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The Astronomical Journal, 133:656 – 664, 2007 February# 2007. The American Astronomical Society. All rights reserved. Printed in U.S.A. LINEAR STABILITY OF RING SYSTEMS Robert J. Vanderbei Operations Research and Financial Engineering, Princeton University, Princeton, NJ, USA; rvdb@princeton.edu and Egemen Kolemen Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ, USA; ekolemen@princeton.edu Received 2006 September 2; accepted 2006 October 27 ABSTRACT We give a self-contained modern linear stability analysis of a system of n equal-mass bodies in circular orbit about a single more massive body. Starting with the mathematical description of the dynamics of the system, we form the linear approximation, compute all of the eigenvalues of the linear stability matrix, and ﬁnally derive inequalities that guarantee that none of these eigenvalues have a positive real part. In the end we rederive the result that Maxwell found for large n in his seminal paper on the nature and stability of Saturn’s rings, which was published 150 years ago. In addition, we identify the exact matrix that deﬁnes the linearized system even when n is not large. This matrix is then investigated numerically (by computer) to ﬁnd stability inequalities. Furthermore, using properties of circulant ma- trices, the eigenvalues of the large 4n ; 4n matrix are computed by solving n quartic equations, which further facil- itates the investigation of stability. Finally, we implement an n-body simulator, and we verify that the threshold mass ratios that we derive mathematically or numerically do indeed identify the threshold between stability and instability. Throughout the paper we consider only the planar n-body problem so that the analysis can be carried out purely in complex notation, which makes the equations and derivations more compact, more elegant, and therefore, we hope, more transparent. The result is a fresh analysis that shows that these systems are always unstable for 2 n 6, and for n > 6 they are stable provided that the central mass is massive enough. We give an explicit formula for this mass- ratio threshold. Key word: planets: rings 1. INTRODUCTION each other: i.e., one leading the other by 60 ). Scheeres Vinh (1991) extended the analysis of Pendse to ﬁnd the stability crite- One hundred and ﬁfty years ago, Maxwell (1859) was awarded rion as a function of the number of satellites when n is small. Thethe prestigious Adams Prize for a seminal paper on the stability ofSaturn’s rings. At that time, neither the structure nor the compo- resulting threshold depends on n, but for n ! 7 it deviates only a small amount from the asymptotically derived value. More re-sition of the rings was known. Hence, Maxwell considered vari- cently, Moeckel (1994) studied the linear stability of n-body sys-ous scenarios such as the possibility that the rings were solid or tems for which the motion is given as a simple rotation about theliquid annuli or a myriad of small boulders. As a key part of this center of mass and under the condition that all masses except onelast possibility, Maxwell studied the case of n equal-mass bodies become vanishingly small. This latter condition greatly simpli-orbiting Saturn at a common radius and uniformly distributed about ﬁes the analysis. Under these assumptions, Moeckel shows, usinga circle of this radius. He concluded that, for large n, the ring ought the invariant subspace of the linearized Hamiltonian, that sym-to be stable provided that the following inequality is satisﬁed: metric ring systems are stable if and only if n ! 7. For n 6 he mass(rings) 2:298 ; mass(Saturn)=n2 : gives some examples of the stable conﬁgurations discussed in Salo Yoder (1988) in which the ring bodies are not uniformly dis-The mathematical analysis that leads to this result has been scru- tributed. Finally, Roberts (2000), expanding on Moeckel’s work,tinized, validated, and generalized by a number of mathemati- obtained stability criteria that match those given by Scheeres cians over the years. Vinh (1991). We summarize brieﬂy some of the key historical developments. In this paper we give a self-contained modern linear stabilityTisserand (1889) derived the same stability criterion using an anal- analysis of a system of equal-mass bodies in circular orbit aboutysis in which he assumed that the ring has no effect on Saturn and a single more massive body. We start with the mathematical de-that the highest vibration mode of the system controls stability. scription of the dynamics of the system. We then form the linearMore recently, Willerding (1986) used the theory of density waves approximation, compute all of the