Restrictions on the Growth of Gas Giant Cores via Pebble Accretion

Abstract

We apply an order-of-magnitude model of gas-assisted growth, known as pebble accretion, in a turbulent medium to suggest a reason why some systems form wide orbital separation gas giants while others do not. In contrast to traditional growth by ballistic collisions with planetesimals, growth by pebble accretion is not necessarily limited by doubling times at the highest core mass. Turbulence, in particular, can cause growth to bottleneck at lower core masses. We demonstrate how a combination of growth by planetesimal and pebble accretion limits the maximum semi-major axis where gas giants can form. We find that, for fiducial disk parameters, strong turbulence ($\alpha \gtrsim 10^{-2}$) restricts gas giant cores to form interior to $a \lesssim 40 \, \text{AU}$, while for weak turbulence gas giants can form out to $a \lesssim 70 \, \text{AU}$. The correspondence between $\alpha$ and semi-major axis depends on the sizes of small bodies available for growth. This dependence on turbulence and small-body size distribution may explain the paucity of wide orbital separation gas giants. We also show that while lower levels of turbulence ($\alpha \lesssim 10^{-4}$) can produce gas giants far out in the disk, we expect these gas giants to be low-mass ($M \lesssim \, 1 M_J$). These planets are not luminous enough to have been observed with the current generation of direct-imaging surveys, which could explain why wide orbital separation gas giants are currently observed only around A stars.