We consider the orthogonal polynomials, {P n (z)} n=0,1,··· , with respect to the measure |z − a| 2c e −N |z| 2 dA(z) supported over the whole complex plane, where a > 0, N > 0 and c > −1. We look at the scaling limit where n and N tend to infinity while keeping their ratio, n/N , fixed. The support of the limiting zero distribution is given in terms of certain "limiting potential-theoretic skeleton" of the unit disk. We show that, as we vary c, both the skeleton and the asymptotic distribution of the zeros behave discontinuously at c = 0. The smooth interpolation of the discontinuity is obtained by the further scaling of c = e −ηN in terms of the parameter η ∈ [0, ∞).
IntroductionConsider the ensemble of n point particles, {z j } n j=1 ⊂ C, distributed according to the probability measure given by 1where Z n is the normalization constant, N > 0 is a (large) parameter, Q : C → R ∪ {∞} is called an external potential and dA is the standard Lebesgue measure on the plane. The statistical behavior of the particles has been studied [1] for a large class of potentials in various contexts including random normal matrices and two-dimensional Coulomb gas. For example, in the scaling limit where n and N tend to infinity while n/N is fixed, it is known [12] that the counting measure of the particles converges weakly,where ∆Q = (∂ 2 x + ∂ 2 y )Q, χ K is the indicator function of the compact set K ⊂ C that we will call a droplet following [12], and the expectation value is taken with respect to the measure in (1).A connection to orthogonal polynomials can be provided by Heine's formula. It says that the averaged characteristic polynomial of the n particles is the (monic) orthogonal polynomial of degree n, i.e., P n (z) = P n,N (z) = E n j=1 (z − z j ) satisfies the orthogonality condition, C P n,N (z)P m,N (z)e −N Q(z) dA(z) = h n,N δ nm (n, m = 0, 1, 2, . . .),where h n,N is a (positive) norming constant. From this connection, one might wonder if the zero distribution of P n would tend to the averaged distribution of the particles. Though this is the case with the orthogonal polynomials on the real line (that corresponds to the particles confined on the line), in the cases of two-dimensional orthogonal polynomials so far studied [3,2,16,14,15,5], the limiting zero distribution is observed to be concentrated on a small subset of the droplet, on some kind of potential-theoretic skeleton of K. 1 A skeleton of K will refer to a subset of (the polynomial hull of) K with zero area, such that there exists a measure that is supported exactly on the skeleton and that generates the same logarithmic potential in the exterior of (the polynomial hull of) K as the Lebesgue measure supported on K. One characteristic of such skeleton is that it can be discontinuous under the continuous variation of the droplet K. A simple example [10] comes from the sequence of polygons converging to a disk. The skeleton of the polygon, which is the set of rays connecting each vertex to the center, does not converge to the skeleton of the disk, the...