The Japanese spacecraft Hayabusa2 visited the asteroid Ryugu and conducted the detailed proximity observations. The observations show that Ryugu is a rubble-pile asteroid and it was formed through a catastrophic collision of a parent body (Watanabe et al. 2019). Ryugu's spectral features and the studies of orbital evolutions of asteroids suggest that Ryugu was a member of the asteroid family Eulalia or Polana (Sugita et al. 2019). A radius of an original parent body of Ryugu is estimated to be about 50 km from the total mass of the Eulalia or Polana family (Walsh et al. 2013).

Hayabusa2 showed that Ryugu's surface is almost homogeneously composed of rocks with C-type spectra and low albedos (Kitazato et al. 2019). On the other hand, the high-resolution observation by ONC-T found that several tens of rocks have S-type spectra and high albedos. These S-type boulders are considered to be fragments of an S-type impactor that caused a catastrophic disruption of a C-type parent body of Ryugu (Tatsumi et al. 2021). The ratio of the volume of whole Ryugu to that of total S-type boulders is estimated to be about 10-5 (Sugimoto et al. 2021).

To investigate how fragments of an impactor are mixed in produced remnants, we conducted numerical simulations of catastrophic collisions of asteroids using the Smoothed Particle Hydrodynamics (SPH) code developed for studies of asteroid collisions (e.g., Sugiura et al. 2018). We focused on a catastrophic collision forming the Eulalia or Polana family and investigated amounts of impactor fragments included in produced remnants. From the total mass of the Eulalia or Polana family and that of the largest member of each family, a catastrophic disruption forming the family is considered to be a collision between a target with about 50 km radius and an impactor, which results in the mass of the largest remnant Mlr to be about 0.1 of the target mass Mt.

We conducted high-resolution simulations with about 3 million SPH particles. We choose the twenty impact conditions with the impact angles of 15 – 60 degrees, the mass ratios between a target to an impactor 4 – 64, and the impact velocities that result in Mlr / Mt ~ 0.1 for each impact simulation. As a result, we found that the low impact angles less than 40 degrees, i.e., nearly head-on collisions, produce only remnants with high impactor fractions of 10-1 - 10-3. We also found that some remnants produced from impacts with impact angles of 50 – 60 degrees have impactor fractions of ~ 5x10-5, which can explain the fraction of S-type boulders on Ryugu. Thus, S-type boulders on Ryugu may be mixed at the Eulalia or Polana family formation impact.