The FU Ori-type young stellar objects are characterized by a sudden increase in luminosity by 1–2 orders of magnitude , followed by slow return to the pre-outburst state on timescales of \sim 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 the 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 ones we identify CO , NH _ { 3 } , C _ { 2 } H _ { 6 } , C _ { 3 } H _ { 4 } , 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 H _ { 2 } CO and NH _ { 2 } OH have longer retention timescales , remaining in the gas phase for \sim 10 - 10 ^ { 3 } yr after the end of the outburst .
Thus H _ { 2 } CO 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 .