Fragmentation favoured in discs around higher mass stars

In work led by James Cadman, we investigate how a protoplanetary disc’s susceptibility to gravitational instability and fragmentation depends on the mass of its host star, finding that discs become increasingly prone to the effects of self-gravity with increasing stellar mass.

 There currently exists no single consistent theory of planet formation. Core accretion theory is able to explain the formation of the less massive terrestrial planets, but struggles to explain how giant planets and brown dwarfs form, as their core accretion formation timescales often exceed typical disc lifetimes. Instead, these massive planets may form rapidly through disc fragmentation; the direct gravitational collapse of disc material, acting to form giant, gaseous bodies. Planet formation through the gravitational instability will only occur whilst the discs are massive, hence whilst still very young, and it is commonly thought that there may be a critical disc-to-star mass ratio above which all discs will become prone to fragmentation.

Recent observations have found that higher mass stars (M_* > 1.5 M_Sun) host a greater frequency of wide-orbit giant planets and brown dwarfs, whilst lower mass stars such as Trappist-1 have been found with as many as 7 terrestrial planets in orbit, both indicating that the gravitational instability may in fact have a stellar mass dependence.

In work led by James Cadman, we investigate how a protoplanetary disc’s susceptibility to gravitational instability and fragmentation depends on the mass of its host star, finding that discs become increasingly prone to the effects of self-gravity with increasing stellar mass. We find that low-mass stars may support high disc-to-star mass ratios without deviating from axisymmetry, whilst discs around ~2 Solar-mass (M_Sun) stars may be prone to fragmentation, which will tend to form objects of initially super-Jupiter and brown dwarf masses. We use 1D disc models, together with 3D SPH simulations, to determine the critical disc-to-star mass ratios at which discs become unstable to fragmentation, exploring stellar masses in the range 0.25-2.0 M_Sun and modelling stellar irradiation in both the optically thin and optically thick limits. Higher disc temperatures act to suppress gravitational instability whilst also pushing up initial planet masses in those discs that do fragment.

The gravitational instability’s stellar mass dependence is demonstrated in the figure on the right. We illustrate that for the same disc radius and disc-to-star mass ratio, only the discs around more massive stars (M_* = 1 M_Sun and M_* = 2M_Sun) fragment, whilst the disc around the less massive star (M_* = 0.25 M_Sun) remains entirely gravitationally stable.

The consequences of this work are twofold. Low-mass stars could in principle support high disc-to-star mass ratios, potentially providing large mass reservoirs for the formation of Trappist-1-like systems. Higher mass stars have discs that are more prone to fragmentation, which is qualitatively consistent with observations that favour high-mass wide-orbit planet and brown dwarf companions around higher mass stars.

Links:

Cadman, J., Rice, K., et al., 2020, Fragmentation favoured in discs around higher mass stars, Monthly Notices of the Royal Astronomical Society, 492, 5041

Haworth, T., Cadman, J., et al., 2020, Massive discs around low-mass stars, Monthly Notices of the Royal Astronomical Society, submitted.