FU Orionis disk outburst: Evidence for a gravitational instability scenario triggered in a magnetically dead zone

International audienceContext. FUors outbursts are a crucial stage of accretion in young stars. However, a complete mechanism at the origin of the outburst still remains missing. Aims. We aim to constrain the instability mechanism in the star FU Orionis itself by directly probing the size and evolution in time of the outburst region with near-infrared (NIR) interferometry, and to confront it with physical models of this region. Methods. As the prototype of the FUors class of objects, FU Orionis has been a regular target of NIR interferometry. In this paper, we analyze more than 20 years of NIR interferometric observations to perform a temporal monitoring of the region of the outburst, and compare it to the spatial structure deduced from 1D magneto-hydrodynamic (MHD) simulations. Results. We measure from the interferometric observations that the size variation of the outburst region is compatible with a constant or slightly decreasing size over time: -0.56 -0.36 +0.14 AU/100 yr and -0.30 -0.19 +0.19 AU/100 yr in the H and K bands, respectively. The temporal variation and the mean size probed by NIR interferometry are consistently reproduced by our 1D MHD simulations. We find that the most compatible scenario is a model of an outburst occurring in a magnetically layered disk, where a magneto-rotational instability (MRI) is triggered by a gravitational instability (GI) at the outer edge of a dead zone. The scenario of a pure thermal instability (TI) fails to reproduce our interferometric sizes because it can only be sustained in a very compact zone of the disk