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# A likely flyby of binary protostar Z CMa caught in action

## Abstract

Close encounters between young stellar objects in star-forming clusters are expected to markedly perturb circumstellar disks. Such events are witnessed in numerical simulations of star formation1,2,3, but few direct observations of ongoing encounters have been made. Here we report sub-0.1″-resolution Atacama Large Millimeter/Submillimeter Array and Jansky Very Large Array observations towards the million-year-old binary protostar Z Canis Majoris in dust continuum and molecular line emission. A point source ~4,700 au from the binary has been discovered at both millimetre and centimetre wavelengths. It is located along the extension of a ~2,000 au streamer structure previously found in scattered light imaging, whose counterpart in dust and gas emission is also newly identified. Comparison with simulations shows signposts of a rare flyby event in action. Z CMa is a ‘double burster’, as both binary components undergo accretion outbursts4, which may be facilitated by perturbations to the host disk by flybys5,6,7,8.

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## Data availability

Raw ALMA data are publicly available via the ALMA archive https://almascience.eso.org/aq/ under project IDs 2016.1.00110.S and 2016.2.00168.S. Raw JVLA data are publicly available via the JVLA archive https://archive.nrao.edu/archive/advquery.jsp under project code 16B-080. Raw Keck data are publicly available via the KOA Data Access Service http://koa.ipac.caltech.edu/ under the programme ID U14N2. Final reduced and calibrated image files are available at https://doi.org/10.6084/m9.figshare.16915327.

## Code availability

The Phantom code is made available at https://github.com/danieljprice/phantom by D. Price. The MCFOST code is made available at https://github.com/cpinte/mcfost by C.P.

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## Acknowledgements

We thank C.-H. Kim, J. Lee and H. Deng for helpful discussions. R.D. would like to thank L. Xu for support and encouragement in the period of this work. R.D. acknowledges financial support provided by the Natural Sciences and Engineering Research Council of Canada through a Discovery Grant, as well as the Alfred P. Sloan Foundation through a Sloan Research Fellowship. N.C. acknowledges support from the European Union’s Horizon 2020 research and innovation programme under Marie Skłodowska-Curie grant agreement 210021. H.B.L. acknowledges support from the Ministry of Science and Technology (MoST) of Taiwan (grant 108-2112-M-001-002-MY3). E.V. acknowledges support from the Russian Fund for Fundamental Research, Russian–Taiwanese project 19-52-52011. Y.H. is supported by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. A.K. and L.C. acknowledge support from the European Research Council under the European Union’s Horizon 2020 research and innovation programme, grant agreement 716155 (SACCRED). L.C. is supported by the Hungarian OTKA grant K132406. M. Takami is supported by the Ministry of Science and Technology (MoST) of Taiwan (grants 106-2119-M-001-026-MY3 and 109-2112-M-001-019). H.B.L. and M.T. and are supported by MoST of Taiwan 108-2923-M-001-006-MY3 for the Taiwanese–Russian collaboration project. The Geryon cluster at the Centro de Astro-Ingenieria UC was extensively used for the calculations performed in this paper. BASAL CATA PFB-06, the Anillo ACT-86, FONDEQUIP AIC-57 and QUIMAL 130008 provided funding for several improvements to the Geryon cluster. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2016.1.00110.S and #2016.2.00168.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 National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

## Author information

Authors

### Contributions

R.D. led the ALMA proposals. H.B.L. led the JVLA proposal and processed the ALMA and JVLA data. N.C. and C.P. performed the hydrodynamics and radiative transfer simulations. R.D., H.B.L., N.C. and C.P. wrote the manuscript. All coauthors provided input to the observational proposals and/or the manuscript.

### Corresponding authors

Correspondence to Ruobing Dong, Hauyu Baobab Liu, Nicolás Cuello or Christophe Pinte.

## Ethics declarations

### Competing interests

The authors declare that they have no competing interests.

## Peer review

### Peer review information

Nature Astronomy thanks Thomas Haworth, Michael Kuffmeier and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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## Extended data

### Extended Data Fig. 1 Integrated intensity maps of the 12CO (2-1), 13CO (2-1), C18O (2-1), and SO 3Σ 6(5)-5(4) transitions.

The beam size (0.19 × 0.18; P.A.=88) is marked at the lower left corner. The Robust=0 weighted 224 GHz continuum image (beam size 0. 075 × 0. 047; P.A.=65) is shown in green contours at 0.13 and 1.3 mJy beam−1 levels (5 and 50 × the root mean square noises) to highlight the four compact continuum sources. The ALMA/VLA source C is labeled.

### Extended Data Fig. 2 Moment-1 maps (intensity-weighted mean velocity) of the gas observations.

The Robust=0 weighted 224 GHz continuum image (beam size 0. 075 × 0. 047; P.A.=65) is shown in white contours at 0.13 and 1.3 mJy beam−1 levels (5 and 50 × the root mean square noises). The systematic velocity is ~ 14 km/s (ref. 81). The ALMA/VLA source C is labeled.

### Extended Data Fig. 3 Gaussian fitting of point-like components detected around Z CMa.

Gaussian fittings may be confused by extended dust emission. While we treat the Gaussian fitted flux as the total flux and use it in dust mass estimates, the true total flux from each source may be in between the peak intensity and the Gaussian fitted flux. (a) Gaussian deconvolved major axis FWHM. (b) Gaussian deconvolved minor axis FWHM. (c) Gaussian deconvolved position angle. (d) Only detected from the Robust = 2 weighted image, with beam=100 × 66 mas (P.A.=19). (e) Gaussian fitting results indicate that the source is consistent with a δ function with non-detectable size.

### Extended Data Fig. 4 Sketch of the flyby simulation geometry.

The perturber’s orbital plane is in orange. The plane of the initial circumprimary disk is in blue. The primary spiral pointing towards the perturber is in solid red and is in the plane of the perturber. The secondary spiral is in dashed red and in the plane of the circumprimary disk. The line of sight is in the z direction.

### Extended Data Fig. 5 Keck/NIRC2 H-band archive data of Z CMa.

The data were taken in 2005 Oct 21. White circle indicates the position of the point source C.

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Dong, R., Liu, H.B., Cuello, N. et al. A likely flyby of binary protostar Z CMa caught in action. Nat Astron 6, 331–338 (2022). https://doi.org/10.1038/s41550-021-01558-y

• Accepted:

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• DOI: https://doi.org/10.1038/s41550-021-01558-y

• ### A massive Keplerian protostellar disk with flyby-induced spirals in the Central Molecular Zone

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