Submillimetre images of transition discs are expected to reflect the distribution of the optically thin dust. Former observation of three transition discs LkHa330, SR21N, and HD1353444B at submillimetre wavelengths revealed images which cannot be modelled by a simple axisymmetric disc. We show that a large-scale anticyclonic vortex that develops where the viscosity has a large gradient (e.g., at the edge of the disc dead zone), might be accountable for these large-scale asymmetries. We modelled the long-term evolution of vortices being triggered by the Rossby wave instability. We found that a horseshoe-shaped (azimuthal wavenumber m=1) large-scale vortex forms by coalescing of smaller vortices within 5x10^4 yr, and can survive on the disc life-time (~5x10^6 yr), depending on the magnitude of global viscosity and the thickness of the viscosity gradient. The two-dimensional grid-based global disc simulations with local isothermal approximation and compressible-gas model have been done by the GPU version of hydrodynamic code FARGO (GFARGO). To calculate the dust continuum image at submillimetre wavelengths, we combined our hydrodynamical results with a 3D radiative transfer code. By the striking similarities of the calculated and observed submillimetre images, we suggest that the three transition discs can be modelled by a disc possessing a large-scale vortex formed near the disc dead zone edge. Since the larger dust grains (larger than mm in size) are collected in these vortices, the non-axisymmetric submillimetre images of the above transition discs might be interpreted as active planet and planetesimal forming regions situated far (> 50 AU) from the central stars.Comment: 13 pages, 14 figures, accepted for publication in MNRA

The merger rate of stellar mass black holes binaries (sBHBs) inferred by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) suggests the need for an efficient source of sBHB formation. Active galactic nucleus (AGN) disks are a promising location for the formation of these sBHBs, as well as binaries of other compact objects, because of powerful torques exerted by the gas disk. These gas torques cause orbiting compact objects to migrate towards regions in the disk where inward and outward torques cancel, known as migration traps. We simulate the migration of stellar mass black holes (sBHs) in an analytically modeled AGN disk using an augmented N-body code that includes migration torques, a stochastic gravitational force exerted by turbulent density fluctuations in the disk, and inclination and eccentricity dampening produced by passages through the gas disk, in addition to the standard gravitational forces between objects. We find that sBHBs form rapidly in the disk as sBHs migrate towards the migration trap. These sBHBs are likely to subsequently merge on short time-scales. The process continues, leading to the build up of a population of over-massive sBHs. sBHBs that form in AGN disks could contribute significantly to the sBHB merger rate inferred by LIGO.

Context. The formation of resonant planet pairs in exoplanetary systems involves planetary migration inside the protoplanetary disc. After a resonant capture, subsequent migration leads to a large increase in planetary eccentricities, if no damping mechanism is applied. This has led to the conclusion that the migration of resonant planetary systems cannot take place across large radial distances, but must be terminated rapidly by disc dissipation. Aims. We investigate if the presence of an inner disc could supply eccentricity damping to the inner planet, and if this effect could explain observed eccentricities in some planetary systems. Methods. We compute hydrodynamic simulations of giant planets, in orbits of a given eccentricity about an inner gas disc, and measure the effect of the gas disc on the planetary orbital parameters. We perform detailed long-term calculations of the GJ 876 system. We complete N-body simulations, which include artificial forces on the planets that recreate the effect of the inner and outer discs.Results. We find that we cannot neglect the influence of the inner disc, and that the disc could explain the observed eccentricities. In particular, we reproduce the orbital parameters of a few systems engaged in 2:1 mean motion resonances: GJ 876, HD 73 526, HD 82 943, and HD 128 311. Analytically, we derive the effect that the inner disc should have on the inner planet to reach a specific orbital configuration, for any given damping effect of the outer disc on the outer planet. Conclusions. We conclude that an inner disc, even though difficult to model properly in hydrodynamical simulations, should be taken into account because of its damping effect on the eccentricity of the inner planet. By including this effect, we can explain quite naturally the observed orbital elements of the pairs of known resonant exoplanets.