eigenvalues of the matrix de-to show that Maxwell’s results are correct in the limit as n goes to ﬁning the linear approximation, and ﬁnally derive inequalitiesinﬁnity. Pendse (1935) reformulated the stability problem so that it that guarantee that none of these eigenvalues have a positive realtook into account the effect of the rings on the central body. He part. In the end we get exactly the same result that Maxwell foundproved that for n 6 the system is unconditionally unstable. In- for large n. But, in addition, we identify the exact matrix that de-spired by this work, Salo Yoder (1988) studied coorbital for- ﬁnes the linearized system even when n is not large. This matrixmations of n satellites for small values of n in which the satellites can then be investigated numerically to ﬁnd stability inequalitiesare not distributed uniformly around the central body. They showed even in cases in which n is not large. Furthermore, using prop-that there are some stable asymmetric formations (such as the well- erties of circulant matrices, the eigenvalues of the large 4n ; 4nknown case of a pair of ring bodies in L4/L5 position relative to matrix can be computed by solving n quartic equations, which 656
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STABILITY OF RING SYSTEMS 657further facilitates the investigation. Finally, we implement an n-body Substituting equations (5) and (6) into equation (4) and settingsimulator based on a leapfrog integrator (see Saha Tremaine this equal to equation (3), we see that1994; Hut et al. 1995), and we verify that the threshold massratios that we derive mathematically or numerically do indeed GM X Gm ie ik=n nÀ1 À! 2 ¼ À þ ð7Þidentify the threshold between stability and instability. r3 k¼1 4r 3 sin2 (k=n) Throughout the paper we consider only the planar n-bodyproblem. That is, we ignore any instabilities that might arise due GM Gm X nÀ1 1 Gm X cos (k=n) nÀ1 ¼À À 3 þi 3 :to out-of-plane perturbations. Maxwell claimed, and others have r 3 4r k¼1 sin (k=n) 4r k¼1 sin2 (k=n)conﬁrmed, that these out-of-plane perturbations are less desta-bilizing than in-plane ones, and hence our analysis, while not fully ð8Þgeneral, does get to the right answer. Our main reason for wishing It is easy to check that the summation in the imaginary part onto restrict to the planar case is that we can then work in the com- the right-hand side vanishes. Hence,plex plane and our entire analysis can be carried out purely in com-plex notation, which makes the equations and derivations more GM Gmcompact, more elegant, and therefore, we hope, more transparent. !2 ¼ þ 3 In ; ð9Þ r3 r Finally, we should point out that the relevance of this work toobserved planetary rings is perhaps marginal, since real ring wheresystems appear to be highly collisional and the energy dissipa-tion associated with such collisions affects their stability in a fun- 1XnÀ1 1 In ¼ : ð10Þdamental way (see, e.g., Salo 1995). 4 k¼1 sin (k=n) 2. EQUALLY SPACED, EQUAL-MASS BODIES With this choice of !, the trajectories given by equations (1) and IN A CIRCULAR RING ABOUT A MASSIVE BODY (2) satisfy Newton’s law of gravitation. Consider the multibody problem consisting of one large central 3. FIRST-ORDER STABILITYbody, for instance, Saturn, having mass M and n small bodies, In order to carry out a stability analysis, we need to counter-such as boulders, each of mass m orbiting the large body in circu- rotate the system so that all bodies remain at rest. We then per-lar orbits uniformly spaced in a ring of radius r. Indices 0 to n À 1 turb the system slightly and analyze the result.are used to denote the ring masses, and index n is used for Saturn. A counterrotated system would be given byThroughout the paper we assume that n ! 2. For the case n ¼ 1,Lagrange proved that the system is stable for all mass ratios m/M . eÀi!t zj (t) ¼ re 2ij=n ¼ zj (0): The purpose of this section is to show that such a ring existsas a solution to Newton’s law of gravitation. In particular, we de- In such a rotating frame of reference, each body remains ﬁxed atrive the relationship between the angular velocity ! of the ring its initial point. It turns out to be better to rotate the different bodiesparticles and their radius r from the central mass. We assume all different amounts so that every ring body is repositioned to lie onbodies lie in a plane and therefore that complex variable notation the x-axis. In other words, for j ¼ 0, : : : , n À 1, n we deﬁneis convenient. So, with i ¼ ðÀ1Þ1 2 and z ¼ x þ iy, we can write =the equilibrium solution for j ¼ 0, 1, : : : , n À 1 as wj ¼ uj þ ivj ¼ eÀi (!tþ2j=n) zj : ð11Þ zj ¼ re i (!tþ2j=n) ; ð1Þ The advantage of repositioning every ring body to the positive real axis is that perturbations in the real part for any ring body represent radial perturbations, whereas perturbations in the imag- zn ¼ 0: ð2Þ inary part represent azimuthal perturbations. A simple counter-By symmetry (and exploiting our assumption that n ! 