Accretion disks are an essential component in the paradigm of the formation of low-mass stars. Recent observations further identify disks surrounding low-mass pre-main-sequence stars perturbed by flybys. Whether disks around more massive stars evolve in a similar manner has become an urgent question. We report the discovery of a Keplerian disk of a few solar masses surrounding a 32 M⊙ protostar in the Sagittarius C cloud around the Galactic Centre. The disk is gravitationally stable with two embedded spirals. A combined analysis of analytical solutions and numerical simulations demonstrates that the most likely scenario to form the spirals is through external perturbations induced by a close flyby, and one such perturber with the expected parameters is identified. The massive, early O-type star embedded in this disk forms in a similar manner as do low-mass stars, in the sense of not only disk-mediated accretion, but also flyby-impacted disk evolution.
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This paper makes use of the following ALMA data: ADS/JAO.ALMA#2018.1.00641.S. The data are available at https://almascience.nao.ac.jp/aq by setting the observation code. The reduced data used for this study are available from the corresponding authors upon reasonable request.
The ALMA data were reduced using CASA versions 5.4.0 and 5.6.1, which are available at https://casa.nrao.edu/casa_obtaining.shtml. The code to make Fig. 3 is available at https://doi.org/10.5281/zenodo.6413326. The 3DBarolo code is available at https://github.com/editeodoro/Bbarolo. The Phantom code is available at https://github.com/danieljprice/phantom. The splash code is available at https://github.com/danieljprice/splash.
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We thank H. B. Liu, Y. Cheng and P. Sanhueza for helpful discussions. X.L. acknowledges support from the Initial Funding of Scientific Research for High-Level Talents at Shanghai Astronomical Observatory, and the Japan Society for the Promotion of Science KAKENHI grant 20K14528. G.-X.L. thanks M. Krause for discussions on the flyby scenario. G.-X.L. acknowledges support from NSFC grants W820301904 and 12033005. This work made use of the High Performance Computing Resource in the Core Facility for Advanced Research Computing at Shanghai Astronomical Observatory, and the Multi-wavelength Data Analysis System operated by the Astronomy Data Center, National Astronomical Observatory of Japan. It made use of the following ALMA data: ADS/JAO.ALMA#2018.1.00641.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan) and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ.
The authors declare no competing interests.
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Nature Astronomy thanks Susanne Pfalzner, Nicolas Cuello and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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Left: the velocity field of the disk and the two condensations, derived from the CH3OCHO line. The blue and red contours show the blue and red-shifted SiO emission from previous ALMA observations5. The blue-shifted SiO emission is integrated between − 80 and − 51 kms−1, and the red-shifted SiO emission between − 48 and − 25 km s−1. The bipolar outflow associated with the disk, which has been identified in Ref. 5 using multiple molecular lines including SiO, is marked by the blue and red arrows. The best-fit kinematic major axis of the disk is denoted by the green dashed line, same as in Fig. 1b. A candidate bipolar outflow associated with condensation A is marked by the dashed blue and red arrows. Right: the radio continuum emission in this region observed by the Very Large Array (VLA). Green contours are the 23 GHz continuum emission37, while red contours are the 5.6 GHz continuum emission4. The contour levels are between 20% and 80% and increment by 20% of the peak intensity. The synthesized beams at the two frequencies are shown in the top left and top right corners, respectively. At both frequencies, the radio continuum emission is unresolved or marginally resolved. The background image and gray contours show the ALMA 1.3 mm continuum emission, same as in Fig. 1.
The two maps are derived from two groups of molecular lines: CH3CN on the left, and 13CH3CN on the right, with the same scale range for comparison. The green contours show the continuum emission, with the same contour levels as in Fig. 1a.
Left: the case when using the sound speed only. Right: the case when using the epicyclic frequency following Equ. (5).
When the periastron distance is small (bottom panels), the perturber penetrates the disk and leaves a strong dynamical impact, thus truncating the disk. When large (top panels), the dynamical effect becomes insignificant. When the angular velocity at the periastron is low (left panels), the perturber is able to resonate only with outer radii of the disk that rotate more slowly. When high (right panels), the perturber resonates with the inner disk, disturbing smaller radii than observed. The only viable solution remains to the one identified in Fig. 3.
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Lu, X., Li, GX., Zhang, Q. et al. A massive Keplerian protostellar disk with flyby-induced spirals in the Central Molecular Zone. Nat Astron (2022). https://doi.org/10.1038/s41550-022-01681-4