4 December, 2014
With new Atacama Large Millimeter/submillimeter Array (ALMA) observations, astronomers led by Shigehisa Takakuwa, Associate Research Fellow at the Institute of Astronomy and Astrophysics, Academia Sinica (ASIAA), Taiwan, have found spiral arms of molecular gas and dust around “baby twin” stars. Gas motions supplying materials to the twin were also identified. These results unveil for the first time, the mechanism of the birth and growth of binary stars, which are ubiquitous throughout the Universe. The study was published on November 20 in The Astrophysical Journal.
Fig 1. Gas and dust disks around L1551 NE spotted by ALMA. Credit: ALMA (ESO/NAOJ/NRAO)/Takakuwa et al.
Stars form in interstellar clouds of molecular gas and dust. Previous studies of star formation focused primarily on single stars like the Sun, and a standard picture of single star formation has been established. According to this picture, a dense gas condensation in an interstellar cloud collapses gravitationally to form a single protostar at the center. Previous observations have found such collapsing gas motions feeding material toward the central protostars.
Compared to single star formation, our understanding of binary star formation has been limited, even though more than half of stars with a mass similar to that of the Sun are known to be binaries. It is thus crucial to observe the physical mechanism of binary formation to obtain a more comprehensive understanding of star formation. Theory suggests that a disk surrounding a young binary will feed material to the central “baby twin” in order for them to grow. While recent observations have found such disks (known as “circumbinary disks”), it was not possible to image the structure and gas motions because of the insufficient imaging resolution and sensitivity.
Fig 2. Comparison of the disks in simulation and observation. The right panel shows the disk image simulated with ATERUI, and the left panel the real ALMA image. Credit: ALMA (ESO/NAOJ/NRAO)/Takakuwa et al.
The research team, led by Shigehisa Takakuwa, used the ALMA telescope to observe the baby-twin star L1551 NE , located in the constellation of Taurus at a distance of 460 light years, with a 1.6 times better imaging resolution and a 6 times better sensitivity than those of their previous observations with the SubMillimeter Array (SMA). They used the emission from dust at a wavelength of 0.9mm to trace the distribution of interstellar material, and emission from carbon monoxide to study gas motions using the Doppler Effect. They found gas associated with each binary star (the two central components can be seen in Figure 1), and a disk surrounding both stars, the circumbinary disk, with a radius of 300 au. The radius corresponds to 10 times the orbital radius of Neptune in our solar system. For the first time, they succeeded in imaging the detailed structure of the circumbinary disk, and found that it consists of a southern U-shaped feature with northern extensions pointing to the northwest and the northeast (Figure 1).
To understand these newly-identified features, the research team constructed a theoretical model of binary formation in L1551 NE, shown in Figure 2 (right, see also the attached movie), using the supercomputer, “ATERUI” at the National Astronomical Observatory of Japan (NAOJ) . As shown in Figure 2, the southern U-shaped feature and northern emission protrusions observed with ALMA can be reproduced with a pair of spiral arms stemming from each of the baby twins. The research team also investigated the gas motion as seen in carbon monoxide, and found the spiral arms to be rotating faster than the regions between the arms. These inter-arm regions show gas falling toward the central baby twins. This is believed to be the ongoing feeding process of the baby twins. These results show that the twins “shake” the surrounding circumbinary disk and induce the falling gas motion. “Our high-resolution ALMA observation has unveiled live images of the growth of the baby twins for the first time”, said Takakuwa.
Tomoaki Matsumoto, a professor at Hosei University, who constructed the theoretical model with the supercomputer, said, “The ALMA results match with our theoretical prediction remarkably accurately” . Kazuya Saigo, co-principal investigator along with Takakuwa, explained, “We succeeded in unveiling the structure and motion in the circumbinary disk with high accuracy, because of the high resolution and sensitivity of ALMA. Combining these high-resolution ALMA observations with thorough numerical simulations using a supercomputer will become more and more important, and can be regarded as an upcoming research trend”.
 The mass of each twin of L1551 NE is 0.67 and 0.13 times the mass of the Sun, and their separation is 145 au (astronomical unit; 1 au is the distance between the Sun and the Earth, approximately 150 million km).
 The supercomputer ATERUI (Cray XC30), operated by NAOJ Center for Computational Astrophysics, is the fastest one dedicated for astrophysics research.
 NAOJ’s Subaru Telescope found a spiral structure around a more evolved binary system SR24. The stars in SR24 are close to the final stage of their growth and the gas envelope has almost dissipated, whereas the twin stars in L1551 NE are in the very early, active stage of their growth. The ALMA observation shows that the spiral structure plays an important role in this very early phase of binary formation.
First Direct Imaging of a Young Binary System http://subarutelescope.org/Pressrelease/2009/11/19/index.html
These observational results were published in The Astrophysical Journal as Takakuwa et al. “Angular Momentum Exchange by Gravitational Torques and Infall in the Circumbinary Disk of the Protostellar System L1551 NE” in November 2014.
The complete list of authors is: Shigehisa Takakuwa (ASIAA), Masao Saito (NAOJ/SOKENDAI (The Graduate University for Advanced Studies)),Kazuya Saigo (NAOJ), Tomoaki Matsumoto (Hosei Univ.), Jeremy Lim (Univ. of Hong-Kong), Tomoyuki Hanawa (Chiba Univ.), and Paul T. P. Ho (ASIAA).
The research was supported by research grants from the Ministry of Science and Technology of Taiwan (MOST 102-2119-M-001-012-MY3), GRF grants of the Government of the Hong Kong SAR under HKU 703512P and the Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Numbers 24244017, 23540270.
The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Organisation for Astronomical Research in the Southern Hemisphere (ESO), the U.S. National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the Ministry of Science and Technology (MOST) in Taiwan and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI).
ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.
Numerical computations were carried out on Cray XC30 at Center for Computational Astrophysics, National Astronomical Observatory of Japan.
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