2), force rotation does not provide such a clear distinction between theis balanced on Saturn itself. Now consider the ring bodies. Dif- two types of perturbations (and the associated stability matrixferentiating equation (1), we see that fails to have the circulant property that is crucial to all later analysis). zj ¼ À! 2 zj : ¨ ð3Þ Differentiating equation (11) twice, we get wj ¼ ! 2 wj À 2i! wj þ eÀi (!tþ2j=n) zj : ¨ ˙ ¨ ð12ÞFrom Newton’s law of gravity we have that zj À zn X zk À zj From Newton’s law of gravity, we see that zj ¼ ÀGM ¨ þ Gm : ð4Þ jzj À zn j 3 jzk À zj j3 X k; j k6¼j;n wj ¼ ! 2 wj À 2i! wj þ ¨ ˙ Gmk ; ð13Þ k6¼j jk; j j3Equations (3) and (4) allow us to determine !, which is our ﬁrstorder of business. By symmetry it sufﬁces to consider j ¼ 0. It is whereeasy to check that m; k ¼ 0; 1; : : : ; n À 1; mk ¼ ð14Þ zk À z0 ¼ re i!t e ik=n 2i sin (k=n); ð5Þ M; k ¼ n;and hence that k; j ¼ e ikÀj wk À wj ; ð15Þ jzk À z0 j ¼ 2r sin (k=n): ð6Þ k ¼ 2k=n: ð16Þ
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658 VANDERBEI KOLEMEN Vol. 133 Let wj (t) denote variations about the ﬁxed point given by From deﬁnition (11) of the wk values in terms of the zk values, it then follows that r; j ¼ 0; 1; : : : ; n À 1; m X ikÀj wj ð17Þ eÀij wn ¼ À e wk : 0; j ¼ n: M k6¼nWe compute a linear approximation to the differential equation Making this substitution for eÀij wn and an analogous substi-describing the evolution of such a perturbation. Applying the quo- tution for e ij wn in equation (19), we see that ¯tient, chain, and product rules as needed, we get 2 wj ¼ ! 2 wj À 2i! wj ¨ ˙wj ¼ ! wj À 2i! wj ¨ ˙ XÀ Á GM À Á 3 À Á Gm X k; j k; j À k; j ð3=2Þk; j k; j k; j þ k; j k; j ¯ ¯ þ 3 e ikÀj wk þ 3eÀikÀj wk þ 3 wj þ 3wj ¯ ¯ 2r k6¼n 2r þ Gmk 6 À Á k; j k6¼j Gm 1 X e ikÀj wk À wj À 3e ikÀj eÀikÀj wk À wj ¯ ¯ 2 À 3 À Á : 2 1X k; j k; j þ 32 k; j k; j ¯ 2r 8 k6¼j;n sin3 kÀj =2 ¼ ! wj À 2i! wj À ˙ Gmk 5 ; 2 k6¼j k; j ð20Þ ð18Þ 5. CIRCULANT MATRIXwhere Switching toÂmatrix notation, let Wj denote a shorthand for the Ã0 ¯ column vector wj wj . In this notation we see that equation (20) k; j ¼ eikÀj wk À wj ; ¯ k; j ¼ eÀikÀj wk À wj : ¯ ¯ together with its conjugates can be written as 2 3 2 32 3 The next step is to use equation (15) to re-express the k; j W0 I W0values in terms of the wk and wj values, and then to substitute in 6 W 7 6 I 76 W 7 6 1 7 6 76 1 7 6 . 7 6 .. 76 . 7the particular solution given by equation (17). Consider the 6 . 7 6 76 . 7 6 . 7 6 . 76 . 7case in which j n. In this case we have 6 7 6 76 7 d 6 WnÀ1 7 6 6 7 6 I 76 WnÀ1 7 76 7 ( À Á 6 ˙ 7%6 7 6 ˙ 7; dt 6 W0 7 6 D N1 ::: NnÀ1 7 6 W0 7 r e ikÀj À 1 ; k n; 6 7 6 76 7 k; j ¼ 6 W 7 6 NnÀ1 76 W 7 6 ˙1 7 6 D ::: NnÀ2 76 ˙ 1 7 Àr; k ¼ n; 6 . 7 6 . . . .. 76 . 7 6 . 7 6 . . . 76 . 7 4 . 5 4 . . . . 54 . 5and therefore, ˙ WnÀ1 N1 N2 ::: D ˙ WnÀ1 ( À Á ð21Þ k; j ¼ 2r sin kÀj =2 ; k n; r; k ¼ n: where the D, , and Nk notations are 2 ; 2 complex matrices given bySubstituting these into equation (18) and simplifying, we get ! ! 3 1 1 Gm 1 À In þ Jn =2 3 À 3Jn =2wj ¼ ! 2 wj À 2i! wj ¨ ˙ D ¼ !2 þ 3 ; 2 1 1 2r 3 À 3Jn =2 1 À In þ Jn =2 GM À Àij Á GM À Á À Á # À 3 e wn þ 3e ij wn þ 3 wj þ 3wj ¯ ¯ Gm e ik 1 À Jk;n =2 3eÀik þ 3Jk;n =2 2r 2r À Á Nk ¼ 3 À Á ; Gm 1 X e ikÀj wk À wj À 3e ikÀj eÀikÀj wk À wj ¯ ¯ 2r 3e ik þ 3Jk;n =2 eÀik 1 À Jk;n =2 À 3 À Á : ! 2r 8 k6¼j;n sin3 kÀj =2 À1 0 ¼ 2i! ; ð19Þ 0 1 4. CHOICE OF COORDINATE SYSTEM and where Without loss of generality, we can choose our coordinate sys- 1 Ik;n ¼ ;tem so that the center of mass remains ﬁxed at the origin. Hav- 4 sinðjkj=nÞ ¯ing done that, the perturbations wn and wn can be computed 1explicitly in terms of the other perturbations. Indeed, conserva- Jk;n ¼ 3 ; 4 sin ðjkj=nÞtion of momentum implies that X and m zk þ M zn ¼ 0: k6¼n X nÀ1 (nÀ1)=2 X 1 1 1 n In ¼ Ik;n % n % n log ; ð22ÞHence, k¼1 2 k¼1 k 2 2 mX X nÀ1 1 3X 1 1 n3 zn ¼ À zk : Jn ¼ Jk;n % n ¼ 3 (3) ¼ 0:01938n3 : ð23Þ M k6¼n 23 k¼1 k 3 2 k¼1
