2023.07.04
[Translated from the original Japanese https://alma-telescope.jp/news/spiralarm-202307?doing_wp_cron=1688489189.5287880897521972656250 ---njg]
Protostellar stage disks play an important role in the study of extrasolar planetary system formation. Not only does it encourage the growth of the central protostar to more advanced stages, but the disk evolves into the protoplanetary disk and becomes the primary site of planet formation. Thanks to the extremely high capabilities (resolution, sensitivity) of the ALMA (Atacama Large Millimeter-Submillimeter Interferometer), a team at the Institute of Astronomy and Astrophysics, Academia Sinica of Taiwan, led by Li Jing-hui, has been able to detect newborn protostars. We have succeeded in examining the extent of dust that makes up the surrounding disk. We clarified the structure of a disk formed around a newly born protostar. What is particularly noteworthy about this observation is that the disk was observed from an almost horizontal direction (edge-on), revealing a part of the spiral structure arm that occurred near the disk's equatorial plane. . This important observation will allow us to constrain the process of disk formation around a newborn protostar and the collapse of disk material into the central star.
The disk lies in the direction of the constellation Perseus, at the center of the Herbig Halo 211 protostellar system about 1,000 light-years from Earth. This system is a system in which the central star was just born, and it is believed that gravitational collapse began only about 35,000 years ago. Figure 1a is a map of the disk created by detecting radio waves emitted from the dust that composes the disk with ALMA. What we can see from this figure is that the distance from the protostar at the center to the outside of the disk is only about the distance between the Sun and Uranus (about 20 times the distance between the Sun and Earth), and the disk observed this time is a very small disk.
Figure 1. A disk around the protostar of Herbig Halo 211 (a) map created from the results of this ALMA observation and a model of the disk considered by the observation team). A map of the disk obtained from telescopic observations. The size of the solar system (range between Sun and Uranus) is also shown for size comparison.
(b) Three linear structures appearing in the map of the disk extending in the direction perpendicular to the disk's rotation axis (three dotted lines in the figure), excluding the vague surrounding area. A high-pass filter was applied to highlight the linear structure. (c) A disk model that reproduces the observed disk. The linear structures on the left and right sides of the map are reproduced by the hotter surfaces above and below the disk. On the other hand, the linear structure in the center of the map is reproduced by the hotter spiral arm formed at the equatorial plane of the disk. (d) View of the disk model from the rotation axis direction of the disk (top view of the disk). A full view of the spiral structure formed in the disk is displayed. Since the inner portion of the spiral structure rotates faster, the outer portion of the spiral structure rotates more inwardly, forming the spiral structure.
The disk is a thick disk, indicating that not enough dust has yet settled on the equatorial plane. Sufficient dust deposition on the equatorial plane is an essential step in planet formation. Interestingly, although the disk was observed almost from the side this time, three bright structures aligned along the equatorial plane are shown (Fig. 1b). It looks just like a three-layered pancake, but such a structure has never been observed before. Since these seemingly three-layered structures appear to be against a background of uniformly spreading disk structures, a high-pass filter was applied to Fig. 1a to enhance the contrast and appear to be three layers. I made the structure more visible. The linear structures visible on the left and right sides of Fig. 1b are thought to capture radio waves emitted from the surface of the disk, which has a higher temperature.
The model of the observed disk that the research team devised is shown in Fig. 1c, which shows the extended structure of the disk surface. More important is the central equatorial structure, which is asymmetrical about the disk's axis of rotation, indicated by the arrow in Fig. 1c. (The length of the upper and lower sides of the axis is different.) It is thought that the arm of the spiral structure in the model of the disk that the research team considered was excited by the gravity of the disk. Previous observations of disks of protostars at more advanced stages of evolution have observed spiral structures. It will support the structure being formed. It is thought that these spiral arms are formed, bring the disk material into the central star, and accelerate the process of the disk material falling into the central star.
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Chin-Fei Lee et al. "First Detection of a Linear Structure in the Midplane of the Young HH 211 Protostellar Disk: A Spiral Arm?" https://iopscience.iop.org/article/10.3847/2041-8213/acdbca
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The original Japanese text was created based on the press release issued on July 4, 2023 by the Institute of Astronomy and Astrophysics, Academia Sinica.
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The members of the research team that conducted this study are: Li Jing-hui (Institute of Astronomy and Astrophysics, Academia Sinica, National Taiwan University) Kai-Hun Zhan (Institute of Astronomy and Astrophysics, Academia Sinica, Taiwan, National Taiwan University) Anthony Moraghan (Institute of Astronomy and Astrophysics, Academia Sinica, Taiwan)