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

¹ Astronomical Institute, Faculty of Mathematics and Physics, Charles University, V Holešovičkách 2, 180, 00 Prague 8, Czech Republic
e-mail: durech@sirrah.troja.mff.cuni.cz
² Astronomical Institute, Academy of Sciences of the Czech Republic, Fričova 1, 251, 65 Ondřejov, Czech Republic
³ Institute of Astronomy of V.N. Karazin Kharkiv National University, 35 Sumska Str., Kharkiv 61022, Ukraine
⁴ Korea Astronomy and Space Science Institute, 776, Daedeokdae-ro, Yuseong-gu, Daejeon 34055, Republic of Korea
⁵ Faculty of Physics, Weizmann Institute of Science, 234 Herzl St., Rehovot 7610001, Israel
⁶ Kharadze Abastumani Astrophysical Observatory, Ilia State University, 3/5 K. Cholokoshvili Av., Tbilisi 0162, Georgia
⁷ Institute of Astronomy and NAO, Bulgarian Academy of Sciences, 72 Tsarigradsko Chaussee Blvd., 1784 Sofia, Bulgaria
⁸ University of Science and Technology, Korea
⁹ Ulugh Beg Astronomical Institute, 33 Astronomicheskaya Str., Tashkent 100052, Uzbekistan
¹⁰ Samtskhe-Javakheti State University, 113 Rustaveli Str., Akhaltsikhe 0080, Georgia
¹¹ Chungbuk National University, 1 Chungdae-ro, Seowon-gu, Cheongju, Chungbuk 28644, Korea
¹² Fesenkov Astrophysical Institute, 23 Observatory, Almaty 050020, Kazakhstan
¹³ Keldysh Institute of Applied Mathematics, RAS, 4 Miusskaya sq., Moscow 125047, Russia
¹⁴ Institute of Astronomy, RAS, 48 Pyatnitskaya str., Moscow 119017, Russia
¹⁵ Petrozavodsk State University, 33 Lenin Str., Petrozavodsk 185910, Republic of Karelia, Russia
¹⁶ Blue Mountains Observatory, 94 Rawson Pde. Leura, NSW 2780, Australia
¹⁷ Physics and Astronomy Department, Appalachian State University, 525 Rivers St, Boone, NC 28608, USA
¹⁸ Crimean Astrophysical Observatory, Nauchny, Crimea

Received: 22 July 2021
Accepted: 5 October 2021

Abstract

Context. The rotation state of small asteroids is affected by the Yarkovsky–O’Keefe–Radzievskii–Paddack (YORP) effect, which is a net torque caused by solar radiation directly reflected and thermally reemitted from the surface. Due to this effect, the rotation period slowly changes, which can be most easily measured in light curves because the shift in the rotation phase accumulates over time quadratically.

Aims. By new photometric observations of selected near-Earth asteroids, we want to enlarge the sample of asteroids with a detected YORP effect.

Methods. We collected archived light curves and carried out new photometric observations for asteroids (10115) 1992 SK, (1620) Geographos, and (1685) Toro. We applied the method of light curve inversion to fit observations with a convex shape model. The YORP effect was modeled as a linear change of the rotation frequency υ ≡ dω∕dt and optimized together with other spin and shape parameters.

Results. We detected the acceleration υ = (8.3 ± 0.6) × 10⁻⁸ rad d⁻² of the rotation for asteroid (10115) 1992 SK. This observed value agrees well with the theoretical value of YORP-induced spin-up computed for our shape and spin model. For (1685) Toro, we obtained υ = (3.3 ± 0.3) × 10⁻⁹ rad d⁻², which confirms an earlier tentative YORP detection. For (1620) Geographos, we confirmed the previously detected YORP acceleration and derived an updated value of υ with a smaller uncertainty. We also included the effect of solar precession into our inversion algorithm, and we show that there are hints of this effect in Geographos’ data.

Conclusions. The detected change of the spin rate of (10115) 1992 SK has increased the total number of asteroids with YORP detection to ten. In all ten cases, the dω∕dt value is positive, so the rotation of these asteroids is accelerated. It is unlikely to be just a statistical fluke, but it is probably a real feature that needs to be explained.