How Flow Isolation May Set the Mass Scale for Super-Earth Planets. (arXiv:1908.06991v2 [astro-ph.EP] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+Rosenthal_M/0/1/0/all/0/1">M. M. Rosenthal</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Murray_Clay_R/0/1/0/all/0/1">R. A. Murray-Clay</a>
Much recent work on planet formation has focused on the growth of planets by
accretion of grains whose aerodynamic properties make them marginally coupled
to the nebular gas, a theory commonly referred to as “pebble accretion”. While
pebble accretion can ameliorate some of the issues presented by growth by
purely gravitational processes, it has other issues when compared with
observations of exoplanetary systems. A particular concern is the preponderance
of planets that end their growth as “super-Earths” or “sub-Neptunes”, with
masses in the range 2-10 $M_oplus$. Once planets reach this mass scale,
timescales for growth by pebble accretion are so rapid that ubiquitously ending
growth here is difficult. In this work, we highlight this issue in detail using
our previously published model of pebble accretion, and also propose a possible
solution: feedback between the growing planet’s atmosphere and the gas disk
inhibits accretion of smaller particle sizes by forcing them to flow around the
growing planet instead of being accreted. For reasonable fiducial disk
parameters this “flow isolation” will inhibit accretion of all available
particle sizes once the planet reaches super-Earth masses. We also demonstrate
that the characteristics of this “flow isolation mass” agree with previously
published trends identified in the textit{Kepler} planets.
Much recent work on planet formation has focused on the growth of planets by
accretion of grains whose aerodynamic properties make them marginally coupled
to the nebular gas, a theory commonly referred to as “pebble accretion”. While
pebble accretion can ameliorate some of the issues presented by growth by
purely gravitational processes, it has other issues when compared with
observations of exoplanetary systems. A particular concern is the preponderance
of planets that end their growth as “super-Earths” or “sub-Neptunes”, with
masses in the range 2-10 $M_oplus$. Once planets reach this mass scale,
timescales for growth by pebble accretion are so rapid that ubiquitously ending
growth here is difficult. In this work, we highlight this issue in detail using
our previously published model of pebble accretion, and also propose a possible
solution: feedback between the growing planet’s atmosphere and the gas disk
inhibits accretion of smaller particle sizes by forcing them to flow around the
growing planet instead of being accreted. For reasonable fiducial disk
parameters this “flow isolation” will inhibit accretion of all available
particle sizes once the planet reaches super-Earth masses. We also demonstrate
that the characteristics of this “flow isolation mass” agree with previously
published trends identified in the textit{Kepler} planets.
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