A Quintillion-to-One: Giant Stars, Tiny Dust

Artist’s impression of WR 112, a binary system containing a massive, evolved Wolf-Rayet star and an OB-type companion. As their stellar winds collide, dust forms and spirals outward, consisting mostly of extremely tiny, nanometer-sized grains along with a secondary population about 100 times larger. Credit: NSF/AUI/NSF NRAO/M. Weiss. Hi-Res File

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ALMA and JWST reveal nanometer-scale carbon dust grains emanating from a massive binary star system

Telescope (JWST) have discovered that some of the most massive stars in our galaxy are emitting unbelievably tiny grains of carbon dust—dust that one day could form future stars and planets. Both powerful telescopes were required for this research, to reveal all of the dust being produced by these stars.

This new research focused on WR 112, a binary star system that contains a very rare, massive, intensely hot, and dying Wolf–Rayet star orbiting another star companion. Together, these stars blast out powerful stellar winds that collide and create dense, cooling regions where dust forms, before this dust is scattered into interstellar space by intense starlight.

While previous mid-infrared images from JWST revealed bright spiral arcs of dust in WR 112, researchers were surprised when they saw no dust at all in ALMA’s sensitive millimeter observations. Only warm, tiny dust grains could hide from ALMA’s view, one of the most powerful millimeter telescopes on Earth. Combined data from JWST and ALMA suggested that the dust grains in the extended spiral structures are largely smaller than one micrometer, and most of them should be only a few nanometers (or billionths of a meter) across.

“It’s amazing to know that some of the most massive stars in the Universe produce some of the tiniest dust particles before they die. The difference in size between the star and the dust it produces is about a quintillion to one,” shared Donglin Wu, an undergraduate at Yale University and the lead author of this new research.

The team also found evidence that the dust is not evenly made up of a range of sizes, but instead comes in two distinct sizes: a larger group of nanometer-sized grains, and a smaller group of grains about 0.1 micrometer across. This discovery reconciled decades of conflicting measurements of similar binary systems: some revealed only very tiny grains, while others only saw larger ones. Now, it is understood that this type of binary system can have both. The team explored several physical processes that can, in principle, break up or evaporate dust grains near the harsh radiation field of the stars, finding that these processes have a tendency to destroy grains that were in between these sizes under certain conditions.

Because WR 112 is one of the most prolific dust producers of its kind—producing as much as three Moons’ worth of dust every year—the new grain-size measurements have big implications for how much carbon dust massive binaries can contribute to the broader galaxy. By revealing that some of the Universe’s biggest stars are factories for some of its smallest solid particles, this study provides an important missing piece in the life cycle of cosmic dust.

Source: National Radio Astronomy Observatory (NRAO)/News

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About NRAO

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

About ALMA

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Southern Observatory (ESO), the U.S. National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the National Science and Technology Council (NSTC) in Taiwan and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI).

ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.

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Unraveling a Spiral Stream of Dusty Embers from a Massive Binary Stellar Forge

A thermal-infrared image of wr 112 captured with keck observatory's lws instrument in august 2004.

Credit: R. Lau et al./ISAS/JAXA/W. M. Keck Observatory

Maunakea, Hawaii – Astronomers using three Maunakea Observatories have discovered one of the most prolific dust-making Wolf-Rayet star systems known, remarkably producing an entire Earth mass of dust every year.

With nearly two decades of images from the world’s largest observatories – including W. M. Keck Observatory, Subaru Telescope, and Gemini Observatory in Hawaii – a research team led by Ryan Lau of Honolulu, Hawaii, an ʻIolani School alumnus and astronomer with the Japan Aerospace Exploration Agency (JAXA) at the Institute of Space and Astronautical Science (ISAS), has captured the beautiful, spiral motion of newly-formed dust streaming from a massive binary star system called Wolf-Rayet (WR) 112.

“WR 112 is incredibly hot and luminous with fast stellar winds ejecting material at high velocities – over thousands of kilometers per second,” said Lau, lead author of the study. “We’d expect dust to incinerate from the intense radiation of heat and violent winds. The fact that we see dust survive in this extreme environment is what makes WR 112 so mysterious and unusual.”

The study published today in The Astrophysical Journal.

Wolf-Rayet stars are one of the most extreme stars known; they are over 20 times more massive and millions of times brighter than the Sun. Because they are in the very late stage of stellar evolution, losing a large amount of mass, Wolf-Rayet stars have short lives and therefore are extremely rare.

