Sahl Rowther
In recent years, observations of protoplanetary discs — the birthplace of planets around stars — have revealed that substructure in the form of rings and gaps, often assumed to result from planet-disc interactions, are common even in young discs. In their youth, protoplanetary discs are expected to be massive enough that spiral arms can form under the influence of the disc’s self-gravity. These are known as gravitationally unstable discs and are rarely observed. We investigate whether the rarity of gravitationally unstable protoplanetary discs can be explained by planet-disc interactions.
We show that a migrating giant planet strongly suppresses the spiral structure of gravitationally unstable discs. We present mock ALMA (Atacama Large Millimeter/submillimeter Array) continuum observations which show that a disc with a giant planet appears completely axisymmetric apart from the spiral arms of the planet. Whereas if the disc had evolved without a planet, large-scale spiral structures due to gravitational instability are easily seen. We show the planet’s influence on the disc structure is detectable in high resolution gas observations of optically thin CO-isotopologues.
The disc’s gas mass is usually inferred using the more easily observed dust muss via a fixed dust-gas mass ratio; canonically 1%. Sometimes, this results in a disc massive enough to be gravitationally unstable, which is expected to show large-scale spiral structures. In the absence of such structures, higher dust-gas mass ratios are assumed when modelling to ensure a less massive gravitationally stable disc. Our results show that it is unnecessary to limit the gas mass of discs by assuming high dust-gas mass ratios to explain a lack of spiral features that would otherwise be expected in massive discs. We show that a giant planet is able to suppress spiral structures resulting in massive discs appearing axisymmetric.
In the earliest stages of a protoplanetary disc’s evolution, their mass can be comparable to its host star. In such discs, the self-gravity of the disc plays an important role in driving its evolution resulting in spiral arms (left image). This is known as gravitational instability.
In recent years, many observations with the millimetre interferometer ALMA have revealed substructure in the form of rings and gaps in young discs. A common explanation for the formation of rings and gaps in discs are through planet-disc interactions. On the other hand, discs with large scale spiral features remain quite rare (right image).
Hence, the aim of this work is to study whether the observed rarity of large scale spiral structure due to young discs being less massive or can spiral structures in massive discs be hidden by planet-disc interactions?
Slide 2
The disc mass is not the only factor for gravitational instability. The disc temperature also plays an important role. There are two important criteria for a disc to be gravitationally unstable. Firstly, the Toomre stability parameter (a measure of the disc’s gravitationally instability) must be small. This is favoured for cold high mass discs. The second criteria is the speed at which the disc cools. If the cooling is too slow, the disc will not remain gravitationally unstable as it will be stabilised by internal heating due to turbulence from gravitational instabilities. Hence, it is important to model the disc thermodynamics realistically.
Traditionally, the cooling is modelled in a simplistic computationally inexpensive method such that the ratio of the cooling and orbital time is constant. However, this results in the entire disc becoming gravitationally unstable (left image) which is not expected in a realistic gravitationally unstable disc. In this work, the aforementioned ratio decreases with radius. This results in a disc which is only gravitationally unstable in the outer regions of the disc (right image), in line with expectations.
Slide 3
We show that a giant planet is able to suppress the spiral structure and stabilises the disc, yielding an axisymmetric disc. Whereas if the disc had evolved without a planet, the disc would have remained gravitationally unstable with large scale spiral structure (left image). This is also evident from the mock ALMA observations (right image).
This has important implications for observations. In some observations of axisymmetric discs, the observed dust mass can be high enough such that inferring the gas mass via a fixed dust-gas mass ratio results in a disc that is massive enough to be gravitationally unstable. But, a gravitationally unstable disc is expected to show spiral structure. Therefore in the absence of spiral structure, a higher dust-gas mass ratio is assumed when modelling the disc to ensure a less massive gravitationally stable disc. However this assumption is unnecessary as in the presence of a giant planet, we show that spiral structures expected from massive discs can be suppressed, thus appearing axisymmetric.
Slide 4
We show that deviations from the Keplerian flow of the disc (expected for a disc without a planet) as a result of the planet can be detected as a kink in high resolution gas observations of optically thin CO-isotopologues.
Summary
We show that the rarity of gravitationally unstable protoplanetary discs can be explained by planet-disc interactions. A giant planet shortens the gravitationally unstable phase, thus strongly suppressing the large-scale spiral structure resulting in an axisymmetric disc. We also show that high dust-gas mass ratios are not required to limit the gas mass of discs to explain a lack of large scale spiral structure.