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تسجيل مستخدم جديدYORP and Yarkovsky effects in asteroids (1685) Toro, (2100) Ra-Shalom, (3103) Eger, and (161989) Cacus
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The rotation states of small asteroids are affected by a net torque arising from an anisotropic sunlight reflection and thermal radiation from the asteroids surfaces. On long timescales, this so-called YORP effect can change asteroid spin directions and their rotation periods. We analyzed lightcurves of four selected near-Earth asteroids with the aim of detecting secular changes in their rotation rates that are caused by YORP. We use the lightcurve inversion method to model the observed lightcurves and include the change in the rotation rate $mathrm{d} omega / mathrm{d} t$ as a free parameter of optimization. We collected more than 70 new lightcurves. For asteroids Toro and Cacus, we used thermal infrared data from the WISE spacecraft and estimated their size and thermal inertia. We also used the currently available optical and radar astrometry of Toro, Ra-Shalom, and Cacus to infer the Yarkovsky effect. We detected a YORP acceleration of $mathrm{d}omega / mathrm{d} t = (1.9 pm 0.3) times 10^{-8},mathrm{rad},mathrm{d}^{-2}$ for asteroid Cacus. For Toro, we have a tentative ($2sigma$) detection of YORP from a significant improvement of the lightcurve fit for a nonzero value of $mathrm{d}omega / mathrm{d} t = 3.0 times 10^{-9},mathrm{rad},mathrm{d}^{-2}$. For asteroid Eger, we confirmed the previously published YORP detection with more data and updated the YORP value to $(1.1 pm 0.5) times 10^{-8},mathrm{rad},mathrm{d}^{-2}$. We also updated the shape model of asteroid Ra-Shalom and put an upper limit for the change of the rotation rate to $|mathrm{d}omega / mathrm{d} t| lesssim 1.5 times 10^{-8},mathrm{rad},mathrm{d}^{-2}$. Ra-Shalom has a greater than $3sigma$ Yarkovsky detection with a theoretical value consistent with observations assuming its size and/or density is slightly larger than the nominally expected values.
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The spin state of small asteroids can change on a long timescale by the Yarkovsky-OKeefe-Radzievskii-Paddack (YORP) effect, the net torque that arises from anisotropically scattered sunlight and proper thermal radiation from an irregularly-shaped ast
eroid. The secular change in the rotation period caused by the YORP effect can be detected by analysis of asteroid photometric lightcurves. We analyzed photometric lightcurves of near-Earth asteroids (1865) Cerberus, (2100) Ra-Shalom, and (3103) Eger with the aim to detect possible deviations from the constant rotation caused by the YORP effect. We carried out new photometric observations of the three asteroids, combined the new lightcurves with archived data, and used the lightcurve inversion method to model the asteroid shape, pole direction, and rotation rate. The YORP effect was modeled as a linear change in the rotation rate in time domega /dt. Values of domega/ dt derived from observations were compared with the values predicted by theory. We derived physical models for all three asteroids. We had to model Eger as a nonconvex body because the convex model failed to fit the lightcurves observed at high phase angles. We probably detected the acceleration of the rotation rate of Eger domega / dt = (1.4 +/- 0.6) x 10^{-8} rad/d (3sigma error), which corresponds to a decrease in the rotation period by 4.2 ms/yr. The photometry of Cerberus and Ra-Shalom was consistent with a constant-period model, and no secular change in the spin rate was detected. We could only constrain maximum values of |domega / dt| < 8 x 10^{-9} rad/d for Cerberus, and |domega / dt| < 3 x 10^{-8} rad/d for Ra-Shalom.
Any population of asteroids, like asteroid families, will disperse in semi-major axis due to the Yarkovsky effect. The amount of drift is modulated by the asteroid spin state evolution which determines the balance between the diurnal and seasonal Yar
kovsky force. The asteroids spin state is, in turn, controlled in part by the YORP effect. The otherwise smooth evolution of an asteroid can be abruptly altered by collisions, which can cause impulsive changes in the spin state and can move the asteroid onto a different YORP track. In addition, collisions may also alter the YORP parameters by changing the superficial features and overall shape of the asteroid. Thus, the coupling between YORP and Yarkovsky is also strongly affected by the impact history of each body. To investigate this coupling we developed a statistical code modeling the time evolution of semi--major axis under YORP-Yarkovsky coupling. It includes the contributions of NYORP (normal YORP), TYORP (tangential YORP) and collisions whose effects are deterministically calculated and not added in a statistical way. We find that both collisions and TYORP increase the dispersion of a family in semi-major axis by making the spin axis evolution less smooth and regular. We show that the evolution of a familys structure with time is complex and collisions randomize the YORP evolution. In our test families we do not observe the formation of a YORP-eye in the semi-major axis vs. diameter distribution, even after a long period of time. If present, the YORP-eye might be a relic of an initial ejection velocity pattern of the collisional fragments.
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