Grain growth in newly discovered young eruptive stars
  FU Orionis-type stars are young stellar objects showing large outbursts due
to highly enhanced accretion from the circumstellar disk onto the protostar.
FUor-type outbursts happen in a wide variety of sources from the very embedded
ones to those with almost no sign of extended emission beyond the disk. The
subsequent eruptions might gradually clear up the obscuring envelope material
and drive the protostar on its way to become a disk-only T Tauri star. We used
VLT/VISIR to obtain the first spectra that cover the 8-13 $\mu$m mid-infrared
wavelength range in low-resolution of five recently discovered FUors. Four
objects from our sample show the 10 $\mu$m silicate feature in emission. We
study the shape and strength of the silicate feature in these objects and find
that they mostly contain large amorphous grains, suggesting that large grains
are typically not settled to the midplane in FUor disks. This is a general
characteristic of FUors, as opposed to regular T Tauri-type stars whose disks
display anything from pristine small grains to significant grain growth. We
classify our targets by determining whether the silicate feature is in emission
or in absorption, and confront them with the evolutionary scenarios on the
dispersal of the envelopes around young stars. In our sample, all Class II
objects exhibit silicate emission, while for Class I objects, the appearance of
the feature in emission or absorption depends on the viewing angle with respect
to the outflow cavity. This highlights the importance of geometric effects when
interpreting the silicate feature.
