Observing planetary gaps in the gas of debris discs

C. Bergez-Casalou & Q. Kral

Oct. 2024 - accepted in A&A - arxiv:2411.14241

Paper Abstract

Recent ALMA observations discovered consequent amounts (i.e., up to a few 0.1 M_Earth) of CO gas in debris disks that were expected to be gas-free. This gas is in general estimated to be mostly composed of CO, C, and O (i.e., H2-poor), unlike the gas present in protoplanetary disks (H2-rich). At this stage, the majority of planet formation already occurred, and giant planets might be evolving in these disks. While planets have been directly observed in debris disks (e.g., β Pictoris), their direct observations are challenging due to the weak luminosity of the planets. In this paper, with the help of hydrodynamical simulations (with FARGO3D) coupled with a radiative transfer code (RADMC-3D) and an observing tool (CASA), we show that planet-gas interactions can produce observable substructures in this late debris disk stage. While it is tricky to observe gaps in the CO emission of protoplanetary disks, the unique properties of the gaseous debris disks allow us to observe planetary gaps in the gas. Depending on the total mass of the gaseous debris disk, kinks can also be observed. We derive a simple criterion to estimate in which conditions gaps would be observable and apply it to the known gaseous debris disk surrounding HD138813. In our framework, we find that planets as small as 0.5 MJ can produce observable gaps and investigate under which conditions (i.e., gas and planets characteristics) the substructure become observable with ALMA. The first observations of planet-gas interactions in debris disks can lead to a new way to indirectly detect exoplanets, reaching a population that could not be probed before, such as giant planets that are too cold to be detected by direct imaging.

Context and setup

Protoplanetary discs have a lifetime of roughly 10 Myrs, during which the gas present in the system is removed by various processes (stellar and planetary accretion, photo-evaporation, winds etc...). The resulting system is composed of the newborn star, newborn planets and the remaining planetesimals and dust. These last two can form discs, equivalent to the Kuiper or asteroid belts in our Solar System, which are called debris discs. Orbiting stars which are older than a few Myrs, these discs where expected to be gas free until the recent observations of consistent amounts of gas, a little more than 10 years ago. The detection of carbon monoxide (CO), neutral and ionized carbon (CI, CII) gases in these old systems expected to be in their debris disc phase raises the question of their origin and interaction with the planets.

Evolution of a protoplanetary disc
Properties of the observed gaseous debris discs


In this study, we focus on the observability of these interactions between the planets and this gas which properties are different to the gas in the protoplanetary disc phase: colder, enhanced in heavy elements (mostly composed of CO) and around 10 to 100 times less massive. With the help of hydrodynamical simulations to study planet/gas interactions coupled to a radiative transfer code and an observing simulation tool, we show that depending on the mass of the gas disc, these interactions are observable. Even better, some interactions that are not observable in the protoplanetary disc phase due to their massive gas disc become observable in the debris disc phase.

Initially: The discs are initialized with characteristics corresponding to the observations regarding total mass, extent, distance etc... A planet of a given mass (0.5, 1 or 5 Mj) is then slowly introduced in the simulation at different distances (10, 50 or 100 AU). Once the discs reach a quasi-steady state, we stop the simulation and use the resulting 2D gas distribution to estimate the emission of the gas. With the help of a radiative transfer code, we estimate the emission of the 12CO(J=2-1) transition in different velocity channels.

Output: The 2D emission maps are coupled to an observing simulation tool able to simulate what ALMA would see if it was observing such a disc in a given configuration. Regarding the properties that we are looking for, we focus on the C6 configuration corresponding to a spatial resolution of 0.13". Realistic noise is added to the image, setting whether or not planetary substructures are observable.




Main results

In intermediate mass discs (~ 0.001 Earth Mass of CO), planetary gaps are observed. Unlike in the protoplanetary disc phase, the gas present inside the gap is not luminous in debris disc compare to the gas outside the gap. On the other hand, in the most massive debris discs, the amount of gas is similar to the protoplanetary disc phase: here, the gas inside the gap is luminous and another substructure induced by the planet becomes visible, the kink. Originating from the velocity deviation induced by the planet's spiral wake, the kink is a deviation from the lobe-shape emission expected for an unperturbed disc in Keplerian rotation.

Mass ratios for different alpha viscosities and different initial configuration

The observability of the substructures (whether the gap or the kink) is dependent on the inclination of the disc relatively to the observer. The gap observability is better when the disc is face-on (i=0°) and is hard to distinguish only for high inclinations (i>70°). On the other hand, the kinks are easily distinguishable only for inclinations around 30° (red circles on the Figure below).

Moment 0 images and channel maps for discs with different inclinations


The gap observability can be estimated from the planet's characteristics. The properties of the gap (width and depth) depend mostly on the planet mass and distance from the star and on the disc's viscosity. In the paper, we show that , as it is possible to determine the gap depth and width from known criteria (e.g., Crida et al. 2006 or Kanagawa et al. 2015), we can derive an observability criterion depending on the properties of the gas inside the gap. The idea is simple: if the flux of the gas inside the gap is below the sensitivity threshold of a given instrument (here ALMA), then the gap will be observable. However, this relies on the assumption that the gas outside the gap is luminous enough (its flux is above the sensitivity threshold) and the gap width is resolved by the instrument (i.e., the gap width is covered by one or more beams).

Main conclusions

Planet/gas interactions in debris discs : planets embedded in debris discs produce observable imprints. Our study show that we can extend our study of planet gas interactions to gaseous debris discs and not only protoplanetary discs, providing us with more opportunities to observe such interactions.

Observing planetary induced gaps in the gas : thanks to the unique properties of the gas in debris discs, we are able to observe planetary induced gaps in the gas distribution of a circumstellar disc. Hardly observed in protoplanetary discs, such observations can help better constrain our models on planet formation.

Planet/gas interactions at a later stage : observing planetary gaps in gaseous debris discs provides us a new indirect way to detect exoplanets at an intermediate stage during their evolution: they are no longer in their protoplanetary disc where they formed, but they are still in young systems that can undergo extrem dynamical events.

© Camille Bergez-Casalou 2020 - Last update Dec. 2024