Title: Improved torque formula for low and intermediate mass planetary migration,
Abstract: The migration of planets on nearly circular, non-inclined orbits in
protoplanetary discs is entirely described by the disc's torque. This torque is
a complex function of the disc parameters, and essentially amounts to the sum
of two components: the Lindblad torque and the corotation torque. Known torque
formulae do not reproduce accurately the torque actually experienced in
numerical simulations by low- and intermediate- mass planets in radiative
discs. One of the main reasons for this inaccuracy is that these formulae have
been worked out in two-dimensional analyses. Here we revisit the torque formula
and update many of its dimensionless coefficients by means of tailored, three-
dimensional numerical simulations. In particular, we derive the dependence of
the Lindblad torque on the temperature gradient, the dependence of the
corotation torque on the radial entropy gradient (and work out a suitable
expression of this gradient in a three-dimensional disc). We also work out the
dependence of the corotation torque on the radial temperature gradient,
overlooked so far. Corotation torques are known to scale very steeply with the
width of the horseshoe region. We extend the expression of this width to the
domain of intermediate mass planets, so that our updated torque formula remains
valid for planets up to typically several tens of Earth masses, provided these
relatively massive planets do not significantly deplete their coorbital region.
Our torque expression can be applied to low- and intermediate-mass planets in
optically thick protoplanetary discs, as well as protomoons embedded in
circumplanetary discs.