Abstract
Context. The formation of gas giant planets by the accretion of 100 km diameter planetesimals is often thought to be inefficient. A diameter of this size is typical for planetesimals and results from self-gravity. Many models therefore use small kilometer-sized planetesimals, or invoke the accretion of pebbles. Furthermore, models based on planetesimal accretion often use the ad hoc assumption of planetesimals that are distributed radially in a minimum-mass solar-nebula way. Aims. We use a dynamical model for planetesimal formation to investigate the effect of various initial radial density distributions on the resulting planet population. In doing so, we highlight the directive role of the early stages of dust evolution into pebbles and planetesimals in the circumstellar disk on the subsequent planet formation. Methods. We implemented a two-population model for solid evolution and a pebble flux-regulated model for planetesimal formation in our global model for planet population synthesis. This framework was used to study the global effect of planetesimal formation on planet formation. As reference, we compared our dynamically formed planetesimal surface densities with ad hoc set distributions of different radial density slopes of planetesimals. Results. Even though required, it is not the total planetesimal disk mass alone, but the planetesimal surface density slope and subsequently the formation mechanism of planetesimals that enables planetary growth through planetesimal accretion. Highly condensed regions of only 100 km sized planetesimals in the inner regions of circumstellar disks can lead to gas giant growth. Conclusions. Pebble flux-regulated planetesimal formation strongly boosts planet formation even when the planetesimals to be accreted are 100 km in size because it is a highly effective mechanism for creating a steep planetesimal density profile. We find that this leads to the formation of giant planets inside 1 au already by pure 100 km planetesimal accretion. Eventually, adding pebble accretion regulated by pebble flux and planetesimal-based embryo formation as well will further complement this picture.
Highlights
A current conundrum of planetesimal accretion in the coreaccretion scenario of planet formation is that for 100 km planetesimals it appears to require an unreasonably high disk mass to be an effective mechanism for giant planet formation within the lifetime of a circumstellar disk (Fortier et al 2013)
The density slope that arises from the pebble flux-regulated model for planetesimal formation can have a slope as steep as ΣP ∝ r−2.1, and it generally depends on the individual evolution of the disk
Because of the steeper slope, we find a remarkable increase in ΣP in the inner regions of a protoplanetary disk and a corresponding decrease farther out
Summary
A current conundrum of planetesimal accretion in the coreaccretion scenario of planet formation is that for 100 km planetesimals it appears to require an unreasonably high disk mass to be an effective mechanism for giant planet formation within the lifetime of a circumstellar disk (Fortier et al 2013). Planetesimals are typically too small for efficient pebble accretion (Ormel & Klahr 2010), a pebble-accreting embryo might well have formed from planetesimal collisions. This crucial step adds room for discussing the formation of planetesimals and subsequently their role in planetary core and planet formation. The observed size distribution might arise from the growth of planetesimals that originally measured 100 m (Weidenschilling 2011).
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