Secular Increase of the Astronomical Unit:a Possible Explanation in Terms ofthe Total Angular-Momentum Conservation Law

摘要

We give an idea and order-of-magnitude estimations to explain a recently reported secular increase of theAstronomical Unit (AU) by Krasinsky and Brumberg (2004, Celest. Mech. Dyn. Astron., 90, 267). The ideaproposed is analogous to tidal acceleration in the Earth-Moon system, which is based on conservation of the totalangular momentum; we apply this scenario to the Sun-planets system. Assuming the existence of some tidal inter-actions that transfer the rotational angular momentum of the Sun, and using a reported value of the positive secular trend in the Astronomical Unit, —dtAU = 15 ±4 (m cy~(-1)), the suggested change in the period of rotation of the Sun is about 21 ms cy~(-1)in the case that orbits of eight planets have the same "expansion rate." This value issufficiently small, and at present it seems there are no observational data that exclude this possibility. The effects ofa change in the Sun's moment of inertia is also investigated. It is pointed out that a change in the moment of inertiadue to radiative mass loss by the Sun may be responsible for the secular increase of AU, if orbital "expansion" ishappening only in the inner-planets system. Although the existence of some tidal interactions is assumed betweenthe Sun and the planets, concrete mechanisms of angular-momentum transfer are not discussed in this paper, whichremain to be pursued as future investigations.