Recent papers, Episode 3

Double Feature on Transition Disks:

This one covers two papers on a similar topic:

Ring shaped dust accumulation in transition disks
P. Pinilla, M. Bensity, and T. Birnstiel, A&A (2012) vol. 545, A81

Can grain growth explain transition disks?
T. Birnstiel, S. Andrews, and B. Ercolano, A&A (2012) vol. 544, A79

So the obvious topic is "transition disks". Transition disk really means "disk with a large inner hole". About 99% of the mass of the disk is molecular (H2) gas which is mostly invisible and only 1% is dust. But it is the dust which is mostly observed and where inner cavities are found (wether the gas is depleted or not is not yet clear). These holes are detected either through imaging in the millimeter wavelength range or by looking at how much energy is emitted at different wavelength ranges. The latter one is called the SED (spectral energy distribution). This is what it schematically looks like:

Sketch of an SED

The big yellow spectrum is the one of the star which is dominating the emission, while the disk radiates at different, lower temperatures. At wavelengths longer than a few micrometers, the emission of the disk becomes dominant, this is the infrared excess. The total sum of the star+disk spectrum then composes the SED (black solid line).

Now what happens if you take away some disk material close to the star? Imagine you remove the orange spectrum which corresponds to the warm dust inside of couple of AU: then you get the dashed black line instead, so there is a dip in the SED. That is what people have looked into during the last two decades. But since a few years, there are powerful millimeter wave interferometers (e.g., the SMA and soon ALMA). These instruments have the sensitivity and angular resolution (and enough baselines) to make images at radio wavelengths, which show us how the dust is distributed in the disk. And indeed the dust clearing was also found in the images.

So what is the physics behind these signatures? The theory of accretion disks predicts a slow fading away of the disk, which is not what is observed. Some of the first suggestions were that the radiation from the star starts to evaporate the disk from the inside out. This does indeed work, but it turns out that some of these disks have holes which are much larger than what could be explained this way, and there are a few other problems with this idea. But thankfully, there are two more explanations floating around: these effects could be caused by the fact that dust grains grow and start to become invisible or what we see is the signature carved by a forming planet.

In these two papers, we investigated those two ideas. Concerning the effects of growing grains alone, we found that they can indeed produce an SED which would look like the one of a transition disk, however the images don't look like the observed mm images because the grains cannot grow large enough to be invisible to the mm observations.

However, the idea of a companion seems to work: a massive planet embedded in the disk gravitationally perturbs the disk which leads to a density bump outside of the orbit of the planet. As discussed in an earlier post, such a pressure bump can trap large particles which would otherwise spiral inward towards the star. The bump thus prevents them from reaching the inner disk, while the grains which are present in the inner disk continue to spiral inward - a hole is formed. We therefore think that a companion (planet or brown dwarf) is the most likely explanation of these observations - let's see what future observations will reveal!
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