Grain growth in newly discovered young eruptive stars

TL;DR

This study presents mid-infrared spectra of five FU Orionis-type young eruptive stars, revealing large amorphous grains and emphasizing the role of viewing angle and evolution in silicate feature appearance.

Contribution

First low-resolution 8-13 μm spectra of newly discovered FUors, analyzing silicate features to understand grain growth and disk evolution in young eruptive stars.

Findings

Most FUors contain large amorphous grains.

Silicate emission is common in Class II objects.

Viewing angle influences silicate feature appearance in Class I objects.

Abstract

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.