disks, disk, envelopes, material, envelope, shells, matter, dust, shell, gas, environment, medium, grains, environments, emission, lines, disc, discs, clouds, masers, extinction, ring, origin, absorption, interaction, features, cloud, nebula, regions, region, chemistry, outflows, nebulae, mass, density, wind, components, media, molecules, shock, structures, particles, zone, silicates, zones, component, condensation, plasma, reddening, outflow, masses, spectra, flows, bubble, rings, bands, atmospheres, evolution, inversions, gases, distributions, polarization