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M. Cuk, J. A. Burns (Cornell U.)
The Yarkovsky force, produced when thermal radiation is re-emitted asymmetrically, causes significant orbital evolution of small asteroids. When acting on a non-spherical body, the momentum carried by this radiation generally produces a torque, the YORP effect, which seems important in re-orienting asteroidal spins. Here we explore a related effect, the ``binary YORP" (BYORP), that can modify the orbit of a synchronously rotating secondary in a binary system. It arises because a locked secondary is effectively an asymmetric appendage of the primary. It should be particularly important for Near-Earth asteroids owing to their small sizes, proximity to the Sun, and benign colllisional environment.
To estimate BYORP's strength, we subjected 100 random gaussian spheroids to the thermal radiation model of Rubincam (2000). For most shapes, a significant torque arose on the secondary's orbit, typically modifying its size, eccentricity and inclination in only 105 years, for components of 1 and 0.4 km radii, separated by 2 km, at 1AU with density 1500 kg/m3. Together YORP and BYORP are capable of synchonizing secondaries and circularizing orbits, making tidal dissipation unnecessary to explain the evolved state of observed NEA pairs.
However, BYORP's rapid timescale poses a problem for the abundance of observed NEA binaries, since their formation rate is thought to be much slower. We briefly present following alternatives: 1. The approximation using gaussian spheroids inadequately models YORP; 2. Most secondaries are not synchronous, but inhabit other spin-orbit resonances (very unlikely); 3. Tidal dissipation is much more efficient than previously estimated, and thus capable of stabilizing observed systems; and 4. Moderately close encounters with planets will re-orient seconaries, turning BYORP into a slower, random-walk-type process.
The author(s) of this abstract have provided an email address for comments about the abstract: cuk@astro.cornell.edu
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.