AST 309 Lecture Notes - Lecture 24: Formins, Grain Growth, Terrestrial Planet
Document Summary
The appearance of their envelopes is not always symmetrical or spherical look like cometary shapes with tails" Uv light from nearby stars (or the parent star itself) Uv light heats the surfaces of the disks, and in the outer portions of the disks where material is less dense, therefore gravity only binds the material weakly together, the gas can escape into the surrounding hii region. Particles located 100 au from a 1-solar mass star have orbital speeds of ~3 km/s. The escape velocity at this distance is only a little over 4 km/s, so if they are heated just a little bit, by about an additional 1 km/s they can escape the disk. Large disks (ones 100"s of au in size) can have mass loss rates of ~ 10-6 solar masses per year. Time (in years) = . 01 /1x10-6 / year =10,000 years. Luckily, as the disk shrinks, their rate of mass loss also declines.
