Ultracool disk around young star contains dusty surprises

3. Februar 2016

While the circumstellar disk in question has been nicknamed "the Flying Saucer," what is most mysterious about it are not extraterrestrials, but tiny particles of cosmic dust. An unusual new measurement of the disk's dust temperature using the ALMA observatory has yielded surprisingly low values, a mere 7 degrees above absolute zero (7 K). The astronomers, including Dmitry Semenov of the Max Planck Institute for Astronomy, found that the only viable explanations involve unusual properties of the disk's dust grains. With these unusual properties, dust disks could be quite generally more massive than previously thought, with consequences for the types of planets that can be born in such disks.

Background information In-depth description of the results

The protoplanetary disk nicknamed the Flying Saucer not only looks unusual on astronomical images. When astronomers observed it with the ALMA observatory, it seemed to have a highly unusual property: at face value, the measurements showed that the astronomers were receiving a negative amount of radiation from the object. While that absurdity was readily explained by peculiarities of the ALMA measuring process, there is a serious background to the negative values: They show that, contrary to expectations, dust in this disk is colder than the molecular clouds in the background.

This is highly unusual in and of itself. After all, the disk is constantly warmed by the star in its center. For its dust to be this cold, a mere 7 degrees Celsius above absolute zero (= 7 Kelvin), the dust grains must have some unexpected properties.

When Stephane Guilloteau of the University of Bordeaux and Thomas Henning of the Max Planck Institute for Astronomy had first proposed these observations, they had indeed intended to measure dust temperatures in the disk. But they had never expected temperatures this low.

Dmitry Semenov of the Max Planck Institute for Astronomy, one of the study's authors, explains: "The usual models assume that dust grains are compact and spherical. But a disk made of such grains, around a Sun-like star, could never have temperatures that low. Hence, the low temperatures that have been measured point towards a more complex situation. The dust grains could be compact but non-spherical - for instance: they could be elongated, or be highly porous aggregates made of small compact grains. Alternatively, the central regions of the Flying Saucer could contain unusually large dust grains that are thermally decoupled from the surrounding gas, and remain very cold."

At present, there is not enough data to decide between those possible explanations. Further observations are planned – not least because of the ramifications: Protoplanetary disks are the birthplaces of planets, after all. Dust properties play a key role in the first steps of planet formation, where grains of dust clump together to form slightly larger compounds, setting off the sequence that ends with the formation of large planets. The surfaces of dust grains are miniature laboratories, the site of complex chemical reactions including those that form organic compounds, which in turn could prove crucial for the formation of life on one of the system's planets.

Last but not least, a widespread method of estimating the total mass of such disks, most of which is in the form of molecular gas, relies on detecting radiation from the dust, estimating the amount of dust present, and using a conversion factor to estimate the gas mass. This conversion, too, might need to be modified when the dust turns out to have unusual properties – altering estimates of disk mass and thus of what planets that can form in a particular disk.

All good reasons to know your dust if you're interested in planet formation – and when it comes to that, the recent Flying Saucer measurements provide a wake-up call, showing that dust could well be less simple than commonly assumed.

Background information

The results described here have been published as S. Guilloteau et al., "The shadow of the Flying Saucer: A very low temperature for large dust grains" in Astronomy & Astrophysics Letters.

An in-depth description of the results can be found here.

The MPIA researchers involved are

Dmitry Semenov, Thomas Henning, and Til Birnstiel

in collaboration with

S. Guilloteau (Université de Bordeaux/CNRS, Floirac), V. Piétu (IRAM, Saint Martin d’Hères), E. Chapillon (Université de Bordeaux/CNRS; IRAM), E. Di and A. Dutrey (beide University of Bordeaux/CNRS), and N. Grosso (Observatoire Astronomique de Strasbourg).

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