Chemical Signatures of the FU Ori Outbursts
  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 $\sim10-10^3$ 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.
