  Context: It is hypothesized that low-mass young stellar objects undergo
eruptive phases during their early evolution. The outburst of V1647 Ori between
2003 and 2006 offered a rare opportunity to investigate such an accretion
event. Aims: By means of our interferometry observing campaign during this
outburst, supplemented by other observations, we investigate the temporal
evolution of the inner circumstellar structure of V1647 Ori We also study the
role of the changing extinction in the brightening of the object and separate
it from the accretional brightening. Methods: We observed V1647 Ori with
MIDI/VLTI at two epochs in this outburst. First, during the slowly fading
plateau phase (2005 March) and second, just before the rapid fading of the
object (2005 September), which ended the outburst. We used the radiative
transfer code MC3D to fit the interferometry data and the spectral energy
distributions from five different epochs at different stages of the outburst.
The comparison of these models allowed us to trace structural changes in the
system on AU-scales. We also considered qualitative alternatives for the
interpretation of our data. Results: We found that the disk and the envelope
are similar to those of non-eruptive young stars and that the accretion rate
varied during the outburst. We also found evidence for the increase of the
inner radii of the circumstellar disk and envelope at the beginning of the
outburst. Furthermore, the change of the interferometric visibilities indicates
structural changes in the circumstellar material. We test a few scenarios to
interpret these data. We also speculate that the changes are caused by the
fading of the central source, which is not immediately followed by the fading
of the outer regions. However, if the delay in the fading of the disk is
responsible for the changes seen in the MIDI data, the effect should be
confirmed by dynamical modeling.
