M. E. van den Ancker
SPATIALLY RESOLVED DIRECT MID-INFRARED IMAGING OF FU ORIONIS STARS (Poster)

SPATIALLY RESOLVED DIRECT MID-INFRARED IMAGING OF FU ORIONIS STARS

M. E. van den Ancker
European Southern Observatory, Garching, Germany

The small group of YSOs know as FU Orionis stars provide striking evidence for the importance and irregularity of disk accretion during early stellar evolution. FU Orionis star were originally identified as a class of young stars with large ($>$ 4 magnitudes) outbursts in optical light. All are surrounded by reflection nebulae. More recently it has been realized that the physical reason for such an FU Orionis outburst is that the accretion rate onto the central star changes, within a period less than a month, from those commonly found around T Tauri stars into values of $10^{-3}$-$10^{-4}$ M$_\odot$ yr$^{-1}$. Intriguingly, meteoritic evidence suggests that chondritic material has formed when our own proto-solar nebula went through an episode of enhanced temperatures. FU Orionis objects may therefore not only constitute a crucial phase in the evolution of proto-planetary disks, but may also be directly relevant for the condensation of the protoplanetary disk into solids.

Here we present diffraction-limited images in the 9.0-18.7 $\mu$m range of a sample of FU Orionis and related objects obtained with the new mid-infrared imager/spectrometer VISIR on the VLT. Apart from the wide-scale morphology due to material heated in the stars natal cloud, evident in most sources in our sample, we clearly resolve two sources into a compact structure with an elliptical shape. This morphology of the region in the mid-infrared is quite distinct from near-IR images which are dominated by scattered light in the dusty envelope.

We interpret the new mid-infrared data as direct evidence for the presence of a disk surrounding these two objects. In both, the drop-off in brightness with radius in the Par 21 disk can be described as a broken power-law, with a sharp edge. These observation represent the first direct imaging of disks around FU Orionis stars at mid-infrared wavelength, vividly illustrating the scientific pay-back of the gain in spatial resolution and sensitivity that can be obtained by a dedicated mid-IR instrument on a 8m class telescope. Although they confirm the existence of disks, the observations presented here are not in agreement with predictions for the extent of these disks based on models for simple gaseous accretion disks, illustrating the importance of re-processing of accretion energy by dust.