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No. 2, 2007 STABILITY OF RING SYSTEMS 659 TABLE 1 Estimates of the Stability Threshold Threshold Value for n Numerical Eq. (43) (Even-n only) Simulator Roberts Results Ã 2............................. 0.000 [0.0, 0.007] ... Ã 6............................. 0.000 [0.0, 0.025] ... 7............................. 2.452 ... [2.45, 2.46] ... 8............................. 2.412 2.4121 [2.41, 2.42] 2.412 10........................... 2.375 2.3753 [2.37, 2.38] 2.375 12........................... 2.354 2.3543 [2.35, 2.36] 2.354 14........................... 2.341 2.3411 [2.34, 2.35] 2.341 20........................... 2.321 2.3213 [2.32, 2.33] 2.321 36........................... 2.306 2.3066 [2.30, 2.31] ... 50........................... 2.303 2.3031 [2.30, 2.31] 2.303 100......................... 2.300 2.2999 [2.30, 2.31] 2.300 101......................... 2.300 ... [2.30, 2.31] 2.300 500......................... 2.299 2.2987 [. . . , . . .] 2.299 Notes.—Stability thresholds are estimated as in an inequality of the type m M /n3 . The second column contains numerically derived values obtained by a brute-force computation of the eigenvalues together with a simple binary search to ﬁnd the ﬁrst point at which an eigenvalue takes on a positive real part. The third column gives thresholds computed using eq. (43). The column of simulator values corresponds to results from running a leapfrog integrator and noting the smallest value of for which instability is clearly demonstrated. This is the larger of the pair of values shown. The smaller value is a nearby value for which the simulator was run 10 times longer with no overt indication of instability. Finally, the column of Roberts results shows values from Table 3 in Roberts (2000). Cells marked with an asterisk are cells for which an entry is not relevant, and cells marked with an ellipsis are cells for which an entry is unavailable.Here the ‘‘approximate’’ symbol is used to indicate asymptotic The ﬁrst n equations can be used to eliminate the ‘‘derivative’’agreement. That is, an % bn means that an /bn ! 1 as n ! 1, variables from the second set. That is,and (3) denotes the value of the Riemann zeta function at 3. 2 ˙ 3 2 3 ´This constant is known as ‘‘Apery’s constant’’ (see, e.g., Arfken W0 W0 6 W 7 6 W 71985). 6 ˙1 7 6 1 7 Finally, note that in deriving equation (21) from equation (20) 6 7 ¼ k6 . 7; 6 . 7 . 5 6 . 7we have made repeated use of the identity 4 . 4 . 5 ˙ WnÀ1 WnÀ1 X nÀ1 e ik ¼ 4Jn À 8In : ð24Þ and therefore, k¼1 sin3 jk j=2 2 32 3 D N1 : : : NnÀ1 W0 6N 76 7 6 nÀ1 D : : : NnÀ2 76 W1 7 Let A denote the matrix in equation (21). We need to ﬁnd the 6 . . 76 . 7 . 76 . 7eigenvalues of A and derive the necessary and sufﬁcient condi- 6 . . . 54 . 5 4 . . .tions under which none of them have a positive real part. At this N1 N2 : : : D WnÀ1point we could resort to numerical computation to bracket a thresh- 2 32 3 2 3old for stability by doing a binary search to ﬁnd the largest value of W0 W0m/M for which none of the eigenvalues have a positive real part. 6 76 W 7 6 W 7 6 76 1 7 6 1 7We did such a search for some values of n. The results are shown þ k6 6 .. 7 6 . 7 ¼ k2 6 . 7 : 76 . 7 6 . 7 ð26Þin Table 1. 4 . 54 . 5 4 . 5 The eigenvalues are complex numbers for which there are non- WnÀ1 WnÀ1trivial solutions to The matrix on the left-hand side is called a ‘‘block circulant2 32 3 2 3 matrix.’’ Much is known about such matrices. In particular, it is I W0 W06 76 W 7 6 W 7 easy to ﬁnd the eigenvectors of such matrices. For general prop-6 I 76 1 7 6 1 76 .. 76 . 7 6 . 7 erties of block circulant matrices, see Tee (2005).6 76 . 7 6 . 76 . 76 . 7 6 . 7 Let denote an nth root of unity (i.e., ¼ e 2ij=n for some6 76 7 6 766 I 76 WnÀ1 7 76 7 6 W 7 6 nÀ1 7 j ¼ 0, 1, : : :, n À 1), and let be an arbitrary complex 2-vector.6 76 ˙ 7 ¼ k6 ˙ 7: We look for solutions of the form6 D N1 ::: NnÀ1 76 W 0 7 6 W0 76 76 7 6 7 2 3 2 36 NnÀ1 76 W 7 6 W 76 D ::: NnÀ2 76 ˙ 1 7 6 ˙1 7 W0 6 . 76 . 7 6 . 76 . . . . . .. 76 . 7 6 . 7 6 W 7 6 74 . . . . 54 . 5 4 . 5 6 1 7 6 7 6 . 7 ¼ 6 . 7: N1 N2 ::: D ˙ WnÀ1 ˙ WnÀ1 6 . 7 6 . 7 4 . 5 4 . 5 ð25Þ WnÀ1 nÀ1
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660 VANDERBEI KOLEMEN Vol. 133 P Substituting such a guess into equation (26), we see that each of 7. SOLVING det D þ nÀ1 k Nk þ k À k2 I ¼ 0 k¼1the n rows reduces to one and the same thing: Assembling the results from the previous sections, we see À Á that D þ N1 þ : : : þ nÀ1 NnÀ1 þ k ¼ k2 : X nÀ1 Dþ k Nk þ k À k2 IThere are nontrivial solutions to this 2 ; 2 system if and only if 23 k¼1 3 1 3 2 3 2 ! 2 þ 2 À
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2 À 2i!k À k2 ! À j À Á 62 ¼4 2 jþ1 2 2 7 5 det D þ N1 þ : : : þ nÀ1 NnÀ1 þ k À k2 I ¼ 0: 3 2 3 2 3 2 1 2 ! À j ! þ jÀ1 À
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2 þ 2i!k À k2 2 2 2 2 !For each root of unity , there are four values of k that solve this Gm nj¼nÀ1 3nj¼1 þ 3 ; ð33Þequation (counting multiplicities). That makes a total of 4n eigen- 2r 3nj¼nÀ1 nj¼1values and therefore provides all eigenvalues for the full system inequation (25). where 2 and
8.
