"Planet-forming disks accrete material onto their central star. For stars somewhat more massive than our sun, internal mixing processes are slow and the freshly accreted material dominates the surface composition of the star, where its composition is easier to measure. The principle is depicted in the illustration accompanying this report. We developed the theoretical foundations of this ""CAM"" method and described the main applications (Jermyn & Kama 2018, MNRAS). In the first application paper (Kama et al. 2019, ApJ, in press), we used the new method to measure the fraction of sulfur atoms locked in rocky solids in protoplanetary disks. CAM now allows very precise quantitative measurements of the abundance and refractory fraction of nearly twenty individual elements in the disk regions where the vast majority of planets are later found.
Significant new data was acquired during the project to deepen and broaden our knowledge of the composition of gas and solids in planet-forming disks. Observing projects were carried out on the sub-millimetre ALMA and APEX telescopes to map carbon, oxygen, sulfur, and other elements in the gas phase on scales of tens to hundreds of astronomical units in several planet-forming disks. A separate effort was focussed on obtaining new stellar spectra for analysis with the CAM-method. We built up a dataset of 16 previously unstudied stars, chosen to maximise their scientific impact in terms of using CAM to characterise the composition of protoplanetary or debris disk material.
The chemical element content of protoplanetary disks is generally measured relative to the total amount of mass present. A molecule closely linked to the dominant but invisible H2, hydrogen deuteride (HD), can provide a very robust total mass measurement. In DISCO, we were able to apply a fresh perspective to archival data obtained before 2013 to constrain the total mass in 15 disks (Kama et al. submitted), most notably HD 163296 where five giant planets have recently been discovered. We can now connect them to the mass reservoir from which they are being born.
A major part of DISCO was focussed on measuring and understanding the carbon and oxygen abundance in a sufficient number of planet-forming regions to allow for general conclusions to be drawn. As a result of this work, we find that planet-forming disks fall into two clearly distinct chemical categories. These results make testable predictions for the chemical composition of planets around specific types of stars, and will be submitted for publication later this year. Predictions from this work can be tested with exoplanet composition measurements from the James Webb Space Telescope once it launches, and from other forthcoming space telescopes. We are on the path to a deeper understanding of the origins of planetary chemical composition, and to unlocking the full scientific potential of major space missions targeting exoplanets.
In addition to the specialist publications already mentioned, DISCO science was communicated at two public talks at the Institute of Astronomy, and at lectures at the Science Society and the Postdoctoral Society at Trinity College. In connection to astrochemistry, planets and life in the Universe, the PI was interviewed for a popular science programme on Estonian national television."
