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 Comisi Nacional de investigaci
Cientifica y Tecnolica (CONiCYT), the Australian Research Council (ARC), the
Argentinean Consejo Nacional de investigaciones Cienticas y Tnicas (CONiCET)
and the Brazilian Conselho Nacional de Desenvolvimento Cientico e Tecnolico
(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]