Rotation acceleration of asteroids (10115) 1992 SK, (1685) Toro, and\n (1620) Geographos due to the YORP effect

Abstract

The rotation state of small asteroids is affected by the\nYarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect, which is a net torque\ncaused by solar radiation directly reflected and thermally reemitted from the\nsurface. Due to this effect, the rotation period slowly changes, which can be\nmost easily measured in light curves because the shift in the rotation phase\naccumulates over time quadratically. We collected archived light curves and\ncarried out new photometric observations for asteroids (10115) 1992 SK, (1620)\nGeographos, and (1685) Toro. We applied the method of light curve inversion to\nfit observations with a convex shape model. The YORP effect was modeled as a\nlinear change of the rotation frequency $\\upsilon \\equiv \\mathrm{d}\\omega /\n\\mathrm{d}t$ and optimized together with other spin and shape parameters. We\ndetected the acceleration $\\upsilon = (8.3 \\pm 0.6) \\times\n10^{-8}\\,\\mathrm{rad}\\,\\mathrm{d}^{-2}$ of the rotation for asteroid (10115)\n1992 SK. This observed value agrees well with the theoretical value of\nYORP-induced spin-up computed for our shape and spin model. For (1685) Toro, we\nobtained $\\upsilon = (3.3 \\pm 0.3) \\times\n10^{-9}\\,\\mathrm{rad}\\,\\mathrm{d}^{-2}$, which confirms an earlier tentative\nYORP detection. For (1620) Geographos, we confirmed the previously detected\nYORP acceleration and derived an updated value of $\\upsilon$ with a smaller\nuncertainty. We also included the effect of solar precession into our inversion\nalgorithm, and we show that there are hints of this effect in Geographos' data.\nThe detected change of the spin rate of (10115) 1992 SK has increased the total\nnumber of asteroids with YORP detection to ten. In all ten cases, the\n$\\mathrm{d}\\omega / \\mathrm{d}t$ value is positive, so the rotation of these\nasteroids is accelerated. It is unlikely to be just a statistical fluke, but it\nis probably a real feature that needs to be explained.\n