Authors: Nienke van der Marel, Ruobing Dong, James di Francesco, Jonathan P. Williams, John Tobin
First Author Institution: Herzberg Astronomy & Astrophysics Programs, National Research Council of Canada
Status: Accepted to ApJ, open access
If you’re well-versed in exoplanets (or even the formation of your own planet), you may have heard the term protoplanetary disk. Protoplanetary disks are disks of gas and dust surrounding a fairly newborn star, although newborn here means up to several million years old. Interestingly, images of protoplanetary disks that astronomers have captured reveal gaps in the disks, or rather separate rings of material, depending on your perspective. These types of disks were once expected to be completely smooth, so why are we seeing gap-like features in essentially all resolved images of them?
Figure 1: Left – Continuum emission of the disk sample. The beam size is shown in the lower left corner with the spectral type of each star in the right. HL Tau, TW Hya and V1247 Ori, which had higher resolution, have been reduced to reflect the 20 AU beam size of the other images in order to make them comparable.
Figure 1: Right – Enhanced image representations for each disk, which were used in the analyses – gaps have been made easier to see using a variety of unsharp masking. Transition disks are RXJ1615, AATau, DMTau, V1247Ori, HD97048, HD100546,TWHya, HD169142, HD135344B.
The authors of today’s paper use ALMA data to explore what could be causing these gaps. They examine images of 16 different protoplanetary (see Figure 1) and transition disks (disks where the material closest to the star has been cleared out). The sample contains stars of various spectral types, each exhibiting multiple gap features. Since these gaps are present throughout the sample, some correlation between them should reveal the responsible mechanism, right? After all, we would expect that these features evolve in similar ways for most disk systems.
Before they can answer this question, the authors first determine each star + disk’s luminosity and use this information to age each star with model evolutionary tracks. The resulting age range gave them a way to classify their disks as older or younger. They also determined approximate gap locations and sizes via a type of intensity profile fitting, which essentially models the light coming from the star + disk in each image (where there is a gap, there is less light detected, etc.).
Figure 2: Gap properties across ages, each red dot representing a gap in the corresponding disk. The gap center radii is raised to the power 3/2, to mimic orbital resonance ratios
Searching for Answers
With this information, the authors searched for trends in
the data – any correlations between the stars’ properties and the disk properties that may point to an origin story. Figures 2 and 3 show these chosen parameters… as well as the obvious dearth of trends. The only visible trend seems to be the decrease in outermost ring radius for the four oldest stars when compared to the rest of the sample (see Figure 3, bottom). This sure does make it hard to imagine a common mechanism responsible for the gaps.
Two of the most common theories for gaps within disks are 1) planets and 2) stuff freezing. Certain compounds are present in the disk, and there should be a point away from the star where the temperature becomes just right for those compounds to freeze into ice crystals. This is called the snow (or frost) line. At that snow line, it is thought that as these crystals all form together, they may stick to each other and the disk’s dust, which could clear out some space and turn into the gaps we see.
The authors looked to 5 different molecules in order to test this hypothesis: N2, CO, CH4, CO2, and NH3. This is the ‘stuff’ we were talking about freezing. They determine the location of each respective snow line for each star and find that these locations don’t seem to have anything to do with the gaps – there are a few instances where the gap and snow line overlap, but it does not seem to be a systematic trend. So it seems the snow line scenario is a no-go.
Planetary formation is another story. It is thought that as planetesimals accrete surrounding disk material, a gap forms as it carves out its orbit. The authors state that they simply don’t have enough information to disprove or support this theory. Neptune-mass planets are currently undetectable and only one disk (HD 163296) in the sample has been suspected of having a planet. They do note however that one model of planet formation (cold-start model) would allow for giant planet formation at the location of many of the observed gaps. So, planet formation is still a likely culprit.
In case you were wondering, authors also ponder the case of graviational instability. Gravitational instabilities in the disks could potentially cause the gaps, but this depends on the gas-to-dust mass ratio. Unfortunately this is very much uncertain and hard to measure, so the trail stops there.
So What Do We Know?
Figure 3: Gap properties across ages and luminosities, each red dot representing a gap.
The only solid correlation present in the sample is that the outer disk ring is much closer in for the four oldest stars, aka, the older disks are smaller in diameter. In that case, it seems we are missing some outer rings. So maybe the outer rings dissipate faster than the inner rings (due to drag forces or radiation pressure). Either that, or these outer dust grains accrete to become planetesimal sized and therefore undetectable at the observed wavelengths.
Although the authors didn’t find the origin story they were looking for, they can say a few things with certainty. The lack of trends in their data show that disk gaps are diverse and their presence is largely independent of stellar properties, like spectral type or age. They also found that snow lines don’t have anything to do with the gaps we observe, but planets very well might. And last but not least, transition disks seem to host these features in the same manner as the truly protoplanetary disks, implying that they evolve in the same way, even if we don’t know what that way is. This is actually quite a big step in the right direction. These clues get astronomers one step closer to to closing the gap on … gaps.