Colder than expected

image credit: Digitized Sky Survey 2/NASA/ESA, inset: ESO/NASA/ESA The image to the left shows a circumstellar disk that is nick named "the flying saucer". The star in the center of the disk is not seen directly, because the thick dusty disk is exactly seen from the side (it is the dark line in the image) and it absorbs all the light that the star shines directly at us. The bright regions above and below that dark line is light that scatters off small dust grains above and below the disk. In addition to being so exactly edge-on, there is another peculiar arrangement: the disk is in front of a cloud of dust and gas. This special arrangement allowed us to directly measure the temperature of the dust using the ALMA telescope array.
It is commonly believed that dust in circumstellar disks, even far away from its central star, cannot be colder than about 15 degrees above absolute zero because there is always interstellar radiation that heats the dust particles to this minimum temperature. With this in mind, our measurements came as a surprise: we measured a dust temperature of only 7 degrees above absolute zero! There are some possible ways to explain this, but they all indicate that the dust grains have different shapes or different optical properties from what was commonly assumed.
Why does that matter? If we want to understand where planets can form and how large they can become, we need to know how much material is out there, in other words, we need to weight the disk. The methods to measure the masses of disks however rely on the knowledge of the dust temperature and other properties of the dust. Our measurements show that the dust seems to be quite different from what astronomers assumed it to be so far and they highlight why understanding the physics of those small dust particles is such an important part of understanding the origins of planets.

MPIA press release: English | German
ESO press release: English | German
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