A story about a haunted protoplanetary disk…

Title: Sulphur monoxide emission tracing an embedded planet in the HD 100546 protoplanetary disk

Authors: Alice S. Booth, John D. Ilee, Catherine Walsh, Mihkel Kama, Luke Keyte, Ewine F. van Dishoeck, Hideko Nomura

First Author’s Institution: Leiden Observatory

Paper Status: Accepted for publication in Astronomy & Astrophysics [open access]

Note from the author of the bite: haunted disks and ghost planets are not actual astronomical terminology. This terminology was made up to go along with the spooky theme of the bite

We received a call that the protoplanetary disk HD 100546 is haunted – it is haunted by a ghost planet, HD 100546 c. 

The authors of today’s paper found evidence for the ghost planet by detecting sulphur monoxide in the haunted disk. Let’s investigate.

Before we dive in, let’s remind ourselves what ghost planets are. Ghost planets are the planets that we cannot see but we suspect are there due to indirect evidence. The most famous investigating tool for haunted disks is ALMA (Atacama Large Millimeter Array). ALMA revolutionized our search for ghost planets by giving us data to investigate the haunted protoplanetary disks. 

Though we cannot observe planet formation in real time, ghost planets want us to know that they are out there – they give us hints through their motion and their chemical signatures. Using ALMA data, for example, we can detect kinematic structures in protoplanetary disks (e.g., gaps or spiral arms) that can hint to a planet in the disk – this was how the planet PDS 70 c was found! Planet formation can affect the disk’s chemistry. We know that haunted disks are inhabited by gas (mostly hydrogen and helium) and dust (usually small, solid silicates). If we look at the chemical composition of the haunted disks, we can link it to the chemistry of comets and planetesimals, and detect a ghost planet hiding there waiting for us to find it. 

So, the authors of today’s paper are interested in ​​understanding how to distinguish between the molecules that might be tracing the presence of ghost planets in disks and the intricate disk chemistry that is happening in the background. They observe that sulphur monoxide (SO) is a major tracer associated with the way ghost planets can affect the gas in the disks. 

Astronomers already know that HD 100546 hosts a giant planet named HD 100546 b, but the planet that this paper is interested in is HD 100546 c (HD 100546 b and HD 100546 c are parts of the same system, HD 100546). The HD 100546 disk’s SO emission is restricted to two rings. The authors think that the chemical SO and H2CO (formaldehyde) abundances are similar to what was found in AB Aur disk. They notice that SO is primarily located in the region called the inner disk (the region of the disk that’s closer to the star) and it seems to be less abundant in the outer edge of the disk (further from the star). They notice an asymmetry in brightness in the spatial distribution of SO emission (one side is brighter than the other), but are unclear on where this asymmetry might have come from. One explanation could be that the disk is warped. It is suggested that SO is a good tracer for warps in disks, but there is no other direct evidence for that. Usually, passing stars or gas clouds can cause warps in protoplanetary disks. However, another explanation for this asymmetry can be an already existing planet (HD 100546 c) that is trying to declare its existence. 

SO is also suggested to be a good tracer for shocks – discontinuities in the gas that arise when the gas flow becomes greater than the speed of sound. Therefore, if there is a clear abundance of SO in the inner disk this might imply that there is an accretion shock (when the gas gets accreted so rapidly that its speed exceeds the speed of sound and it creates shocks) there around a potential giant planet. The peak of the SO emission is spatially associated with the potential location of the planet HD 100546 c. On Figure 1, the authors are showing where the planet might be hiding in the disk. The contours show the SO emission, and the white star mark shows the CO enhancement. The location of the planet is, however, not really consistent among different groups of astronomers. One group suggests that the planet is located where the purple circle is, and another group suggests that the planet is located where the green circle is, because these two are completely two different sets of observations. So, there’s not a clear agreement where the planet might be located but there’s a ton of evidence that it should be there! Forming planets in disks has been demonstrated in simulations to heat the nearby disk environment, which would change the observable disk chemistry. The authors also noticed that other tracers, like H2S and CS, exhibit increases in abundance and line strength, which also leads to a potential planet there. 

Figure 1: The image of the disk HD 100546 with the location of the potential planet HD 100546 c. Two groups suggest two different locations for this planet – the purple circle and the green circle. The contours show the SO emission, and the white star mark shows the CO emission. Figure 5 in the paper.

The authors argue that the most feasible explanation for their data is that HD 100546 c might have increased the amount of SO in the disk as a whole. More precisely, they analyzed the data from observations that are 8.5 years apart from each other. If the planet is there, it is probably located at about 10 AU from the star, and 8.5 Earth years for this planet would be 1/5th of its orbit, which can explain the inconsistency in the SO spectra between the two rounds of observations. On Figure 2, Cycle 0 and Cycle 7 are the two rounds of different observations discussed above. The peaks in the two graphs correspond to potential locations of the planet, but, , as you can see on the figure, they do not coincide. This implies the anti-clockwise orbit of the potential planet in the disk.

Figure 2: Spectra of SO lines extracted from the disk from two different observation rounds – Cycle 0 and Cycle 7. These observations are 8.5 years apart. The peaks indicate a potential planet there. Because the peaks do not coincide, this might imply that the planet has moved in these 8.5 years. The x-axis shows the relative velocity, and the y-axis shows the flux (the light). Figure 2 from the paper.

Though we cannot see the planet HD 100536 c directly, it clearly wants to communicate with us, thus all the indirect evidence! There’s still work that needs to be done, but we’ll certainly continue investigating haunted disks! Happy ghost hunting!

Astrobite is edited by Ryan Golant

Featured image credit: NASA / Jet Propulsion Laboratory – Caltech

About Sabina Sagynbayeva

I'm a graduate student at Stony Brook University and my main research area is planet formation. I'm currently working on planetary migration using hydrodynamical simulations. I'm also interested in protoplanetary disks but nearly any topic related to planets is fascinating to me! In addition to doing research, I'm also a singer-songwriter. I LOVE writing songs, and you can find them on any streaming platform.

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