Formation of planetary populations – I. Metallicity and envelope opacity effects
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abstract
We present a comprehensive body of simulations of the formation of
exoplanetary populations that incorporate the role of planet traps in slowing
planetary migration. The traps we include in our model are the water ice line,
the disk heat transition, and the dead zone outer edge. We reduce our model
parameter set to two physical parameters: the opacity of the accreting
planetary atmospheres ($\kappa_{\rm{env}}$) and a measure of the efficiency of
planetary accretion after gap opening ($f_{\rm{max}}$). We perform planet
population synthesis calculations based on the initial observed distributions
of host star and disk properties - their disk masses, lifetimes, and stellar
metallicities. We find the frequency of giant planet formation scales with disk
metallicity, in agreement with the observed Jovian planet frequency-metallicity
relation. We consider both X-ray and cosmic ray disk ionization models, whose
differing ionization rates lead to different dead zone trap locations. In both
cases, Jovian planets form in our model out to 2-3 AU, with a distribution at
smaller radii dependent on the disk ionization source and the setting of
envelope opacity. We find that low values of $\kappa_{\rm{env}}$ (0.001-0.002
cm$^2$ g$^{-1}$) and X-ray disk ionization are necessary to obtain a separation
between hot Jupiters near 0.1 AU, and warm Jupiters outside 0.6 AU, a feature
present in the data. Our model also produces a large number of super Earths,
but the majority are outside of 2 AU. As our model assumes a constant dust to
gas ratio, we suggest that radial dust evolution must be taken into account to
reproduce the observed super Earth population.
