Dust processing in protoplanetary envelopes as the origin of hot minerals in comets

Crystalline silicates are found in a large number of comets. These pose a long-standing conundrum for Solar system formation models as they can only be created in the inner hot disc at temperatures higher than 800 K, and there is no obvious mechanism to transport them out into the comets formation region. Here, we propose that these particles could have formed inside the hydrostatic envelopes surrounding young protoplanets still embedded in the protoplanetary disc. Using a simplified 1D model we investigate the thermal structure of these envelopes, and find that for core masses ranging from 0.08 to 1.5 M·, located anywhere between 1 and 30 au, the temperature and pressure at the base of the envelopes are high enough to quickly vapourize silicate particles of various sizes. Moreover, if the grain abundance is atleast solar, these envelopes become fully convective, allowing for dust ejection across the Bondi radius back into the disc. Amorphous silicates are hence thermally processed into crystalline particles in these envelopes, and then transported back to disc through convective diffusion to be finally incorporated into the cometary building blocks.