Constraining the initial planetary population in the gravitational instability model. (arXiv:1907.07584v1 [astro-ph.EP])
<a href="http://arxiv.org/find/astro-ph/1/au:+Humphries_J/0/1/0/all/0/1">Jaxk Humphries</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Vazan_A/0/1/0/all/0/1">Allona Vazan</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bonavita_M/0/1/0/all/0/1">Mariangela Bonavita</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Helled_R/0/1/0/all/0/1">Ravit Helled</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Nayakshin_S/0/1/0/all/0/1">Sergei Nayakshin</a>
Direct imaging (DI) surveys suggest that gas giants beyond 20 AU are rare
around FGK stars. However, it is not clear what this means for the formation
frequency of Gravitational Instability (GI) protoplanets due to uncertainties
in gap opening and migration efficiency. Here we combine state-of-the-art
calculations of homogeneous planet contraction with a population synthesis
code. We find DI constraints to be satisfied if protoplanet formation by GI
occurs in tens of percent of systems if protoplanets `super migrate’ to small
separations. In contrast, GI may occur in only a few percent of systems if
protoplanets remain stranded at wide orbits because their migration is
`quenched’ by efficient gap opening. We then use the frequency of massive
giants in radial velocity surveys inside 5 AU to break this degeneracy –
observations recently showed that this population does not correlate with the
host star metallicity and is therefore suspected to have formed via GI followed
by inward migration. We find that only the super-migration scenario can
sufficiently explain this population whilst simultaneously satisfying the DI
constraints and producing the right mass spectrum of planets inside 5 AU. If
massive gas-giants inside 5 AU formed via GI, then our models imply that
migration must be efficient and that the formation of GI protoplanets occurs in
at least a tens of percent of systems.
Direct imaging (DI) surveys suggest that gas giants beyond 20 AU are rare
around FGK stars. However, it is not clear what this means for the formation
frequency of Gravitational Instability (GI) protoplanets due to uncertainties
in gap opening and migration efficiency. Here we combine state-of-the-art
calculations of homogeneous planet contraction with a population synthesis
code. We find DI constraints to be satisfied if protoplanet formation by GI
occurs in tens of percent of systems if protoplanets `super migrate’ to small
separations. In contrast, GI may occur in only a few percent of systems if
protoplanets remain stranded at wide orbits because their migration is
`quenched’ by efficient gap opening. We then use the frequency of massive
giants in radial velocity surveys inside 5 AU to break this degeneracy –
observations recently showed that this population does not correlate with the
host star metallicity and is therefore suspected to have formed via GI followed
by inward migration. We find that only the super-migration scenario can
sufficiently explain this population whilst simultaneously satisfying the DI
constraints and producing the right mass spectrum of planets inside 5 AU. If
massive gas-giants inside 5 AU formed via GI, then our models imply that
migration must be efficient and that the formation of GI protoplanets occurs in
at least a tens of percent of systems.
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