Mid-infrared images of beta Pictoris and the possible role of planetesimal collisions in the central disk

When viewed in optical starlight scattered by dust, the nearly edge-on debris disk surrounding the A5V star beta Pictoris (distance 19.3 pc; ref. 1) extends farther than 1,450 au from the star. Its large-scale complexity has been well characterized, but the detailed structure of the disk's central approximately 200-au region has remained elusive. This region is of special interest, because planets may have formed there during the star's 10-20-million-year lifetime, perhaps resulting in both the observed tilt of 4.6 degrees relative to the large-scale main disk and the partial clearing of the innermost dust. A peculiarity of the central disk (also possibly related to the presence of planets) is the asymmetry in the brightness of the 'wings', in which the southwestern wing is brighter and more extended at 12 microm than the northeastern wing. Here we present thermal infrared images of the central disk that imply that the brightness asymmetry results from the presence of a bright clump composed of particles that may differ in size from dust elsewhere in the disk. We suggest that this clump results from the collisional grinding of resonantly trapped planetesimals or the cataclysmic break-up of a planetesimal.     

Transformation by concanavalin A of the response of molluscan neurones to L-glutamate.

Concanavalin A causes in all Aplysia and Helix neurones a depolarising response to L-glutamate. The Con A-induced glutamate response is pharmacologically distinct from the three cell-specific glutamate responses (two inhibitory, one excitatory) that can be elicited from untreated molluscan neurones. The transformation in glutamate sensitivity brought about by Con A does not seem to be related to the lectin's capacity to produce redistribution of receptor sites in cell membranes.