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Compact systems: Products of early accretion during stellar infall?

Presentation #102.362 in the session Poster Session.

Published onJun 20, 2022
Compact systems: Products of early accretion during stellar infall?

A surprising discovery has been the existence of compact systems of Earth to super-Earth sized planets. These multi-planet systems have nearly circular, coplanar orbits at distances of only ~ 0.01 – 0.1 AU, a region devoid of planets in our Solar System. Although compact systems are common, their origin remains debated.

Prior models of compact system origin assume that infall to the stellar disk ends before planets form. Infall of gas and grains occurs over the first few ×105 to 106 yr of a star’s life, as material flows into the circumstellar disk from a precursor molecular cloud. Our terrestrial planets completed their accretion in a few × 107 to 108 yr, long after the Sun’s infall phase. However, planets in compact systems may accrete in only ~ 102 to 104 yr, which is shorter than the infall timescale. Thus in compact systems, planet formation may occur during infall to the stellar disk, rather than after infall ended as previously assumed. Indeed, recent observations and other arguments now suggest that planet accretion may commence in disks as young as ~105 to 106 yr.

Motivated by these findings, we propose and are developing a new model in which accretion that commences during stellar infall yields long-lived compact systems. As the cloud core collapses, gas and entrained grains flow into the disk. Solids that accrete into pebbles and larger particles decouple from the gas and build-up in the inner disk region supplied by the infall. The inner disk region becomes solid-enhanced, producing favorable conditions for the formation and survival of close-in planets, even with Type-I migration. In many systems, including perhaps our Solar System and those with long nebular lifetimes, planets formed during infall may well have been lost. However, we argue that key properties of compact systems may be most easily explained if they are surviving remnants of the infall era.

We present results of N-body simulations of planet accretion during infall that demonstrate that such planets may survive until the nebula disperses. We identify favorable parameter regimes for producing compact systems, and compare these to a model of disk evolution during infall and other constraints. Finally, we identify distinguishing features of compact systems produced during infall for comparison with observed systems. A key prediction is that such systems display a similar ratio between the planet system mass and the mass of the central star.

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