Abstract. We study the class of compact complex manifolds whose first Chern class vanishes in the Bott-Chern cohomology. This class includes all manifolds with torsion canonical bundle, but it is strictly larger. After making some elementary remarks, we show that a manifold in Fujiki's class C with vanishing first Bott-Chern class has torsion canonical bundle. We also give some examples of non-Kähler CalabiYau manifolds, and discuss the problem of defining and constructing canonical metrics on them.
IntroductionIn this paper, Calabi-Yau manifolds are defined to be compact Kähler manifolds M with c 1 (M ) = 0 in H 2 (M, R). Thanks to Yau's theorem [84] these are precisely the compact manifolds that admit Ricci-flat Kähler metrics. Using this, Calabi proved a decomposition theorem [13] which shows that any such manifold has a finite unramified cover which splits as a product of a torus and a Calabi-Yau manifold with vanishing first Betti number. From this one can easily deduce that Calabi-Yau manifolds have holomorphically torsion canonical bundle (see Theorem 1.4).One can ask how much of this theory carries over to the case of non-Kähler Hermitian manifolds. Simple examples, such as a Hopf surface diffeomorphic to S 1 × S 3 , show that the condition that c 1 (M ) = 0 in H 2 (M, R) is too weak in general (see Example 3.3). On the other hand, much interest has been devoted to studying non-Kähler compact complex manifolds with holomorphically trivial (or more generally torsion) canonical bundle, and many examples can be found in [6,15,17,20,23,33,35,36,37,40,43,53,60,78,83,86] and references therein. For example, every compact complex nilmanifold with a left-invariant complex structure has trivial canonical bundle, and it is always non-Kähler unless it is a torus [6]. A lot of interest in the subject was generated by "Reid's fantasy" [64] that all Calabi-Yau threefolds with trivial canonical bundle should form a connected family provided one allows deformations and singular transitions through non-Kähler manifolds with trivial canonical bundle. The geometry of compact complex manifolds with trivial canonical bundle has been investigated for example by [2,6,12,19,21,25,26,27,30,33,45,52,62,65] and others. In this paper we will consider a more general class of manifolds, that we now define, and argue that they can naturally be considered as "non-Kähler Calabi-Yau" manifolds.On any compact complex manifold there is a (finite-dimensional) cohomology theory called Bott-Chern cohomology. We will need only the real (1, 1) Bott-Chern cohomologyThere is a "first Bott-Chern class" map c BC 1 : Pic(M ) → H 1,1 BC (M, R), which can be described as follows. Given any holomorphic line bundle L → M and any Hermitian metric h on the fibers of L, its curvature form R h is locally given by − √ −1∂∂ log h. Then R h is a closed real (1, 1)-form and if we choose a different metricIf g is any Hermitian metric on M , with fundamental 2-form ω, then its first Chern form given locally byWe will call Ric(ω) the Chern-Ricci form of ω.We the...