WR 112 is composed of a Wolf-Rayet star and a companion star that’s also much more massive than the Sun. A sequence of images taken since 2001, including observations using Keck Observatory’s Long Wavelength Spectrometer (LWS), shows this system moving over time, with the two stars orbiting around each other at timescales of about 20 years, thus causing the appearance of a spiral rotation.

“Keck Observatory’s LWS was one of the few instruments capable of capturing high-resolution thermal-infrared images and Maunakea is an exceptional site for such observations,” said Lau. “These combined capabilities allowed us to trace the decades-long evolution of the dusty nebula around WR 112.” 

Sequence of 7 mid-infrared (~10 micrometers) images of WR 112 taken between 2001 – 2019 by Gemini North, Gemini South, Keck Observatory, the Very Large Telescope (VLT), and Subaru Telescope. The length of the white line on each image corresponds to about 6800 astronomical units. “Spurs” are the structures formed in the past 20 years showing variations between observations. “Nested shells” are expanding structures formed previously. The X-like signature in the Subaru Telescope image is an artifact due to property of the instrument. Credit: R. Lau et al./ISAS/JAXA

The team determined dust forms in the region where stellar winds from these two stars interact.

“When the two winds collide, all hell breaks loose, including the release of copious shocked-gas X-rays, but also the (at first blush surprising) creation of copious amounts of carbon-based aerosol dust particles in those binaries where one of the stars has evolved to helium-burning, which produces 40% of carbon in their winds,” said co-author Anthony Moffat, emeritus professor of astronomy at the University of Montreal.

This binary dust formation phenomenon has been revealed in other systems such as WR 104 by co-author Peter Tuthill, professor of physics at the University of Sydney. WR 104, in particular, reveals an elegant trail of dust resembling a ‘pinwheel’ that traces the orbital motion of the central binary star system.

However, the dusty nebula around WR 112 is far more complex than a simple pinwheel pattern. Decades of multi-wavelength observations presented conflicting interpretations of its dusty outflow and orbital motion. After almost 20 years uncertainty on WR 112, images from Subaru Telescope’s COMICS instrument taken in Oct 2019 provided the final—and unexpected—piece to the puzzle.

“We published a study in 2017 on WR 112 suggesting the dusty nebula was not moving at all, so I thought our COMICS observation would confirm this,” said Lau. “To my surprise, the COMICS image revealed the dusty shell had definitely moved since the last image we took with the Very Large Telescope in 2016. It confused me so much that I couldn’t sleep after the observing run—I kept flipping through the images until it finally registered in my head that the spiral looked like it was tumbling towards us.”

Lau collaborated with researchers at the University of Sydney, including Tuthill and undergraduate student Yinuo Han, who are experts at modeling and interpreting the motion of the dusty spirals from binary systems like WR 112.

“I shared the images of WR 112 with Peter and Yinuo and they were able to produce an amazing preliminary model that confirmed the dusty spiral stream is in fact revolving in our direction along our line of sight,” said Lau.

With the revised picture of WR 112, the research team was able to deduce how much dust this binary system is forming. To their surprise, the team found WR 112’s dust output rate of 3×10^-6 solar mass per year was unusual given its 20-year orbital period—the most efficient dust producers in this type of WR binary star system tend to have shorter orbital periods of less than a year, like WR 104 with its 220-day period.

WR 112 therefore demonstrates the diversity of WR binary systems capable of being highly-efficient dust factories and highlights their potential role as significant sources of dust not only in the Milky Way, but galaxies beyond our own.

Massive binary star systems like WR 112, as well as supernova explosions, are regarded as sources of dust in the early universe, but the process of dust production and the amount of the ejected dust are still open questions. With the discovery of WR 112, astronomers now have new insight into the origin of dust in the young universe.

Wolf-Rayet 112 from Keck Observatory on Vimeo.

Above: animated model of the spiral dust nebula around WR 112 (left) and the actual corresponding observations (right).  The φ symbol on the model animation indicates the orbital phase of the central binary, where φ = 0 is at the beginning of its 20-yr orbit, and φ = 1 is at the end of its orbit. The animation pauses at each phase that is displayed in the real observations. Credit: R. Lau et al./ISAS/JAXA 

Related Links

• Subaru Telescope Press Release, September 15, 2020
• NAOJ Press Release, September 15, 2020

 About W. M. Keck Observatory

The W. M. Keck Observatory telescopes are among the most scientifically productive on Earth. The two 10-meter optical/infrared telescopes on the summit of Maunakea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrometers, and world-leading laser guide star adaptive optics systems. 

Some of the data presented herein were obtained at Keck Observatory, which is a private 501(c) 3 non-profit 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.

Source: W. M. Keck Obsertory

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