Massive Discs around low-mass stars

Planets form within flattened discs of material around young stars. How these discs evolve and how they might form planets remains a mystery. In recent years exoplanet detections have shown planets around low-mass stars (stars lower in mass than about half the sun’s) are extremely common, even more common than around sun-like stars. How these discs’ retain sufficient mass to form these planetary systems is an open questions.

The problem of the requirement for massive discs around low-mass stars is personified by the famous Trappist-1 planetary system that boasts 7 rocky planets that all orbit their 0.1 solar mass star in periods less than a month. Standard models of the disc that formed the Trappist-1 planetary system suggest they should be so massive it was unstable and would have destroyed itself on a short-timescale. However, young stars are much brighter when they are first born, heating this surrounding disc, increasing its temperature and providing pressure support against the gravity that wants to destroy it.

TJ Haworth led a collaboration as part of the DiRAC project dp100, to use Smooth Particle Hydrodynamics simulations to show that the luminosity emitted by bright stars was strong enough to stabilise these discs, allowing them to retain the mass necessary to form the observed planetary systems.

Disc structures around low mass stars determined using SPH simulations. The left panel shows a smooth stable disc heated by a young low-mass star. The middle panel shows the disc structure if the low-mass star’s main sequence properties were used. This disc shows spiral structures indicative of gravitational instabilities that rapidly drain it of material. The right panel shows a disc around a low-mass star that is not emitting, this disc has destroyed itself within a few orbital periods.

References

Haworth et al. 2020 MNRAS 494 4130