Size and surface area of icy dust aggregates after a heating event at a protoplanetary nebula

Abstract

The activity of a young star rises abruptly during an FU Orionis outburst. This event causes a temporary temperature increase in the protoplanetary nebula. H2O icy grains are sublimated by this event, and silicate cores embedded inside the ice are ejected. During the high-temperature phase, the silicate grains coagulate to form silicate core aggregates. After the heating event, the temperature drops, and the ice recondenses onto the aggregates. I determined numerically the size distribution of the ice-covered aggregates. The size of the aggregates exceeds 10 μm around the snow line. Because of the migration of the ice to large aggregates, only a small fraction of the silicate core aggregate is covered with H2O ice. After the heating event, the surface of an ice-covered aggregate is totally covered by silicate core aggregates. This might reduce the fragmentation velocity of aggregates when they collide. It is possible that the covering silicate cores shield the UV radiation field which induces photodissociation of H2O ice. This effect may cause the shortage of cold H2O vapor observed by Herschel. © 2013. The American Astronomical Society. All rights reserved.