An Extraordinary Response of Iron Emission to the Central Outburst in a
  Tidal Disruption Event Candidate
  Understanding the origin of \feii\ emission is important because it is
crucial to construct the main sequence of Active Galactic Nuclei (AGNs).
Despite several decades of observational and theoretical effort, the location
of the optical iron emitting region and the mechanism responsible for the
positive correlation between the \feii\ strength and the black hole accretion
rate remain open questions as yet. In this letter, we report the optical \feii\
response to the central outburst in PS1-10adi, a candidate tidal disruption
event (TDE) taking place in an AGN at $z = 0.203$ that has aroused extensive
attention. For the first time, we observe that the \feii\ response in the
rising phase of its central luminosity is significantly more prominent than
that in the decline phase, showing a hysteresis effect. We interpret this
hysteresis effect as a consequence of the gradual sublimation of the dust
grains situating at the inner surface of the torus into gas when the luminosity
of the central engine increases. It is the iron element released from the
sublimated dust that contributes evidently to the observed \feii\ emission.
This interpretation, together with the weak response of the \hb\ emission as we
observe, naturally explains the applicability of relative \feii\ strength as a
tracer of the Eddington ratio. In addition, optical iron emission of this
origin renders the \feii\ time lag a potential "standard candle" with
cosmological implications.
