The influence of a static planetary atmosphere on spin transfer during pebble accretion

Context. Pebble accretion has been used to explain the small size of Mars, the heavy element contents of the gas giants, and the size distribution of asteroids. More recently, pebble accretion has been proposed as a means to explain not only the growth but also the prograde spin preference of most larger bodies in the Solar System. Pebble accretion could induce planetary and asteroid spin equal to or exceeding the spins currently measured. However, as these planetesimals grow, they start condensing the gas of the disc around them, forming an atmosphere within their Bondi radius.
Aims. We study the effect an atmosphere has on the pebble orbits and spin build-up on the planet's surface during pebble accretion in the extreme case of a static atmosphere. Pebble feedback to the gas is not taken into account. Methods. The equations of motion for pebbles in a planar, global frame with a planet and a central star were integrated using the AIS15 integrator of REBOUND. An adiabatic atmosphere was then placed around the planet, and the spin deposited on the planet's surface was measured. These simulations were evaluated for different distances to the star, Stokes numbers, and planet masses.
Results. In general, an atmosphere dampens the spin the planet's surface receives by absorbing part of the angular momentum of the pebbles and circularising their orbits. This could prevent the excessive spin values predicted in some 3D pebble accretion simulations without an atmosphere. For planets larger than 0.5 M_⊕, a stationary atmosphere absorbs all angular momentum, leaving no spin for the surface. Significant quantities of angular momentum are stored in the inner and intermediate atmosphere (