Shape Model And Spin Direction Analysis Of Pha (436724) 2011 Uw158: A Large Superfast Rotator

Observations of the large superfast rotator (436724) 2011 UW158 were carried out at the Observatorio Astronomico do Sertao de Itaparica (OASI, Brazil) between May and October 2015, before and after it made a close approach to Earth in July 2015. These observations allowed us to obtain 11 light curves, and additional observations at the San Pedro Martir Observatory (Baja California, Mexico) in March 2017 provided a light curve. From the obtained light curves we could confirm the fast rotation, 0.61071 h, of the near-Earth object (NEO) and by applying the inversion method, we derived a prograde sense of rotation and a quite elongated shape model with rough dimensions a/b = 2.0, a/c = 4.2, and b/c = 2.1. The best determined pole directions suggest that the maximum amplitude of the light curves was obtained from an equatorial view. The reconstructed shape models are in good agreement with the shape elongation and asymmetric shape reported by radar observations. As 2011 UW158 has an uncommon rotation period for asteroids larger than similar to 200 m, we used the determined parameters to calculate the minimum internal cohesion strength required to keep its structure intact. We have found that a minimum cohesion ranging from 176 to 295 Pa is required in case the NEO has a composition similar to that of C-type asteroids, and from 364 to 451 Pa for the E-type. Therefore, we suggest that 2011 UW158, if not monolithic, requires a significant cohesion force to keep it spinning so fast.