Gas in Planetary Systems

Planetary systems around main sequence stars were thought to be gas-free after the dispersal of primordial gas (via accretion onto the host star and photoevaporation) at the end of the protoplanetary disc phase (when giant planets have already formed). However, this picture has changed as we detect gas around a growing number of these evolved stars. My work aims to use these newly available gas observations around Kuiper-belt-like planetesimal belts (debris discs) to further our understanding of planetary systems.

Gas is created from exocomets in these extrasolar Kuiper belts and thus is unique in its ability to probe their composition via emission lines of key species. The conventional route using dust observations in debris disc systems to infer hidden components (like planets) in planetary systems is essential, but now that gas observations are available, it is timely to start using them to complement our knowledge of these systems.

I have started building the essential tools to study this new gas component and here I list a few questions that I am trying to answer to:

1) Do all planetary systems contain gas and can we detect it? 2) What is the origin of the gas that is observed? 3) What do gas observations around debris discs tell us about the magneto-rotational instability theory (see the MRI in debris disc section)? 4) Can we detect new planets in the inner regions of planetary systems using high-resolution observations of these gas discs? 5) Can the observed gas explain the ubiquity of hot dust observed around one third of stars (see here)? 6) What kind of new gaseous species can be detected using ALMA, the VLT or the HST?

 

The first paper that appears down below describes the new gas model I developed that can follow the dynamics and thermal state of gas with time. This model is the first to provide a self-consistent explanation for the gas observations in the famous beta Pic system and can be used to make predictions for future observations. Our model assumes that CO gas is released from volatile-rich exocomets in extrasolar Kuiper belts, which photodissociates rapidly into atomic carbon and oxygen. The atomic gas then evolves by viscous spreading resulting in an accretion disc inside the parent belt and a decretion disc outside. The ionisation fraction, temperature and population levels are followed with the PDR (photodissociation region) code Cloudy, which is coupled to a dynamical viscous model.

 

In the beta Pic paper, we show a new detection of OI with Herschel and a new observation of CI with APEX. These observations and the published CII and CO can all be explained within the frame of this new model if the viscosity (parameterised with alpha) in the gas disc is sufficiently high. We find that alpha is required to be greater than 0.1, which is similar to values found in sufficiently ionised discs of other astronomical objects. We propose that the magnetorotational instability (MRI) may be responsible for this high viscosity in this highly ionised medium (more details here). We also constrain the water content in beta Pic exocomets and the quantity of hydrogen in the gas disc (highly depleted compared to solar abundances) and its accretion rate onto the central star. The accretion rate we predict was recently confirmed with new HST/COS observations of the HI Lyman alpha line by Wilson et al. (2017).

The second paper that appears down below describes a new semi-analytical gas model I developed that can predict the amount of gas around a given debris disc. It makes predictions for observation of gas with ALMA and predicts that we could detect at least 30 new systems with gas using the CI 610 micron line on ALMA. We also, here, confirm that a secondary gas production model is a viable way to explain all gas observations so far except for 3 systems that are likely to be of primordial origin.

“That is where my dearest and brightest dreams have ranged — to hear for the duration of a heartbeat the universe and the totality of life in its mysterious, innate harmony.” Hermann Hesse