Planetary core formation via multispecies pebble accretion

Abstract

In the general classical picture of pebble-based core growth, planetary cores grow by accretion of single pebble species. The growing planet may reach the so-called pebble isolation mass, at which it induces a pressure bump that blocks inward drifting pebbles exterior to its orbit, thereby stalling core growth by pebble accretion. In recent hydrodynamic simulations, pebble filtration by the pressure bump depends on several parameters including core mass, disc structure, turbulent viscosity and pebble size. We investigated how accretion of multiple, instead of single, pebble species affects core growth rates, and how the dependence of pebble isolation mass on turbulent viscosity and pebble size sets the final core masses. We performed numerical simulations in viscous 1D disc, where maximal grain sizes were regulated by grain growth, fragmentation and drift limits. We confirm that core growth rates and final core masses are sensitive to three key parameters: threshold velocity at which pebbles fragment on collision, turbulent viscosity and distribution of pebble species, which yield a diversity of planetary cores. With accretion of multiple pebble species, planetary cores can grow pretty fast, reaching over 30 – 40 ME in mass. Potential cores of cold gas giants were able to form from embryos initially implanted as far as 50 AU. Our results suggest that accretion of multi-species pebbles could explain: the estimated 25 – 45 ME heavy elements abundance inside Jupiter’s core; massive cores of extrasolar planets; disc rings and gaps at wider orbits; early and rapid formation of planetary bodies.

The role of density perturbation on planet formation by pebble accretion

“…In this study we adopt the formulation in Bitsch et al (2018) as used in our previous works (Andama et al 2022;Ndugu et al 2022). We remark here that the results of the formulations in Bitsch et al (2018) and Ataiee et al (2018) were in close agreement and we therefore think that our choice of the former formula should not qualitatively influence our results.…”

“…In our growth model for the cases of A = 0 and A = 0.1, the planetary core accretes until it reaches the pebble isolation mass of pebble species with the smallest Stokes number in the grain size distribution as in Andama et al (2022). This is because, as discussed in Section 3.1, the dust evolution for both cases is similar.…”

“…In our simulations, we start pebble accretion at the transition mass, which is the mass at which accretion transitions from Bondi to Hill regime (Lambrechts & Johansen 2012). We follow accretion model described in Andama et al (2022) where the planetary core grows by concurrent accretion of different pebble species. The contribution of each pebble species to core growth is calculated via the classical pebble accretion formulation as (Morbidelli et al 2015)…”

Planet population synthesis: The role of stellar encounters

“…We modelled the pebble isolation mass via the criterion that allow diffusion of particles across the pressure bump (Bitsch et al 2018). We remind the reader that the pebble accretion scheme in this work follows the concurrent accretion of multiple pebble sizes (Andama et al 2022), with pebble size distributions derived from the full grain size distribution (see sub section 2.3).…”

“…We also switched from accreting the dominant pebble size to accreting realistically full pebble size distributions in discs onto planetary cores via multi-species pebble accretion paradigm of Andama et al (2022) that built from the existing pebble accretion formalism (Ormel & Klahr 2010;Johansen & Lacerda 2010;). The gas accretion scheme follows Ndugu et al (2021), which show that additional accretion of gas from the horseshoe region allows reduced migration of massive planets compared to the classical gas accretion formalism.…”