2 are shorthand for the expressions j PnÀ1 Gm À ˜ Á 6. EXPLICIT EXPRESSION FOR k Nk 2 ¼ j 3 Jn À Jj;n ! 0; k¼1 2r In order to compute the eigenvalues, it is essential that we P Gmcompute nÀ1 k Nk as explicitly as possible. To this end, we note k¼1
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2 ¼ In ! 0;the following reduction and new deﬁnition: 2r3 and, as a reminder, In and Jn are deﬁned by equations (22) and ˜ (23), respectively, whereas Jj;n is deﬁned by equation (27). X nÀ1 1X nÀ1 e 2ijk=n 1 X cosð jk Þ nÀ1 k Jk;n ¼ ¼ ˜ Jj;n : It turns out in our subsequent analysis that the root of unity k¼1 4 k¼1 sin3 ðk =2Þ 4 k¼1 sin3 ðk =2Þ given by j ¼ n/2 is the most critical one for stability, at least for n ! 7. For n ¼ 2, : : : , 6 the instability stems from the eigen- ð27Þ vectors associated with j ¼ 1 and j ¼ n À 1. We analyze the key cases. But ﬁrst we note that the critical j ¼ n/2 case correspondsSimilarly, to perturbations in which every other body is perturbed in the opposite direction. And, more importantly, it does not matter what X nÀ1 the direction of the perturbation is. That is, if body 0 is advanced ˜ k e ik Jk;n ¼ Jjþ1;n ; ð28Þ azimuthally, then all of the even-numbered bodies are advanced k¼1 azimuthally, and all of the odd-numbered bodies are retarded by the same amount. Similarly, if body 0 is pushed outward radially, X nÀ1 then all of the even-numbered bodies are also pushed outward, k eÀik Jk;n ¼ JjÀ1;n : ˜ ð29Þ whereas the odd-numbered bodies are pulled inward. Azimuthal k¼1 and radial perturbations contribute equally to instability.We also compute 7.1. The Case in Which n Is Arbitrary and j Is neither 1 nor n À 1 X nÀ1 X nÀ1 X nÀ1 Assuming that j is neither 1 nor n À 1, we see that k e ik ¼ e ijk e ik ¼ e i ( jþ1)k ! k¼1 k¼1 k¼1 X nÀ1 k 2 det D þ Nk þ k À k I n À 1; j ¼ n À 1; ¼ ð30Þ k¼1 À1; otherwise: 3 2 1 2 ¼ ! þ jþ1 À
10.
2 À 2i!k À k2 X nÀ1 2 2 k Àik n À 1; j ¼ 1; e ¼ ð31Þ 3 2 1 2 k¼1 À1; otherwise: ; 2 ! þ jÀ1 À
11.
þ 2i!k À k 2 2 2 PnÀ1 Substituting the deﬁnition of Nk into k Nk and making 3 2 3 2 2 k¼1 À ! À j : ð34Þuse of equations (27)–(31), we get 2 2XnÀ1 Expanding out the products on the right-hand side in equation (34), k Nk we get thatk¼1 ! 2 1˜ 3˜ 3 XnÀ1 det D þ k Nk þ k À k I ¼ k4 þ Aj k2 þ iBj k þ Cj ¼ 0; 2 Gm 6 À1 þ nj¼nÀ1 À 2 Jjþ1;n À3 þ 3nj¼1 þ Jj;n 2 7¼ 34 5; k¼1 2r 3˜ 1˜ À3 þ 3nj¼nÀ1 þ Jj;n À1 þ nj¼1 À JjÀ1;n ð35Þ 2 2 ð32Þ wherewhere j¼k denotes the Kronecker delta (i.e., 1 when j ¼ k and 1 2 Aj ¼ ! 2 À jÀ1 þ 2 2 jþ1 þ 2
17.
2 4 It is the greater-than constraint that is relevant, and so we take 1 2 9 the positive root. Finally, we recall that À 2 À 2 jÀ1 jþ1 À j : 4 ð38Þ 16 4 GM Gm Gm À Á !2 ¼ þ 3 In ; 2 ¼ ˜ Jn À Jn=2;n ; n=2 r3 r 2r 3 7.1.1. The Subcase in Which n Is Even and j ¼ n/2 Gm
18.
