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8.3. Degenerations of Ricci-flat Kähler manifolds [0188]

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8.3. Degenerations of Ricci-flat Kähler manifolds

Let MM be a Ricci-flat Kähler manifold, i.e. a compact Kähler manifold with trivial first Chern class c1​(M)∈H2​(M,ℂ)c_{1}(M)\in H^{2}(M,{\mathbb{C}}). Then MM carries a canonical probability measure μ\mu, given by μ=e2​ψ/∫Me2​ψ\mu=e^{2\psi}/\int_{M}e^{2\psi} where ψ\psi is a Hermitian metric on KMK_{M} with curvature 00 (and hence unique up to a constant).

By the Calabi-Yau theorem, each Kähler (1,1)(1,1)-class on MM further contains a unique Ricci-flat Kähler metric ω\omega, characterized by

ωn∫Mωn=μ.\frac{\omega^{n}}{\int_{M}\omega^{n}}=\mu.

Recall also that KMK_{M} is torsion, i.e. r​KM≃𝒪MrK_{M}\simeq{\mathcal{O}}_{M} for some positive integer rr. Indeed, this is a consequence of the Beauville-Bogomolov theorem [Beau83, Bog74], which implies that MM admits a finite étale cover p:M′→Mp:M^{\prime}\to M with KM′=p∗​KMK_{M^{\prime}}=p^{*}K_{M} trivial. A trivializing section η\eta of r​KMrK_{M} defines a metric ψ=1r​log⁡|η|\psi=\tfrac{1}{r}\log|\eta| on KMK_{M} as above, and hence μ=|η|2/r/∫|η|2/r\mu=|\eta|^{2/r}/\int|\eta|^{2/r}.

As a consequence of Theorem A, we shall prove:

Theorem 8.5.

Let π:X→𝔻∗\pi\colon X\to{\mathbb{D}}^{*} be a holomorphic family of Calabi-Yau Kähler manifolds XtX_{t}, meromorphic at t=0t=0, and let μt\mu_{t} be the corresponding family of canonical probability measures. For any snc model 𝒳{\mathcal{X}}, μt\mu_{t} converges in 𝒳hyb{\mathcal{X}}^{\mathrm{hyb}} to a skeletal measure μ0\mu_{0} supported in Δ⁡(𝒳)\Delta({\mathcal{X}}).

Proof.

As recalled above, KXtK_{X_{t}} is torsion for each fixed tt. Equivalently, h0​(Xt,r​KXt)=1h^{0}(X_{t},rK_{X_{t}})=1 for some positive integer rr. Since t↦h0​(Xt,r​KXt)t\mapsto h^{0}(X_{t},rK_{X_{t}}) is upper semicontinuous in the Zariski topology, it follows that r​KXtrK_{X_{t}} is trivial for a fixed rr independent of tt. Given any snc model π:𝒳→𝔻\pi\colon{\mathcal{X}}\to{\mathbb{D}}, π∗​𝒪​(r​K𝒳/𝔻)\pi_{*}{\mathcal{O}}(rK_{{\mathcal{X}}/{\mathbb{D}}}) is torsion free of rank one, and hence a line bundle. The choice of a trivializing section yields a holomorphic section η\eta of K𝒳/𝔻K_{{\mathcal{X}}/{\mathbb{D}}}, inducing a holomorphic family ηt\eta_{t} of trivializing sections of r​KXtrK_{X_{t}} for t≠0t\neq 0. As a consequence, the family of volume forms νt:=|ηt|2/r\nu_{t}:=|\eta_{t}|^{2/r} has analytic singularities at t=0t=0, and the result is thus a consequence of Theorem A, since μt=νt/νt​(Xt)\mu_{t}=\nu_{t}/\nu_{t}(X_{t}). ∎

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