Gas and dust hydrodynamical simulations of massive lopsided transition discs – I. Gas distribution

Motivated by lopsided structures observed in some massive transition discs, we have carried out 2D numerical simulations to study vortex structure in massive discs, including the effects of disc self-gravity and the indirect force which is due to the displacement of the central star from the barycentre of the system by the lopsided structure. When only the indirect force is included, we confirm the finding by Mittal & Chiang that the vortex becomes stronger and can be more than two pressure scale heights wide, as long as the disc-to-star mass ratio is ≳1 per cent. Such wide vortices can excite strong density waves in the disc and therefore migrate inwards rapidly. However, when disc self-gravity is also considered in simulations, self-gravity plays a more prominent role on the vortex structure. We confirm that when the disc Toomre Q parameter is smaller than π/(2h), where h is the disc's aspect ratio, the vortices are significantly weakened and their inward migration slows down dramatically. Most importantly, when the disc is massive enough (e.g. Q ∼ 3), we find that the lopsided gas structure orbits around the star at a speed significantly slower than the local Keplerian speed. This sub-Keplerian pattern speed can lead to the concentration of dust particles at a radius beyond the lopsided gas structure (as shown in Paper II). Overall, disc self-gravity regulates the vortex structure in massive discs and the radial shift between the gas and dust distributions in vortices within massive discs may be probed by future observations.