2 ¼ In ; In the subcase in which n is even and j ¼ n/2, it is easy to see 2r 3by symmetry that n/2þ1 ¼ n/2À1 . To emphasize the equality,we denote this common value by n/2Æ1. Equations (36)–(38) and so the inequality on ! 2 reduces tosimplify signiﬁcantly. The result is ! M À Á 9À Á XnÀ1 ˜ ˜ !2 Jn À Jn=2Æ1;n þ Jn À Jn=2;n À 5In k 2 m 2 det D þ Nk þ k À k I ( !2 k¼1 À Á 9À Á þ ˜ ˜ 2 Jn À Jn=2Æ1;n þ Jn À Jn=2;n À 4In 4 ¼ k þ !2 À 2 n=2Æ1 þ 2
20.
2 ˜ À Jn À Jn=2;n : ð43Þ 2 2 n=2 4 2 1 2 9 þ À
21.
2 À 4 : ð39Þ 2 n=2Æ1 4 n=2 The third column in Table 1 shows thresholds computed using this inequality. It is clear that for even values of n greater than 7,For a moment let us write this biquadratic polynomial (in k) in this threshold matches the numerically derived threshold showna simple generic form and equate it to 0: in the second column in the table. This suggests that inequalities analogous to equation (43) derived for j 6¼ n/2 are less restrictive k4 þ Ak2 þ C ¼ 0: than equation (43). The proof of this statement is obviously more complicated than the j ¼ n/2 case because the general case in-The quadratic formula then tells us that cludes a linear term (Bj 6¼ 0) that vanishes in the j ¼ n/2 case. The linear term makes it impossible simply to use the quadratic pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ ÀA Æ A2 À 4C formula, and therefore, any analysis involves a more general anal- k2 ¼ : ysis of a quartic equation. Scheeres Vinh (1991) analyzed the 2 general case. Although their notations are different, the funda-To get the eigenvalues we need to take square roots one more mental quantities are the same, and so their analysis is valid heretime. The only way for the resulting eigenvalues not to have a as well. Rather than repeating their complete analysis, we simplypositive real part is for k2 to be real and nonpositive. Necessary outline the basic steps in x 7.1.2.and sufﬁcient conditions for this are that 7.1.2. The Subcase in Which j 6¼ n/2 A ! 0; ð40Þ Let jÆ1 denote the average of jþ1 and jÀ1 : C ! 0; ð41Þ jÆ1 ¼ ( jþ1 þ jÀ1 )=2: A2 À 4C ! 0: ð42Þ If we were to assume for the moment, incorrectly, that terms in-It turns out that the third condition implies the ﬁrst two (we leave volving the difference jþ1 À jÀ1 were not present in equa-veriﬁcation of this fact to the reader). In terms of computable tions (36)–(38), then an analysis analogous to that given in x 7.1.1quantities, this third condition can be written, after simpliﬁcation, would give us the inequalityas r ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ 2 ! 2 ! 4 2 þ 9 2 À 8
24.
2 2 ! þ 9 4 n=2 ! 0: Next one uses the fact that 2 is unimodal as a function of jÆ1Again we use the quadratic formula to ﬁnd that j, taking its maximum value at j ¼ n/2. Hence, the inequality associated with j ¼ n/2 is the strictest of these inequalities. Fi- ! 2 !4 2 n=2Æ1 þ 9 n=2 À 8
25.
2 2 nally, the difference terms are treated as small perturbations to r ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ 2 this simple case, and a homotopy analysis shows that the j ¼ n/2 þ 4 2 2 2 À9 4 ; case remains the strictest case even as the difference terms are n=2Æ1 þ 9 n=2 À 8
27.
662 VANDERBEI KOLEMEN Vol. 133 7.1.3. Large n tion (50) form a conjugate pair, and therefore, the correspond- ˜ ˜ When n is large, Jn=2Æ1;n % Jn=2;n and Jn 3 In . Furthermore, ing pair of values for k are such that one has a positive real part and the other a negative real part. Hence, in that case the system n=2 is demonstrably unstable. Simple numerical investigation reveals ˜ 1 X ( À 1)k n3 X ( À 1)k 1 Jn=2;n % % 3 that this is precisely what happens when 2 n 6 regardless of 2 k¼1 sin3 (k=n) 2 k¼1 k 3 the mass ratio M /m. n=2 To see why, let us consider just the case in which M /m is very 3 n3 X 1 1 31X 1 3 large, and hence the ratio ¼À %À % À Jn : 3 4 2 k¼1 k 3 4 2 k¼1 sin3 (k=n) 4 r ¼ m=MHence, equation (43) reduces to is very close to 0. In this asymptotic regime, M 7 pﬃﬃﬃﬃﬃ ! 13 þ 4 10 Jn ; pﬃﬃﬃﬃﬃ m 8 A1 ¼ aM ; B1 ¼ b M m; C1 ¼ cMm;or, equivalently, where a 0 and the sign of c is the same as the sign of (1/4) 2 þ 2 M 2:299M (3/2) 2 À
28.
