Dissipation timescale of dust in protoplanetary disks revealed with Japanese satellite AKARI

Dec. 15, 2021 | GATEWAY to Academic Articles

MAESHIMA Hiroshi
At the time: Dept. of Physics, Graduate School of Science, The University of Tokyo/ISAS

Protoplanetary disks are circumstellar disks that surround young stars. The disks consist of dust and gas, which are the raw materials for creating planetary systems. Thus, the dissipation timescale for the disks is essential to study planet formation. However, the timescale for the outer region of these disks (about 10 au from the central star, where 1 au is the average distance between the Earth and Sun) has not been revealed observationally due to the relatively low sensitivity of detectors. From all-sky data from the Japanese infrared astronomical satellite AKARI, and NASA’s Wide-field Infrared Survey Explorer (WISE), we obtained a number of infrared images of young stars previously detected at optical wavelengths. We estimate the decay timescale of the infrared radiation emitted from the dust in the disks surrounding these stars by making averaged infrared images, including those for stars that could not be resolved individually.

Our result shows that the dust in the disk middle regions (about 1 au) and the outer regions (approximately 10 au) dissipates on almost the same timescales, contrary to the expectation of inside-out disk clearing, where the middle region would dissipate first. Moreover, we reveal that the dominant dust changes from primordial dust, which has existed since the disk formation, to secondary dust, which is formed by collisional destruction of small planets (planetesimals) in the disks on the timescales of about 8 million years.

Research Summary

Protoplanetary disks are circumstellar disks that surround young stars. The disk consists of gas and dust*1, which are the raw materials of planetary systems. Because the gas and dust in disks dissipate with the evolution of the central star, the study of the dissipation process in each disk region is essential for understanding how planet formation occurs; a process that must be mainly complete before dissipation of the disk. Although previous studies discuss the dissipation timescale in the inner-middle region (less than about 1 au*2 from the central star), the timescale in the outer region (about 10 au) has not been researched due to the relatively low sensitivity of detectors that make this region difficult to see. Also, it is known that the dominant dust type must change from primordial dust, which has existed since the disk formation, to secondary dust, which is formed through collisional destruction of small planets (planetesimals) as the disk evolves. However, the timescale for this change has also been unclear.

Fig. 1 Schematic view of a protoplanetary disk. The closer region to the central star has a higher temperature and emits infrared radiation with a shorter-wavelength peak. In particular, the far-infrared light (about 100 µm) observed by AKARI is mainly emitted from the outer region (about 10 au), while the mid-infrared light (about 10 µm) observed by WISE is from the middle region (about 1 au).

To reveal the average dissipation timescale, we need statistical sample of many disks. Currently, spatially resolved observations of dust disks is usually performed at sub-mm or mm wavelengths, but the observations of many disks are not practical because of the vast amounts of time required. On the other hand, the dust farther from the central star is cooler and emits infrared radiation with at a longer-wavelength peak (Fig. 1). Therefore, observations at several wavelengths in the infrared give information on the spatial distribution of dust at these distances. In this study, we obtain a number of infrared images of young stars from the all-sky images captured by the Japanese infrared astronomical satellite AKARI*3 and by the Wide-Infrared Survey Explorer (WISE)*4. However, evolved disks are difficult to detect due to their weak infrared emission. Therefore, we classified young stars into three groups according to their age and averaged the infrared images of the young stars, including images where the star itself cannot be directly seen, in the same age group. Because random noise can be reduced by the averaging process, we can obtain a more sensitive average infrared image than through imaging these systems individually. From the averaged images of three age groups, we estimated the decay timescales of the infrared intensities.

This study revealed two results:
(1) Dust in the middle and outer regions of the disks dissipates on almost the same timescale, about 1.4 million years, and there is no timescale difference in this region. The dust disks are traditionally considered to dissipate from inside to outside, but our result is contrary to this expectation. This discovery was possible because our study was able to give a quantitive timescale for the outer disk region.
(2) The dominant dust changes from primordial dust to secondary dust on the timescale of about 8 million years (Fig. 2). Revealing the transition of the dust sources is expected to lead to the clarification of the disk evolution.

Fig. 2 Comparison between the average disk dust mass (red points) obtained by our study and the individually detected disk dust masses (blue and orange points) of previous studies (Maeshima et al. 2021, partially changed). The red line shows the fitted decay curve of the average dust mass. The black dotted line shows the fitted model curve of the secondary dust. The intersection of the red line and the black dotted line is the transition timescale of the dominant dust from primordial to secondary dust (about 8 million years).

This study reveals the dissipation timescales of dust in disks quantitatively. However, what physical process causes the disk dissipation is still under discussion. By revealing the constraint of the dissipation timescale, this result can aid theoretical studies of the disk evolution and planet formation process.

Terminologies

  • *1 dust : Small solid particles in space.
  • *2 au (astronomical unit) : A unit of length, roughly the distance from the earth to the sun (about 150 million kilometers).
  • *3 AKARI : Japanese infrared astronomical satellite launched in 2006. AKARI made all-sky surveys and spectroscopic observations in the infrared wavelength range. In particular, all-sky surveys were performed in six wavelength bands (9, 18, 65, 89, 140, and 160 µm).
  • *4 WISE (Wide-field Infrared Survey Explorer) : American infrared astronomical satellite launched in 2009. All-sky surveys were performed in four infrared bands (3.4, 4.3, 12, and 22 µm).

Information

Journal Title Publications of the Astronomical Society of Japan
Full title of the paper Dust dissipation timescales in the intermediate and outer regions of protoplanetary disks
DOI https://doi.org/10.1093/pasj/psab095
Publish date 28 October 2021
Author(s) Hiroshi Maeshima, Takao Nakagawa, Takuya Kojima, Satoshi Takita, Jungmi Kwon
ISAS or JAXA member(s) among author(s) Hiroshi Maeshima (Dept. of Space Astronomy and Astrophysics, ISAS / Dept. of Physics, Graduate School of Science, The University of Tokyo), Takao Nakagawa (Dept. of Space Astronomy and Astrophysics, ISAS), Takuya Kojima (Dept. of Space Astronomy and Astrophysics, ISAS / Dept. of Physics, Graduate School of Science, The University of Tokyo)

Links

Author

MAESHIMA Hiroshi
-2016 Department of Physics, Faculty of Science, The University of Tokyo
2016-2021 Department of Physics, Graduate School of Science, The University of Tokyo
2016-2021 Nakagawa Lab., Laboratory of Infrared Astrophysics, Department of Space Astronomy and Astrophysics, ISAS, JAXA
2021 Ph. D. The University of Tokyo
2021- A corporate employee