Bouncing Grains Keep Protoplanetary Disks Bright

Abstract Protoplanetary disks display the so-called size–luminosity relation, where their millimeter wavelength fluxes scale linearly with their emitting areas. This suggests that these disks are optically thick in millimeter band, an interpretation further supported by their near-blackbody spectral indexes. Such characteristics are seen not only among disks in very young star-forming regions like Lupus (1–3 Myr), but, as we demonstrate here, also among disks in the much older Upper Scorpius region (5–11 Myr). How can disks shine brightly for so long, when grain growth and subsequent radial drift should have quickly depleted their solid reservoir? Here, we suggest that the “bouncing barrier” provides the answer. Even colliding at very low speeds (below 1 cm s −1 ), grains already fail to stick to each other but instead bounce off in elastically. This barrier stalls grain growth at a near-universal size of ∼100 μ m. These small grains experience much reduced radial drift, and so are able to keep the disks bright for millions of years. They are also tightly coupled to gas, offering poor prospects for processes like streaming instability or pebble accretion. We speculate briefly on how planetesimals can arise in such a bath of 100 μ m grains.