2 À (1/2)n. Substituting rM for m and making the pﬃﬃﬃﬃﬃ m À pﬃﬃﬃﬃﬃÁ % ; ð44Þ 1 ð7=8Þ 13 þ 4 10 Jn n3 change of variables deﬁned by ¼ / M , we getwhich is precisely the answer Maxwell obtained 150 years ago. 4 À a 2 þ br þ cr ¼ 0:Of course, we have assumed here that n is even. For the oddcase, as n ! 1, j jÀ1 À jþ1 j ! 0, so that the odd quartic For r ¼ 0 this equation reduces to 4 À a 2 ¼ 0, which hasequation for j ¼ (n À 1)/2 reduces to the even equation for three real roots: a positive one, a negative one, and a root of mul-j ¼ n/2, giving the same stability criteria as the even-particle tiplicity 2 at ¼ 0. By continuity, for r small but nonzero thecase. This can be seen in our simulations as well. The cases of quartic still has a positive root and a negative root, but the doublen ¼ 100 and 101 give the same threshold to several signiﬁcant root at ¼ 0 can either disappear or split into a pair of realﬁgures. roots. Since this bifurcation takes place in the neighborhood of the origin, the quartic term can be ignored, and the equation in 7.2. The Case in Which j ¼ 1 or j ¼ n À 1 the neighborhood of zero reduces to a quadratic equation: By symmetry, these two cases are the same. Hence, we con-sider only j ¼ 1. Again after some manipulation in which we Àa 2 þ br þ cr ¼ 0:exploit the fact that 2 ¼ 0, we arrive at 0 ! This equation has two real roots if and only if its discriminant X nÀ1 is nonnegative:det D þ k Nk þ k À k2 I ¼ k4 þ Aj k2 þ iBj k þ Cj ¼ 0; À Á k¼1 r rb2 þ 4ac ! 0: ð45Þ Hence, if c is negative there will not be a full set of real roots forwhere r very small, and hence, the ring system will be unstable in that case. In other words, the system will be unstable if 1À Á A1 ¼ ! 2 À n þ 2 þ 2
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4 2 2 1 2 4 X nÀ1 1 1 1 À Á2 9 2 À 2 Á À n À cot 0: ð51Þ À n À 2 À 1 1 À n ; 2 ð48Þ sinðk=nÞ 2 2n 16 4 k¼1 Gm ¼ 3 : ð49Þ It is easy to check that the expression is negative for n ¼ 2, : : : , r 6 and positive for n ! 7. Therefore, we have proved that ring Note that the coefﬁcient B1 is imaginary, whereas the other systems are unstable for n ¼ 2, : : : , 6, at least when m is verythree coefﬁcients are real. This suggests making the substitution small relative to M. We have not proved the result for larger ¼ ik. In terms of , equation (45) becomes a quartic equation values of m, but it seems that such a case should be even morewith all real coefﬁcients: unstable, which is certainly veriﬁed by our simulator. 4 À A1 2 þ B1 þ C1 ¼ 0: ð50Þ 8. COMPARISON WITH PRIOR RESULTS Of the prior work summarized in x 1, the work of RobertsThis equation has at most four real roots. If it does have four real (2000) is the most recent, most rigorous, and most self-contained.roots, then the corresponding values for k are purely imaginary In this section we compare our results with his. Like us, Robertsand the system could be stable. If, on the other hand, there are shows that the ring conﬁguration is linearly unstable when n 6.two or fewer real roots, then at least one pair of roots to equa- Furthermore, for n ! 7 Roberts shows that the ring is linearly
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No. 2, 2007 STABILITY OF RING SYSTEMS 663stable if and only if M hn m, where hn is a bifurcation value in equal to Saturn’s mass and the ring’s radius is about the radiusa certain quartic polynomial. For n even, the formula he derives of the Cassini division (120,000 km), we see that the upper boundfor hn is on the linear density of boulders is pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ hn ¼ A þ A2 À B; 1=3 M =mE rE k 8 ¼ 0:219:where 25:65 ; 0:01938 r A ¼ 2 þ 8P þ 18Q; B ¼ 36Q 2 À 8P; In other words, the linear density cannot exceed 22%; otherwise, the ring will be unstable. Of course, this is for a one-dimensionaland circular ring of ice boulders. Analysis of a two-dimensional annulus or the full three-dimensional case is naturally more com- 1XnÀ1 1 plicated. Nonetheless, the 22% linear density ﬁgure matches ¼ ¼ 2In ; surprisingly well with the measured optical density, which hovers 2 k¼1 sin k =2 around 0.05–2.5. X 1 À ( À 1)k cos k nÀ1 1À Á P¼ ¼ ˜ Jn À Jn=2Æ1;n ; 3 4 10. NUMERICAL RESULTS k¼1 16 sin k =2 X cos k À ( À 1)k nÀ1 1À Á We have computed stability thresholds three different ways for Q¼ ˜ ¼ Jn À 2In À Jn=2;n : various ﬁnite n. First we numerically solved for all eigenvalues 3 4 k¼1 16 sin k =2 of the 4x ; 4n matrix in equation (25) and did a binary search to locate the smallest mass ratio M /m for which no eigenvalue hasIn the above expressions we have used formulae (22), (23), (24), a positive real part. We then translated this threshold into a value(27), and (28) to establish the alternate form of these expres- of , expressed assions. From these alternate forms it is easy to see that the right-hand side in equation (43) is precisely hn. Furthermore, the fact that the asymptotic expression given by m M =n3 ;Roberts matches ours is easy to verify