Some Rare Abnormal Stars may have White Dwarf Parents to Blame(Forwarded)

Gemini Observatory
Hilo, Hawaii

Science Contacts:

Dr. Geoffrey C. Clayton
Louisiana State University,
Baton Rouge, LA
gclayton @ fenway.phys.lsu.edu

Dr. Thomas R. Geballe
Gemini Observatory, Hilo, HI
(808) 974-2519 (desk)

Media Contact:

Peter Michaud
Gemini Observatory, Hilo HI, USA
(808) 974-2510 (desk)
www.gemini.edu

For embargoed release at 9:30 AM (Pacific Time) on Tuesday, January 9,
2007

Some Rare Abnormal Stars may have White Dwarf Parents to Blame

Astronomers have announced the discovery of huge quantities of an unusual
variety of oxygen in two very rare types of stars. The finding suggests
that the origin of these oddball stars may lie in the physics behind the
mergers of white dwarf star pairs.

The unusual stars are known as hydrogen-deficient (HdC) and R Coronae
Borealis (RCB) stars. Both types have almost no hydrogen -- an element
that makes up about 90% of most stars. Surprisingly, they contain up to a
thousand times more of the isotope oxygen-18 than normal stars like our
Sun. The discovery of abnormal quantities of oxygen-18 is based on
near-infrared spectroscopic observations from the Gemini Near-Infrared
Spectrograph (GNIRS) on the 8-meter Gemini-South telescope in Chile.

The findings were presented today at the 209th meeting of the American
Astronomical Society in Seattle Washington by a team consisting of: Dr.
Geoffrey C. Clayton (Louisiana State University, Baton Rouge, LA), Dr.
Thomas R. Geballe (Gemini Observatory, Hilo, HI), Dr. Falk Herwig (Keele
University, UK) and Dr. Christopher Fryer (Los Alamos National Laboratory,
Los Alamos, NM), and Dr. Martin Asplund (Mount Stromlo Observatory,
Australia).

Prompted by the discovery, the team roughly simulated the nuclear
reactions that would occur during a merger of two types of white dwarfs,
an idea originally proposed for the origin of RCB stars in 1984 by Prof.
Ronald F. Webbink (University of Illinois). According to Clayton
conditions had to be just right to yield the oxygen-18 observed in these
stars. "It's like the porridge in Goldilocks and the Three Bears. During
the merger process, when nuclear reactions were taking place, the
temperature was neither too hot, nor too cold, but just right for the
production of large amounts of oxygen-18."

One of the challenges in understanding these stars is how oxygen-18 can be
formed from nitrogen in the star while maintaining more normal amounts of
the isotope oxygen-16 made from the star's preexisting carbon. "It's
really the ratio of oxygen-18 to oxygen-16 that is important and in these
stars that ratio is very lopsided. Although we need to do more precise
modeling, it appears that the white dwarf merger theory might just allow
this to occur," said Clayton.

RCB stars are a small group of carbon-rich supergiants that undergo
spectacular declines in brightness at irregular intervals, typically a few
years in duration, before returning to their initial brightnesses. It is
now thought that carbon grains intermittently condensing in the gas
ejected by the star are responsible for dimming the star's light. On the
other hand, the HdC stars, although resembling the RCB stars in their
elemental abundances, do not eject gas and thus do not make dust or appear
to vary in brightness.

An alternative theory to the merging of white dwarf pairs, originally
proposed by Icko Iben (University of Illinois), is that oxygen-18 rich
stars could be formed when a single star on the verge of becoming a white
dwarf undergoes a final flash of thermonuclear burning near its surface.
This inflates the star to supergiant size and cools off its outer
atmosphere.

"This final-flash model is a tempting explanation because two stars known
as V605 Aquilae and Sakurai's Object have recently been discovered going
through the final flash phase where they resembled RCB stars in
abundances, temperature, and brightness," said team member Geballe.
"However, both of these stars are now known to have spent only a few years
in this phase and given this extremely short period as cool supergiants
this makes it unlikely that they can account for even the small number of
RCB stars currently known in the Milky Way Galaxy." These stars are so
rare that a total of only 55 HdC and RCB stars have been identified in our
galaxy.

"The properties and antics of these weird stars have been the subject of
intense observation and discussion for generations of astronomers," said
Geballe. "This discovery should help us pinpoint how the combination of
two degenerate stars is different than the sum of their parts."

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.

Background

The atom that we think of as oxygen has eight protons and eight neutrons
and is called "oxygen-16" or O-16. This form of oxygen is by far the
dominant form of oxygen everywhere throughout our Milky Way, it's found in
interstellar clouds and distant stars, as well as on Earth and in the Sun.
Two other stable forms of oxygen exist (isotopes), with one and two extra
neutrons, known as oxygen-17 and oxygen-18 (O-17 and O-18). However, both
of these isotopes are extremely rare. Our Earth, Sun, and most other stars
and clouds in interstellar space studied to date have about 2700 times as
much O-16 as O-17 and about 500 times as much O-16 as O-18.

The unpredictable variability of RCB stars has made them popular targets
for measurement by amateur astronomers and the source of much discussion
by professionals seeking an explanation for their behavior. RCB star
atmospheres are also extremely deficient in hydrogen, but very rich in
carbon.

Two different evolutionary models have been suggested for the origin of
RCB stars. Both theories invoke objects known as white dwarfs, the
ultra-dense cores of previously normal stars like the Sun. They typically
have masses about half that of the Sun, and their sizes are close to that
of the Earth. In one model an RCB star is formed when two white dwarf
stars merge. In the other model the RCB star is formed when a single star
on the verge of becoming a white dwarf undergoes a final flash of
thermonuclear burning near its surface, blowing the star up to supergiant
size and cooling off its outer atmosphere.

In 1984 Prof. Ronald F. Webbink (University of Illinois) proposed that an
RCB star is formed from the merger of a helium-rich white dwarf and a
carbon/oxygen-rich white dwarf. He suggested that as the binary white
dwarf coalesces into one object, the helium-white dwarf is disrupted, with
part of it accreting onto the carbon/oxygen-white dwarf and undergoing
thermonuclear "burning." The remainder forms an extended atmosphere around
the object. Webbink proposed that this structure, a star with an
He-burning outer shell in the center of a ~100 solar radii H-deficient
envelope, is a RCB star.

Additionally, in 2002, Dr. Simon Jeffery (Armagh Observatory, Northern
Ireland) and Dr. Hideyuki Saio (Tohoku University, Japan), suggested that
a white dwarf pair merger could also account for the abundances of
elements such as hydrogen, helium, carbon, nitrogen and oxygen seen in RCB
stars. However, little is known about how the isotopes of these elements
were created in these stars.

IMAGE CAPTION:
[http://www.gemini.edu/oxygen18]
A pair of white dwarf stars in a close binary system are brought ever
closer to each other, either by magnetic braking or gravitational wave
emission, until one of the stars is disrupted and then merges with the
other star. The gas becomes hot enough for nuclear reactions to take
place. The energy produced causes the new merged star to expand and become
a supergiant star, about a thousand times larger than the white dwarfs
that formed it.

Gemini Artwork by Jon Lomberg

[NOTE: A GIF animation of the artwork is available at
http://www.gemini.edu/images/stories...ite_dwarf.gif]