Planets are formed when dust grains from a protoplanetary disk turn into pebbles and then into planets. Because the small grains of dust interact with the gas (via the drag it imparts), the gas in the protoplanetary discs influences the distribution of the small grains and therefore the growth of the planets. Astronomers trying to understand how dust-gas interactions affect the development of the planet are particularly interested in studying the thickness of the disc (its “vertical height”) in relation to the distance from the star; the disc flares outward in most cases where the central star dominates the mass of the system. By independently measuring the heights of gas and small grains of dust, astronomers can study fundamental characteristics of the disk such as gas-to-dust mass ratio and turbulence in the disk.
CFA astronomers Richard Teague and David Wilner and a team of colleagues made the first direct comparison of vertical heights of gas and dust. They modeled multi-wavelength archival observations from ALMA, Hubble and Gemini in three planetary disks particularly suitable for such measurements: the systems are moderately tilted with respect to the line of sight to provide a 3D perspective, they have enough carbon monoxide and dust. for those components to be measured, and the discs display multiple rings. The rings scatter light and are necessary for estimating the vertical heights of small grains (the origins of the rings are uncertain, possibly sculpted by planets or by a temperature transition that produces ice).
Astronomers find that in two systems, gas and dust at distances from the star of about one hundred astronomical units are collocated with the same structure, but further away the dust grains have a smaller vertical height than gas. CO. In the third system, the two components have the same shape at all distances. Scientists argue that a gas / dust mass ratio greater than 100 (the typical value for the interstellar medium) could explain the behavior of the first two. The team also concludes that the vertical heights of gas and dust are not just functions of the star’s mass, age, or spectral type, but that they hope in future work to clarify the dependencies. .
Scientists warn that with just three examples, it is premature to generalize their findings. They also note that the production mechanisms of the rings are uncertain and that there could have been an unidentified selection effect in these systems. For example, these disks are relatively large, and smaller, more typical disks may behave differently. Finally, the effects of turbulence and dust sedimentation remain uncertain. These first results however demonstrate the feasibility of the techniques. Further observations and modeling should help characterize disks from other systems and trace more details about the process of planet formation.
Reference: “Investiging the Relative Gas and Small Dust Grain Surface Heights in Protoplanetary Disks”, Evan A. Rich, Richard Teague, John D. Monnier, Claire L. Davies, Arthur Bosman, Tim J. Harries, Nuria Calvet, Fred C. Adams, David Wilner and Zhaohuan Zhu, The Astrophysical Journal 913, 138, 2021.