Outwards migration for planets in stellar irradiated 3D discs

TL;DR

This study presents 3D simulations of planet migration in stellar irradiated discs, showing that planets can migrate outwards in shadowed regions due to entropy gradients, with results aligning with analytical predictions.

Contribution

First 3D simulations of planet migration considering stellar irradiation, confirming analytical models and exploring migration regions for planets up to 60 Earth masses.

Findings

Outward migration occurs only in shadowed disc regions.

Numerical results agree qualitatively with analytical predictions.

Quantitative agreement for planets over 20 Earth masses.

Abstract

For the very first time we present 3D simulations of planets embedded in stellar irradiated discs. It is well known that thermal effects could reverse the direction of planetary migration from inwards to outwards, potentially saving planets in the inner, optically thick parts of the protoplanetary disc. When considering stellar irradiation in addition to viscous friction as a source of heating, the outer disc changes from a shadowed to a flared structure. Using a suited analytical formula it has been shown that in the flared part of the disc the migration is inwards; planets can migrate outwards only in shadowed regions of the disc, { because the radial gradient of entropy is stronger there}. In order to confirm this result numerically, we have computed the total torque acting on planets held on fixed orbits embedded in stellar irradiated 3D discs using the hydrodynamical code FARGOCA. We find qualitatively good agreement between the total torque obtained with numerical simulations and the one predicted by the analytical formula. For large masses (>20 Earth masses) we find quantitative agreement, and we obtain outwards migration regions for planets up to 60 Earth masses in the early stages of accretional discs. We find nevertheless that the agreement with the analytic formula is quite fortuitous because the formula underestimates the size of the horseshoe region; this error is compensated by imperfect estimates of other terms, most likely the cooling rate and saturation.