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Abstract

The FU Ori-type young stellar objects are characterized by a sudden increase in luminosity by 1-2 orders of magnitude followed by a slow return to the pre-outburst state on timescales of ̃10-100 yr. The outburst strongly affects the entire disk, changing its thermal structure and radiation field. In this paper, using a detailed physical- chemical model, we study the impact of the FU Ori outburst on the disk chemical inventory. Our main goal is to identify gas-phase molecular tracers of the outburst activity that could be observed after the outburst with modern telescopes such as ALMA and NOEMA. We find that the majority of molecules experience a considerable increase in total disk gas-phase abundances due to the outburst, mainly due to the sublimation of their ices. Their return to the pre-outburst chemical state takes different amounts of time, from nearly instantaneous to very long. Among the former, we identify CO, NH₃, C₂H₆, C₃H₄, etc. Their abundance evolution tightly follows the luminosity curve. For CO, the abundance increase does not exceed an order of magnitude, while for other tracers, the abundances increase by 2-5 orders of magnitude. Other molecules, like H₂CO and NH₂OH, have longer retention timescales, remaining in the gas phase for ̃10-10³ yr after the end of the outburst. Thus, H₂CO could be used as an indicator of the previous outbursts in the post-outburst FU Ori systems. We investigate the corresponding time-dependent chemistry in detail and present the most favorable transitions and ALMA configurations for future observations.