  Luminosity bursts in young FU Orionis-type stars warm up the surrounding
disks of gas and dust, thus inflicting changes on their morphological and
chemical composition. In this work, we aim at studying the effects that such
bursts may have on the spatial distribution of dust grain sizes and the
corresponding spectral index in protoplanetary disks. We use the numerical
hydrodynamics code FEOSAD, which simulates the co-evolution of gas, dust, and
volatiles in a protoplanetary disk, taking dust growth and back reaction on gas
into account. The dependence of the maximum dust size on the water ice mantles
is explicitly considered. The burst is initialized by increasing the luminosity
of the central star to 100-300 L_sun for a time period of 100 yr. The water
snowline shifts during the burst to a larger distance, resulting in the drop of
the maximum dust size interior to the snowline position because of more
efficient fragmentation of bare grains. After the burst, the water snowline
shifts quickly back to its preburst location followed by renewed dust growth.
The timescale of dust regrowth after the burst depends on the radial distance
so that the dust grains at smaller distances reach the preburst values faster
than the dust grains at larger distances. As a result, a broad peak in the
radial distribution of the spectral index in the millimeter dust emission
develops at \approx 10 au, which shifts further out as the disk evolves and
dust grains regrow to preburst values at progressively larger distances. This
feature is most pronounced in evolved axisymmetric disks rather than in young
gravitationally unstable counterparts, although young disks may still be good
candidates if gravitational instability is suppressed. Abridged.

Earth and Planetary Astrophysics;Solar and Stellar Astrophysics