Press Release

Massive Star Pair Raises Dust While Doing the Tango

By SpaceRef Editor
February 27, 2002
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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 Universite de
Montreal (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 Universite de Montreal, 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. Cote. Herzberg Inst. of Astrophysics,
National Research Council Canada/Victoria, R. Doyon, Universite de
Montreal. 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.

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 Pachon 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 Comision Nacional de
Investigacion Cientifica y Tecnologica (CONICYT), the Australian Research
Council (ARC), the Argentinean Consejo Nacional de Investigaciones
Cientificas y Tecnicas (CONICET) and the Brazilian Conselho Nacional de
Desenvolvimento Cientifico e Tecnologico (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.

Science Contacts:

Dr. Sergey V. Marchenko

Department of Physics and Astronomy

Western Kentucky University

Bowling Green, Kentucky

(270)-745-6201 (Office)

E-mail: sergey@astro.wku.edu

Dr. Anthony F.J. Moffat

Departement de physique

Universite de Montreal

(514) 343-6682 (Office)

E-mail: moffat@astro.umontreal.ca

Media Contacts:

Peter Michaud

Gemini Observatory, Hilo HI

(808) 974-2510 (Desk), (808) 987-5876 (Cell)

E-mail: pmichaud@gemini.edu

Dr. Dennis Crabtree

National Research Council

(250) 363-0024 (Office)

E-mail: Dennis.Crabtree@nrc.ca

SpaceRef staff editor.