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Unveiling a Structure of a Protoplanetary Disk

It has become evident that the planetary systems like our solar system are ubiquitous in the universe since Mayor and Queloz had first discovered an extra-solar planet in 1995. Subsequent observations have identified a large number of extrasolar planets and extrasolar planetary systems. Previous studies have shown that planets like the earth or Jupiter form in a disk: a so-called “protoplanetary disk”, which is composed of gas and dust. Therefore, it is very important to study the protoplanetary disk as the progenitor for planets and planetary systems, as well as for the formation of planetary systems.

Using the Nobeyama 45-m radio telescope and the Atacama Submillimeter Telescope Experiment (ASTE) 10m radio telescope, a research group led by AKIYAMA Eiji (National Astronomical Observatory of Japan, NAOJ) observed a protoplanetary disk around a young star MWC 480 of approximately 2 solar masses, located in Taurus, revealing the surface density and temperature distributions of the gas. The several theoretical calculations show the vertical temperature gradient in the disk: it is warmer in the disk surface where the radiation from the central star directly hits and cooler in the disk interior where the radiation can not reach to. It becomes also evident the gas extends even farther outward than expected. In the outer region of the disk, the gas appears to migrate from the inner region and diffuse into the ambient interstellar space. Planetary systems like our own solar system can form by collecting and clearing gas at specific regions within the protoplanetary disk.

ASTE 10 m telescope measures radio emission in sub-millimeter wavelengths. It operates differently from other telescopes in Japan and therefore detects gas in relatively high temperature in a protoplanetary disk. As a result, we succeeded in detecting the extended gas, proven difficult to see in past observations. The same results were indicated during the early science testing phase of Atacama Large Millimeter/submillimeter Array (ALMA) and the results play a pioneering role in this research.

Reference: Akiyama et al. (2013) PASJ, 65, 123

Figures : Curves in dark color in the upper panels show the emission from the disk in the observation. Curves in green color show the results of theoretical calculation. Figures in lower panels shows the residuals after subtracting observational curve (dark color) from the theoretically calculated curve (green color). The smaller the residual after subtraction, the better the theoretical model fits the observational data. (a): CO(J=1-0), (b): CO(J=3-2), (c): 13CO(J=1-0), (d): C18O(J=1-0). CO line traces the low density region, C18O line traces the high density region, and 13CO line traces the region in between. The notation of (J=1-0) or (J=3-2) is associated with the temperature and density of the molecules. Generally large number suggests high temperature and density. It is possible to estimate temperature and surface density distributions by combining these observational results.