Massive Star Pair Raises Dust While Doing the Tango

February 25, 2002

A Canadian-led research team using the Gemini Observatory has released tantalizing evidence that tiny dust particles ejected by hot, massive stars, may survive long enough to reach the interstellar medium. This kind of process might have provided some of the materials necessary for the early formation of planetary systems in the young Universe.

The research team used the advanced mid-infrared imaging capabilities of the Gemini North Telescope, on Mauna Kea in Hawaii, to study the dynamic interaction between a massive binary star pair engaged in a dusty orbital tango. The star system, named "WR 112", pits stellar winds from one star against the other to produce a bow shock where the stronger wind pushes back the weaker. The extreme compression at the bow shock forms dust that subsequently flows out from the system, tracing a giant spiral that hints at the star pair's ongoing orbital dance.

"These massive, most unsuspecting dust-producing Wolf-Rayet stars have been observed as they orbit in binary pairs before, but this is the first time that we have imaged one at multiple, mid-infrared wavelengths at this resolution," says Dr. Sergey Marchenko, formerly of the Université de Montréal (now at the Western Kentucky University) and lead-author of the paper published in the January 20, 2002 Astrophysical Journal Letters. "Looking at this system with Gemini we have revealed that the carbon dust particles, while tiny, are about 100 times larger than state-of-the-art theory predicts. In addition, a significant portion of the dust appears to be escaping into interstellar space before it can be destroyed by the lethal radiation field emanating from the hot, massive stars of the binary system."

Theory predicts that very early in the history of the Universe, the majority of stars may have been very massive, like those that become Wolf-Rayet (WR) stars. Because of their high mass, these stars burn rapidly and intensely, living lives about 1000 times shorter than stars like our Sun. It is therefore likely that this process could have injected a large amount of heavy-element (mainly carbon from the nuclear fusion of helium) dust into the interstellar medium while the Universe was still relatively young. "As a result, we might need to consider a relatively early epoch in the history of the Universe when the necessary ingredients first became available in the interstellar medium to seed and form planetary systems," said Marchenko.

The images produced by Gemini of this system clearly show the spiraling dust cloud formed by the dance of these two giant stars. Anthony Moffat also of the Université de Montréal, and co-PI with Marchenko, describes the result of this interaction in more earthly terms, "If you look downstream, beyond the central region where the winds from the two stars collide, we see a trail of dust that spirals out due to the combined orbital motions of the two stars. This outflowing is much like the path that water takes as a playful gardener swings around a high pressure garden hose!"

One mystery that remains is how the amorphous carbon dust particles form and survive in the harsh environment surrounding these stars. It is also unknown what processes lead to the formation of dust grains that are almost two orders of magnitude larger than theory predicts. Even at this size, each dust particle is still only about the size of cigarette smoke particles, or about 1 micron across.

What is understood is that the stellar wind from the carbon-rich Wolf-Rayet star in the WR112 pair is much stronger than that of the companion. As the wind from the Wolf-Rayet star encounters the weaker wind from its companion, a "shock-zone" is formed that bends back around the companion. The increased pressure in the shock-zone is believed to spark the formation of these larger grains of amorphous carbon dust. The dust then is obliged by the stronger WR stellar wind to flow away from and out of the system in the distinctive spiral pattern that was revealed by the Gemini mid-infrared images. See http://www.gemini.edu/media/MSImages.html for illustrations and data showing this process.

WR112 is thought to lie at about 14,000 light-years from the Earth and consist of one fairly massive Wolf-Rayet star that is gravitationally bound to another more normal yet very massive "O" type star. The two stars orbit each other with an orbital period that is estimated to be about 25 years, based on the known wind speed of the WR star, the form of the spiral and the estimated distance from Earth. The detectable dust spiral extends at least to a radius of about 12000 AU or over 100 times the radius of our solar system assuming the estimated distance to the system is accurate.

Members of the team which conducted this research led by Marchenko and Moffat included W.D Vacca – Max-Planck-Institut fuer extraterrestrische, Astrophysik Germany/Garching, S. Côté – Herzberg Inst. of Astrophysics, National Research Council Canada/Victoria, R. Doyon – Université de Montréal. The instrument used to make these observations on Gemini was the "Observatory Spectrometer and Camera for the Infrared" (OSCIR) that was built by the University of Florida, funded by the United States' National Science Foundation (NSF) and NASA and operated by the OSCIR Team led by Dr. Charles Telesco. The research was based upon images that were obtained at wavelengths of 7.9, 12.5 and 18.2 microns in the mid-infrared region of the electromagnetic spectrum. Additional near-infrared images of WR112 were obtained in 1999 and 2000 at the Canada-France-Hawaii Telescope and NASA Infrared Telescope Facility. Those shorter wavelength images helped to constrain characteristics of the hotter dust particles closer to the stars.

Previous observations of massive binary pairs have been made by other research groups, most notably the observations of WR98a and WR104 by P. Tuthill, J. Monnier and W. Danchi using the W.M. Keck Observatory, exposing beautiful, ever-changing dust spirals emanating from the binaries. These two systems also contain relatively cool carbon-rich WR stars, but in smaller orbits with periods of about a year. These observations have been crucial to our understanding of the dynamics around these systems by providing data at shorter infrared wavelengths that revealed details on the hotter dust in the vicinity of the binary star, but did not place any firm restrictions on the size of the particles or the full extent of the dust shell.

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The Gemini Observatory is an international collaboration that has built two identical 8-meter telescopes. The telescopes are located at Mauna Kea, Hawaii (Gemini North) and Cerro Pachón in central Chile (Gemini South), and hence provide full coverage of both hemispheres of the sky. Both telescopes incorporate new technologies that allow large, relatively thin mirrors under active control to collect and focus both optical and infrared radiation from space. Gemini North began science operations in 2000 and Gemini South began limited scientific operations in late 2001.

The Gemini Observatory provides the astronomical communities in each partner country with state-of-the-art astronomical facilities that allocate observing time in proportion to each country's contribution. In addition to financial support, each country also contributes significant scientific and technical resources. The national research agencies that form the Gemini partnership include: the US National Science Foundation (NSF), the UK Particle Physics and Astronomy Research Council (PPARC), the Canadian National Research Council (NRC), the Chilean Comisión Nacional de Investigación Cientifica y Tecnológica (CONICYT), the Australian Research Council (ARC), the Argentinean Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) and the Brazilian Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq). The Observatory is managed by the Association of Universities for Research in Astronomy, Inc. (AURA) under a cooperative agreement with the NSF. The NSF also serves as the executive agency for the international partnership.

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