BSc project: An investigation into the correlation between CO and H2O emission in protoplanetary disks

The correlation between the CO P12 and H20 emission at 29.85 micron gives a clue to the the geometry of the protoplanetery disk itself.

As for many students, the bachelor’s project was my first taste of what some would call ‘real’ science. While the subject of protoplanetery disks sounds quite interesting, it wasn’t my preferred subject for a BSc project. Actually, I was simply a bit late in finding a subject. Fortunately Prof. Dr. Inga Kamp had a project ready to be tackled by a new student and, without much time left, I decided that it could be fun.

I still remember how much time it took to read and (somewhat) understand a scientific publication. After reading a small set of papers by Salyk et al. (2011) and Pontoppidan et al. (2010) I slowly started to understand that protoplanetary disks, the birthplaces of planets around newly formed stars, are a forest of mid infrared emission lines. These lines trace the various elements that are present in these systems and some of them appeared to be correlated, suggesting a common origin. The question I was to investigate was whether there existed such a correlation between water and carbon monoxide emission lines.

As is typically done, I first had to understand the physical processes underlying these emission lines. What exactly caused these water and carbon monoxide moluces to emit mid infrared light?
Electrons orbiting the nucleus of an atom can only exist at given energy levels, or quantum levels. Besides the quite well known electronic transitions for single atoms, these energy levels are also applicable to certain rotational and vibrational states for molecules. Since the transition energy from one rotational or vibrational state to the other is generally quite small, the released energy can be seen as infrared light.

Carbon monoxide is a relatively simple molecule of only two atoms. It should therefore be of no surprise that water, with three different atoms, has many more rotational and vibrational modes. This leads to many emission lines with similar (but not the same!) wavelengths. Due to the limited resolution of space based telescopes and because this emission is completely blocked by the atmosphere, making observations from the ground impossible, Salyk and his colleagues decided to treat a whole blend of emission lines as a single entity.

As I just mentioned, the detection of water in protoplanetary disks is difficult and only a few detections had been made at the time of my research into this topic. The Protoplanetary Disk Model (ProDiMo) simulation code provided a solution to this problem. In this simulation, developed by Woitke et al. (2009) and applied by Hein-Bertelsen (2015), you can give a set of geometry parameters and then the physical properties, including emission line fluxes, are returned.


The H2O blend fluxes plotted against the CO line fluxes in units of 10^−14 erg cm^−2 s^−1. T-Tauri stars are represented by blue dots, Herbig stars are represented by red dots and the model fluxes are green. Triangles represent upper limits in the direction they are pointing. Grey ‘+’s represent systems for which both CO and H2O fluxes are upper limits.
Based on the spread of the data points, a linear correlation is found.

Now came the time to start doing something myself. With the data I was handed I started to select the emission lines that we were interested in and plotted the water components against the CO components.
From the figure above we can see a that there is definitely a clustering of sorts for the different type of stars. Once a best fit was applied through these data points it was clear that the coefficient of the fits was similar and there definitively seemed to be a correlation!

Further analysis on the data then showed that the fluxes of the emission lines for different disk geometries tended to cluster together. This is also seen in the figure at the top of this page, which strengthens the suspicion of a correlation even more.

At this point of my BSc research project, the time was almost up and concluded here. Of course, the question of where this correlation was actually originating was still open. Possibly someone has found the answer to this by now. But that has been something that I did not follow up on.

For those of you that are interested in reading the full thesis: Download

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