On the gap-opening criterion of migrating planets in protoplanetary disks

TL;DR

This study uses hydrodynamical simulations to refine the understanding of gap-opening criteria for migrating planets in protoplanetary disks, emphasizing the importance of the timescale for gap formation relative to planetary crossing time.

Contribution

It introduces an additional timescale condition to the existing torque balance criterion, highlighting its significance for planet survival and migration in various disk environments.

Findings

Gap-opening depends on rapid enough formation, not just ability.

The timescale criterion affects planet survival in low and high mass disks.

Implications for observed transition disks and planet mass estimates.

Abstract

We perform two-dimensional hydrodynamical simulations to quantitatively explore the torque balance criterion for gap-opening (as formulated by Crida et al. 2006) in a variety of disks when considering a migrating planet. We find that even when the criterion is satisfied, there are instances when planets still do not open gaps. We stress that gap-opening is not only dependent on whether a planet has the ability to open a gap, but whether it can do so quickly enough. This can be expressed as an additional condition on the gap-opening timescale versus the crossing time, i.e. the time it takes the planet to cross the region which it is carving out. While this point has been briefly made in the previous literature, our results quantify it for a range of protoplanetary disk properties and planetary masses, demonstrating how crucial it is for gap-opening. This additional condition has important implications for the survival of planets formed by core accretion in low mass disks as well as giant planets or brown dwarfs formed by gravitational instability in massive disks. It is particularly important for planets with intermediate masses susceptible to Type III-like migration. For some observed transition disks or disks with gaps, we expect that estimates on the potential planet masses based on the torque balance gap-opening criterion alone may not be sufficient. With consideration of this additional timescale criterion theoretical studies may find a reduced planet survivability or that planets may migrate further inwards before opening a gap.