# Spiral structures in an embedded protostellar disk driven by envelope accretion

## Abstract

Hydrodynamical simulations show that a pair of spiral arms can form in the disk around a rapidly growing young star and that the arms are crucial in transporting angular momentum as the disk accretes material from the surrounding envelope1,2,3,4. Here we report the detection of a pair of symmetric spiral structures in a protostellar disk, supporting the formation of spiral arms in the disk around a forming star. The HH 111 VLA 1 source is a young Class I source embedded in a massive infalling protostellar envelope and is actively accreting, driving the prominent HH 111 jet. Previous observations showed a ring of shock emission around the disk’s outer edge5, indicating accretion of the envelope material onto the disk at a high rate. Now with ALMA observations of thermal emission from dust particles, we detect a pair of spiral arms extending from the inner region to the disk’s outer edge, similar to that seen in many simulations1,2,3,4. Additionally, the disk is massive, with a Toomre Q parameter near unity in the outer parts where the spiral structures are detected, supporting the notion that envelope accretion is making the outer disk gravitationally unstable. In our observations, another source, HH 111 VLA 2, is spatially resolved for the first time, showing a disk-like structure with a diameter of ~26 au and an orientation nearly orthogonal to that of the HH 111 VLA 1 disk.

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## Data availability

This Letter makes use of the following ALMA data: ADS/JAO.ALMA#2015.1.00037.S and 2017.1.00044.S. The data that support the plots within this paper and other findings of this study are available from the corresponding author on reasonable request.

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## Acknowledgements

This Letter makes use of the following ALMA data: ADS/JAO.ALMA#2015.1.00037.S and 2017.1.00044.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), NSC 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. C.-F.L. acknowledges grants from the Ministry of Science and Technology of Taiwan (MoST 107-2119-M- 001-040-MY3) and the Academia Sinica (Investigator Award AS-IA-108-M01). Z.-Y.L. is supported in part by NSF grants AST-1716259, 1815784 and 1910106 and NASA grant 80NSSC18K1095. N.J.T.’s contribution was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA and with the support of NASA grant 17-XRP17_2-0081.

## Author information

C.-F.L. led the project, analysis and discussion and drafted the manuscript. Z.-Y.L. and N.J.T. commented on the manuscript and participated in the discussion.

Correspondence to Chin-Fei Lee.

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### Competing interests

The authors declare no competing interests.

Peer review information Nature Astronomy thanks John Ilee and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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## Supplementary information

### Supplementary Information

Supplementary Figs. 1–3.

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Lee, C., Li, Z. & Turner, N.J. Spiral structures in an embedded protostellar disk driven by envelope accretion. Nat Astron (2019) doi:10.1038/s41550-019-0905-x