event
Thursday 19 Jul 2012: The radial distribution of dust species in young brown dwarf disks
Dr Basmah Riaz - University of Hertfordshire
Physics, 4th floor 14:00-14:30
I'll present results from a study of the radial distribution of dust species in young brown dwarf disks. Our work is based on a compositional analysis of the 10 and 20 mu silicate emission features for brown dwarfs in the Taurus star-forming region. A fundamental finding of our work is that brown dwarfs exhibit stronger signs of dust processing in the cold component of the disk, compared to the higher mass T Tauri stars in Taurus. The crystallinity levels at larger radii (~1-2 AU) in these brown dwarf disks are found to be higher than the fractions in the inner disk regions (< 0.1 AU), which indicate efficient diffusion of crystalline material from the warm/inner region to the cold/outer parts in the disks. For nearly 80% of the disks, the mass fraction of small ISM-like dust grains is negligible (< 5%) in the outer disk region. Most disks show similar large-grain mass fractions in the inner and outer disk regions. The disk structure in both the inner and outer regions is found to be affected by the crystallinity level in the disk. For the case of one brown dwarf, 2M04230607, we find the forsterite mass fraction to be a factor of ~3 higher in the outer disk compared to the inner disk region. Simple large-scale radial mixing cannot account for this gradient in the dust chemical composition, and some local crystalline formation mechanism may be effective in this disk. A weak anti-correlation between the X-ray emission strength and the extent of crystallinity in the disk is observed, suggesting X-rays to be an important dust amorphization agent in these disks. The relatively high abundance of crystalline silicates in the outer cold regions of brown dwarf disks provides an interesting analogy to comets. In this context, I will discuss the applicability of the various mechanisms that have been proposed for comets on the formation and the outward transport of high-temperature material.