directly. Roberts wrotethe asymptotic expression as and tabulated those results in the ‘‘Numerical’’ column in Table 1. pﬃﬃﬃﬃﬃ 1 Second, for even values of n we used equation (43) to derive 13 þ 4 10 X 1 threshold values. These values are reported in the third column hn % : 23 (2k À 1)2 in Table 1. Note that for even values of n larger than 7, these 1 results agree with those obtained numerically.To verify the agreement, it sufﬁces to note that Finally, and perhaps most interestingly, the fourth column in the table are stability thresholds that were estimated using a sim-X 1 X 1 1 1 X1 1 1X 1 X 1 1 1 ulator based on a leapfrog integrator (Saha Tremaine 1994; Hut 3 ¼ 3 þ 3 ¼ 3 þ 3 ; et al. 1995). In this column two values are given. For the larger k k¼1 (2k) k¼1 (2k À 1) 8 k¼1 k k¼1 (2k À 1)k¼1 value, instability has been decisively observed. However, verify- ing stability is more challenging since one should in principle runand therefore, the simulator forever. Rather than waiting that long, we use the X rule of thumb that if the system appears intact for a period of time 1 1 7X 1 1 10 times greater than the time it took to demonstrate instability, 3 ¼ : k¼1 (2k À 1) 8 k¼1 k 3 then we deem the system stable at that mass. This is how the lower bounds in the table were obtained. It is our belief that a symplecticWith this identity, it is easy to see that his expression matches simulator provides the most convincing method of discriminatingours given in equation (44). between stable and unstable orbits, at least when the number of bodies remains relatively small, for instance, up to a few hun- 9. RING DENSITY dred. Unstable orbits reveal themselves quickly, as the initial inaccuracy of double-precision arithmetic quickly cascades into Suppose that the linear density of the boulders is k. That is, k is something dramatic if the system is unstable. If, on the otherthe ratio of the diameter of one boulder to the separation between hand, the system is stable, then the initial imprecisions simplythe centers of two adjacent boulders. Then the diameter of a result in an orbit that is close to but not identical to the intendedsingle boulder is k(2r/n). Hence, the volume of a single boulder orbit. The situation does not decay. Any reader who has neveris (4/3)(kr/n)3 . Let denote the density of a boulder. Then the experimented with a good symplectic integrator is strongly en-mass of a single boulder is (4/3)(kr/n)3 . If we assume that the couraged to experiment with the Java applet posted online,1 asdensity of Earth is about 8 times that of a boulder (Earth’s density hands-on experience can be very convincing.is 5.5 and Saturn’s moons have a density of about 0.7, being com- Of course, the amazing thing about the simulator results isposed of porous water ice), then we have that they match the numerical results in the second column. The thresholds determined by linear stability analysis only tell us 1 mE deﬁnitively that for m larger than the threshold, the system is ¼ 3 ; 8 (4=3)rE necessarily unstable. But for m smaller than the threshold, the mathematical /numerical analysis says nothing, since in thosewhere mE denotes the mass of Earth and rE denotes its radius. 1Combining all of these factors and assuming the central mass is See http://www.princeton.edu /$rvdb/JAVA /astro/galaxy/StableRings.html.
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664 VANDERBEI KOLEMENcases the eigenvalues are all purely imaginary. Yet simulation are unstable. The case n ¼ 2 is especially interesting. For thisconﬁrms that the thresholds we have derived are truly necessary case the two small bodies are at opposite sides of a commonand sufﬁcient conditions for stability. orbit. Essentially they are in the L3 position with respect to each As shown in x 7.2 for 2 n 6, the system is unstable. The other. For the restricted three-body problem (in which one bodysimulator veriﬁes this. In these cases there is lots of room to roam has zero mass) it is well known that L3 is unstable no matter whatbefore one body catches up to another. Even the tiniest masses the mass ratio. Our results bear this out. REFERENCESArfken, G. 1985, Mathematical Methods for Physicists (3rd ed.; Orlando: Saha, P., Tremaine, S. 1994, AJ, 108, 1962 Academic) Salo, H. 1995, Icarus, 117, 287Hut, P., Makino, J., McMillan, S. 1995, ApJ, 443, L93 Salo, H., Yoder, C. F. 1988, AA, 205, 309Maxwell, J. C. 1859, On the Stability of Motions of Saturn’s Rings (Cambridge: Scheeres, D. J., Vinh, N. X. 1991, Celest. Mech. Dyn. Astron., 51, 83 Macmillan) Tee, G. J. 2005, Res. Lett. Inf. Math. Sci., 8, 123Moeckel, R. 1994, J. Dyn. Diff. Eqs., 6, 37 ´ ´ ´ Tisserand, F. 1889, Traite de Mechanique Celeste ( Paris: Gauthier-Villars)Pendse, C. G. 1935, Philos. Trans. R. Soc. London A, 234, 145 Willerding, E. 1986, AA, 161, 403Roberts, G. 2000, in Hamiltonian Systems and Celestial Mechanics, ed. J. Delgado (Singapore: River Edge), 303
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