Formation of single-moon systems around gas giants

Context. Several mechanisms have been proposed to explain the formation process of satellite systems, and relatively large moons are thought to be born in circumplanetary disks. Making a single-moon system is known to be more difficult than multiple-moon or moonless systems. Aims. We aim to find a way to form a system with a single large moon, such as Titan around Saturn. We examine the orbital migration of moons, which change their direction and speed depending on the properties of circumplanetary disks. Methods. We modeled dissipating circumplanetary disks with taking the effect of temperature structures into account and calculated the orbital evolution of Titan-mass satellites in the final evolution stage of various circumplanetary disks. We also performed N -body simulations of systems that initially had multiple satellites to see whether single-moon systems remained at the end. Results. The radial slope of the disk-temperature structure characterized by the dust opacity produces a patch of orbits in which the Titan-mass moons cease inward migration and even migrate outward in a certain range of the disk viscosity. The patch assists moons initially located in the outer orbits to remain in the disk, while those in the inner orbits fall onto the planet. Conclusions. We demonstrate for the first time that systems can form that have only one large moon around giant planet. Our N -body simulations suggest satellite formation was not efficient in the outer radii of circumplanetary disks.