Growth and Settling of Dust Particles in Protoplanetary Nebulae: Implications for Opacity, Thermal Profile, and Gravitational Instability

Debanjan Sengupta1,2, Sarah E. Dodson-Robinson1,3, Yasuhiro Hasegawa2, and Neal J. Turner2
Astrophysical Journal 874, 26 Link to Article [DOI: 10.3847/1538-4357/aafc36 ]
1Department of Physics & Astronomy, University of Delaware, Newark, DE 19716, USA
2Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
3Bartol Research Institute, Department of Physics & Astronomy, University of Delaware, Newark, DE 19716, USA

Despite making a small contribution to total protoplanetary disk mass, dust affects the disk temperature by controlling the absorption of starlight. As grains grow from their initial interstellar-medium-like size distribution, settling depletes the disk’s upper layers of dust and decreases the optical depth, cooling the interior. Here we investigate the effect of collisional growth of dust grains and their dynamics on the thermal and optical profile of the disk, and explore the possibility that cooling induced by grain growth and settling could lead to gravitational instability. We develop a Monte Carlo dust collision model with a weighting technique and allow particles to collisionally evolve through sticking and fragmentation, along with vertical settling and turbulent mixing. We explore three disk models and perform simulations for both constant and spatially variable turbulence profile. We then calculate mean wavelength-dependent opacities for the evolving disks and perform radiative transfer to calculate the temperature profile. Finally, we calculate the Toomre Q parameter, a measure of the disk’s stability against self-gravity, after it reaches a steady-state dust-size distribution. We find that even weak turbulence can keep submicrometer-sized particles stirred in the disk’s upper layer, affecting its optical and thermal profiles, and the growth of large particles in the midplane can make a massive disk optically thick at millimeter wavelengths, making it difficult to calculate the surface density of dust available for planet formation in the inner disk. Also, for all our initially marginally stable annuli, we find a small but noticeable reduction in Q.

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