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
"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."
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.
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.
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.
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.
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.
For full-resolution science images, illustrations and additional information see: http://www.gemini.edu/media/MSImages.html.
Dr. Sergey V. Marchenko
Dr. Anthony F.J. Moffat