Researchers using the Atacama Large Millimeter/submillimeter Array (ALMA) have, for the first time, achieved a precise size measurement of small dust particles around a young star through radio-wave polarisation. ALMA’s high sensitivity for detecting polarised radio waves made possible this important step in tracing the formation of planets around young stars.
Astronomers have believed that planets are formed from gas and dust particles, although the details of the process have been veiled. One of the major enigmas is how dust particles as small as 1 micrometre aggregate to form a rocky planet with a diameter of 10 thousand kilometres (6213 miles). Difficulty in measuring the size of dust particles has prevented astronomers from tracing the process of dust growth.
Akimasa Kataoka, a Humboldt Research Fellow stationed at Heidelberg University and the National Astronomical Observatory of Japan, tackled this problem. He and his collaborators have theoretically predicted that, around a young star, radio waves scattered by the dust particles should carry unique polarisation features. He also noticed that the intensity of polarised emissions allows us to estimate the size of dust particles far better than other methods.
To test their prediction, the team led by Kataoka observed the young star HD 142527 with ALMA and discovered, for the first time, the unique polarisation pattern in the dust disc around the star. As predicted, the polarisation has a radial direction in most parts of the disc, but at the edge of the disc, the direction is flipped perpendicular to the radial direction.
Comparing the observed intensity of the polarised emissions with the theoretical prediction, they determined that the size of the dust particles is at most 150 micrometres. This is the first estimation of the dust size based on polarisation. Surprisingly, this estimated size is more than 10 times smaller than previously thought.
“In the previous studies, astronomers have estimated the size based on radio emissions assuming hypothetical spherical dust particles,” explains Kataoka. “We observed the scattered radio waves through polarisation, which carries independent information from the thermal dust emission. Such a big difference in the estimated size of dust particles implies that the previous assumption might be wrong.”
The team’s idea to solve this inconsistency is to consider fluffy, complex-shaped dust particles, not simple spherical dust. In the macroscopic view, such particles are indeed large, but in the microscopic view, each small part of a large dust particle scatters radio waves and produces unique polarisation features. According to the present study, astronomers obtain these “microscopic” features through polarisation observations. This idea might prompt astronomers to reconsider the previous interpretation of observational data.
“The polarisation fraction of radio waves from the dust disc around HD 142527 is only a few percent. Thanks to ALMA’s high sensitivity, we have detected such a tiny signal to derive information about the size and shape of the dust particles,” says Kataoka. “This is the very first step in the research on dust evolution with polarimetry, and I believe the future progress will be full of excitement.”