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Disk-driven rotating bipolar outflow in Orion Source I


One of the outstanding problems in star formation theory concerns the transfer of angular momentum so that mass can accrete onto a newly born young stellar object (YSO). From a theoretical standpoint, outflows and jets are predicted to play an essential role in the transfer of angular momentum1,2,3,4 and their rotations have been reported for both low-5 and high-mass6,7 YSOs. However, little quantitative discussion on outflow launching mechanisms has been presented for high-mass YSOs due to a lack of observational data. Here we present a clear signature of rotation in the bipolar outflow driven by Orion Source I, a high-mass YSO candidate, using the Atacama Large Millimeter/Submillimeter Array (ALMA). A rotational transition of silicon monoxide (Si18O) reveals a velocity gradient perpendicular to the outflow axis, which is consistent with that of the circumstellar disk traced by a high excitation water line. The launching radii and outward velocity of the outflow are estimated to be >10 au and 10 km s−1, respectively. These parameters rule out the possibility that the observed outflow is produced by the entrainment of a high-velocity jet8, and that contributions from the stellar wind9 or X-wind10, which have smaller launching radii, are significant in the case of Source I. Thus these results provide convincing evidence of a rotating outflow directly driven by the magneto-centrifugal disk wind launched by a high-mass YSO candidate6,11.

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Figure 1: Moment maps of the observed lines and continuum emissions.
Figure 2: Position–velocity diagrams parallel to the disk mid-plane.
Figure 3: Derived outflow parameters from position–velocity diagrams of the 484 GHz Si18O line.


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We are grateful to R.L. Plambeck, Y. Oya, N. Sakai and S. Yamamoto for valuable discussions. The ALMA is a partnership of the European Southern Observatory (representing its member states), the National Science Foundation (USA) and the National Institutes of Natural Sciences (Japan), together with the National Research Council Canada, National Science Council of Taiwan and Academia Sinica Institute of Astronomy and Astrophysics (Taiwan) and the Korea Astronomy and Space Science Institute (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by the European Southern Observatory, Associated Universities Inc. (AUI)/National Radio Astronomy Observatory and the National Astronomical Observatory of Japan (NAOJ). We thank the staff at ALMA for making the observations and reducing the data. T.H. is supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT)/Japan Society for the Promotion of Science (JSPS) KAKENHI grant numbers 21224002, 24684011, 25108005 and 15H03646 and the ALMA Japan Research Grant of the NAOJ Chile Observatory, NAOJ-ALMA-0006, -0028 and -0066. M.N.M. is supported by MEXT/JSPS KAKENHI grant numbers 15K05032 and 17K05387. K.M. is supported by MEXT/JSPS KAKENHI grant number 15K17613. M.H. is supported by MEXT/JSPS KAKENHI grant numbers 24540242 and 25120007. Data analyses were in part carried out on the common use data analysis computer system at the Astronomy Data Center, NAOJ.

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T.H. led the project as a principal investigator of the ALMA observations and performed the data analysis. M.N.M. and Y.M. interpreted the ALMA results from the theoretical point of view. K.M. analysed part of the ALMA data and checked the results. N.M., M.K.K., R.A.B. and M.H. contributed to writing the paper. All the authors discussed the results and commented on the paper.

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Correspondence to Tomoya Hirota.

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The authors declare no competing financial interests.

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Hirota, T., Machida, M., Matsushita, Y. et al. Disk-driven rotating bipolar outflow in Orion Source I. Nat Astron 1, 0146 (2017).

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