Revealing the Accelerating Wind in the Inner Region of Colliding-wind Binary WR 112

Creators: Monnier, John D.¹; Han, Yinuo²; Corcoran, Michael F.^{3, 4}; Bloot, Sanne^{5, 6}; Callingham, Joseph R.^{5, 7}; Danchi, William³; Edwards, Philip G.⁸; Greenhill, Lincoln⁹; Hamaguchi, Kenji^{3, 10}; Hankins, Matthew J.¹¹; Lau, Ryan¹²; Miller, Jon M.¹; Moffat, Anthony F. J.¹³; Ruane, Garreth¹⁴; Russell, Christopher M. P.¹⁵; Soulain, Anthony¹⁶; Tinyanont, Samaporn¹⁷; Tuthill, Peter¹⁸; Wang, Jason J.¹⁹; Williams, Peredur M.²⁰

1. University of Michigan–Ann Arbor
2. California Institute of Technology
3. Goddard Space Flight Center
4. Catholic University of America
5. Netherlands Institute for Radio Astronomy
6. University of Groningen
7. University of Amsterdam
8. Australia Telescope National Facility
9. Harvard-Smithsonian Center for Astrophysics
10. University of Maryland, Baltimore County
11. Arkansas Tech University
12. NOIRLab
13. University of Montreal
14. Jet Propulsion Lab
15. University of Delaware
16. French National Centre for Scientific Research
17. National Astronomical Research Institute of Thailand
18. University of Sydney
19. Northwestern University
20. Royal Observatory

Abstract

Colliding winds in massive binaries generate X-ray-bright shocks, synchrotron radio emission, and sometimes even dusty “pinwheel” spirals. We report the first X-ray detections of the dusty WC+O binary system WR 112 from Chandra and Swift, alongside 27 yr of Very Large Array/Australia Telescope Compact Array radio monitoring and new diffraction-limited Keck images. Because we view the nearly circular orbit almost edge-on, the colliding-wind zone alternates between heavy Wolf–Rayet wind self-absorption and near-transparent O-star wind foreground each 20 yr orbit, producing phase-locked radio and X-ray variability. This scenario leads to a prediction that the radio spectral index is flatter from a larger nonthermal contribution around the radio intensity maximum, which indeed was observed. Existing models that assume a single dust-expansion speed fail to reproduce the combined infrared (IR) geometry and radio light curve. Instead, we require an accelerating postshock flow that climbs from near-stationary to ∼1350 km s⁻¹ in about one orbital cycle, naturally matching the IR spiral from 5″ down to within 0."1, while also fitting the phase of the radio brightening. These kinematic constraints supply critical boundary conditions for future hydrodynamic simulations, which can link hot-plasma cooling, nonthermal radio emission, X-ray spectra, and dust formation in a self-consistent framework. WR 112 thus joins WR 140, WR 104, and WR 70-16 (Apep) as a benchmark system for testing colliding-wind physics under an increasingly diverse range of orbital architectures and physical conditions.

Copyright and License

© 2025. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Acknowledgement

The authors would like to thank a few colleagues who have contributed to obtaining this data presented herein: Anthony Pollack, Andreas Sander, and David Espinoza. We also thank the anonymous referee for their help in improving the manuscript.

Support for this work was provided by the National Aeronautics and Space Administration through Chandra Award Number 23200246 issued by the Chandra X-ray Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of the National Aeronautics Space Administration under contract NAS8-03060.

The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

The Australia Telescope Compact Array is part of the Australia Telescope National Facility (grid.421683.a), which is funded by the Australian Government for operation as a National Facility managed by CSIRO. We acknowledge the Gomeroi people as the traditional owners of the Observatory site.

Some of the data presented herein were obtained at Keck Observatory, which is a private 501(c)3 nonprofit organization operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the Native Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.

M.F.C. and K.H. are supported by NASA under grant Nos. 80GSFC21M0002 and 80GSFC24M0006. J.J.W. acknowledges support by the Heising-Simons Foundation (grant No. 2023-4598) and the Alfred P. Sloan Foundation. S.B. acknowledges funding from the Dutch research council (NWO) under the talent program (Vidi grant VI.Vidi.203.093). J.D.M. and M.C. acknowledge support from Chandra Award No. GO2-23012B. J.R.C. acknowledges funding from the European Union via the European Research Council (ERC) grant Epaphus (project number 101166008). C.M.P.R. acknowledges support from NASA Chandra Theory grant No. TM3-24001X.

Facilities

Keck:I - KECK I Telescope (NIRC), Keck:II - KECK II Telescope (NIRC2), VLA - Very Large Array, EVLA - Expanded Very Large Array, ATCA - Australia Telescope Compact Array, CXO - Chandra X-ray Observatory satellite, Swift - Swift Gamma-Ray Burst Mission.

Software References

astropy (Astropy Collaboration et al. 2013,2018, 2022).

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Resource type: Journal Article
Publisher: American Astronomical Society
Published in: Astronomical Journal, 170(4), 218, ISSN: 0004-6256.
Languages: English