Speaker
Description
Protoplanetary disks (PPDs) are dominated by molecular $H_{2}$ and $He$, with minor species serving as essential tracers of the disk structure and evolution. Rich ALMA-based evidence strongly suggests that different gaseous species in the disk are vertically stratified. However, the classical thermal scale height $H_\text{therm} = c_s/\Omega$ is fundamentally limited in explaining this phenomenon due to its dependence on a global sound speed $c_s$, in a hydrogen-gas-dominated environment. Here we present an optical analogue of the sound speed that is governed by the molecular mass of the gaseous species and the energy of the incident photon. We find that lighter molecular species are elevated to higher radiative scale heights ($H_g^\gamma$), due to photon-momentum coupling, while heavier species remain gravitationally confined near the disk's midplane. Results from these methods generally show excellent agreement with the elevated CO emission surfaces observed by ALMA in Herbig and T Tauri stars. We observe a correlation between CO emission height and disk radius (( 0.04 < H_g^\gamma/R < 0.20)) attributed to radiative pressure. This is consistent with the wide diversity in line-emitting heights (( H_\text{therm}/R \sim 0.01\text{--}0.50)) hinted in previous studies. Our analysis of ALMA archival data yields CO emission surfaces tracing $0.21 < H_\text{tracer} < 0.54$ for the disks around HD 100546, HD 169142 and V1094 Sco. On average, the CO emission surface traces ($\sim 2{-}5 \, H_{\rm{therm}}$). We give an outlook on the future possibilities of this method and its direct application as a kinetic tracer for other gaseous species in radiation-dominated disk environments.
| Stream | Science or Engineering |
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