisions, leading to a larger dispersion of the semimajor axis, through direct and indirect effects can be tracked and understood. Separating the diurnal and the seasonal Yarkowski effects, it is observed how the latter is shifting the family members inwards with some small members drifting significantly far from the initial location. We expect further tests to be able to understand whether these combined effects can erase the initial features of a realistic family, dominating its evolution.