Majority of Planetary Nebulae May Arise from Binary Systems (Forwarded)

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EMBARGOED FOR RELEASE: 10 a.m. EST Thursday, January 8, 2004

RELEASE NO: NOAO 04-02

Majority of Planetary Nebulae May Arise from Binary Systems

Near the end of its lifetime, a star like the Sun ejects its outer
layers into space, producing a hazy cloud of material called a
planetary nebula. The complex shapes and dazzling colors of planetary
nebulae make them some of the most popular objects in the night
sky, for both amateur observing and scientific study.

New research suggests that many if not most of the stellar corpses at the
centers of these wildly varied cosmic objects have companion stars, a
surprising finding that will influence how astronomers explain their
origins.

Astronomers used the Wisconsin-Indiana-Yale-NOAO 3.5-meter telescope at
the National Science Foundation's Kitt Peak National Observatory to take
radial velocity measurements of 11 central stars of planetary nebulae
(PNe), looking for the telltale, repeatable wobble that indicates the
presence of a companion's gravitational influence. This technique is also
used to search for extrasolar planets around nearby stars. Ten of the 11
central stars of the PNe in the recent study showed clear evidence for
radial velocity oscillations.

"If our current results are confirmed with further observations, we could
be at the start of a revolution in the study of the origin of planetary
nebulae," says Howard Bond of the Space Telescope Science Institute in
Baltimore, the principal investigator of the results presented today in
Atlanta at the 203rd meeting of the American Astronomical Society. "If
these nebulae arise from binary stars, it implies a very different origin
for these systems than what most astronomers had thought."

It might be expected that nebulae ejected from spherical stars would be
spherical, but many years of telescope observations show this not to be
the case. In fact, most PNe are either elliptical or have pronounced
lobes, often accompanied by jet-like structures.

There is general agreement that in order to eject gas with these observed
morphologies, single stars would have to rotate rather rapidly or have
reasonably strong magnetic fields, which themselves are the product of
stellar rotation. However, the stars that most commonly eject PNe are
large, bloated giants, indisposed to fast rotation.

"The most direct way to spin up these vast, fluffy stars is by the action
of an orbiting companion. In extreme cases, as a red giant star gradually
increases in size, it may actually swallow a companion star, which would
then spiral down inside the giant and eventually eject its outer layers,"
explains Orsola De Marco, an astronomer at the American Museum of Natural
History (AMNH) in New York and the lead author of the publication
reporting the first results of this project. "Despite this, the
mainstream astronomical view remains rooted in single star theories for
the evolution of planetary nebulae, supported by the small percentage of
planetary nebulae central stars that that were previously known to be
binaries. However, our new research threatens to turn this viewpoint on
its head."

Astronomers currently believe that the majority of stars -- those that
begin with no more than eight times the mass of our Sun -- end their
lives by ejecting a planetary nebula and becoming a cosmic ember called a
white dwarf. However, the new results from the WIYN telescope suggest
that the story may be more complicated, in that an interaction with a
companion star may be required to produce most planetary nebulae.

"We need more data to determine the exact periods of the binary central
stars, since this is the only way to be sure of their binarity and
eliminate other possible physical sources that could simulate the stellar
wobble," De Marco says. "We are reasonably sure that these variations are
due to binarity, but determination of their precise periods is the only
way to be sure. We must also increase the size of our sample."

Among the objects observed in this initial study are Abell 78, NGC 6891,
NGC 6210, and IC 4593. The new radial velocity measurements were taken by
the WIYN Hydra spectrographic instrument.

A previously released Hubble Space Telescope image of NGC 6210 is
available at:
http://hubblesite.org/newscenter/new...998/36/image/a

Co-authors of this work are Dianne Harmer of the National Optical
Astronomy Observatory (NOAO) in Tucson, AZ, and Andrew Fleming of
Michigan Technological University in Houghton, MI, an NSF Research
Experiences for Undergraduates (REU) student at AMNH during the summer of
2003.

These results (Abstract 127.03 in the AAS meeting program) will be
discussed in an oral session that begins at 10:00 a.m. on Thursday,
January 8, in Regency VI. This research has been accepted for publication
in the February 1, 2004, issue of Astrophysical Journal Letters.

Images of other planetary nebulae taken by Kitt Peak telescopes are
available in the NOAO Image Gallery at:

http://www.noao.edu/image_gallery/pl...y_nebulae.html
and
http://www.noao.edu/outreach/aop/observers/pn.html

The Wisconsin-Indiana-Yale-NOAO (WIYN) 3.5-meter telescope is located at
Kitt Peak National Observatory, 55 miles southwest of Tucson, AZ. Kitt
Peak National Observatory is part of the National Optical Astronomy
Observatory, which is operated by the Association of Universities for
Research in Astronomy (AURA), Inc., under a cooperative agreement with
the National Science Foundation (NSF).