Chronicle of a Death Foretold (Forwarded)
- From: Andrew Yee <ayee@xxxxxxxxxxxxxxxxxxxxxx>
- Date: Mon, 25 Jun 2007 16:16:18 GMT
ESO Education and Public Relations Dept.
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Contacts:
Markus Wittkowski
ESO
Phone: +49 89 3200 6769
David A. Boboltz
U.S. Naval Observatory, USA
Phone: +1 202 762 1488
For Immediate Release: 31 May 2007
ESO Science Release 25/07
Chronicle of a Death Foretold
Two of the World's Largest Interferometric Facilities Team-up to Study a
Red Giant Star
Using ESO's VLTI on Cerro Paranal and the VLBA facility operated by NRAO,
an international team of astronomers has made what is arguably the most
detailed study of the environment of a pulsating red giant star. They
performed, for the first time, a series of coordinated observations of
three separate layers within the star's tenuous outer envelope: the
molecular shell, the dust shell, and the maser shell, leading to
significant progress in our understanding of the mechanism of how, before
dying, evolved stars lose mass and return it to the interstellar medium.
S Orionis (S Ori) belongs to the class of Mira-type variable stars. It is
a solar-mass star that, as will be the fate of our Sun in 5 billion years,
is nearing its gloomy end as a white dwarf. Mira stars are very large and
lose huge amounts of matter. Every year, S Ori ejects as much as the
equivalent of Earth's mass into the cosmos.
"Because we are all stardust, studying the phases in the life of a star
when processed matter is sent back to the interstellar medium to be used
for the next generation of stars, planets ... and humans, is very
important," said Markus Wittkowski, lead author of the paper reporting the
results. A star such as the Sun will lose between a third and half of its
mass during the Mira phase.
S Ori pulsates with a period of 420 days. In the course of its cycle, it
changes its brightness by a factor of the order of 500, while its diameter
varies by about 20%.
Although such stars are enormous -- they are typically larger than the
current Sun by a factor of a few hundred, i.e. they encompass the orbit of
the Earth around the Sun -- they are also distant and to peer into their
deep envelopes requires very high resolution. This can only be achieved
with interferometric techniques.
"Astronomers are like medical doctors, who use various instruments to
examine different parts of the human body," said co-author David Boboltz.
"While the mouth can be checked with a simple light, a stethoscope is
required to listen to the heart beat. Similarly the heart of the star can
be observed in the optical, the molecular and dust layers can be studied
in the infrared and the maser emission can be probed with radio
instruments. Only the combination of the three gives us a more complete
picture of the star and its envelope."
The maser emission comes from silicon monoxide (SiO) molecules and can be
used to image and track the motion of gas clouds in the stellar envelope
roughly 10 times the size of the Sun.
The astronomers observed S Ori with two of the largest interferometric
facilities available: the ESO Very Large Telescope Interferometer (VLTI)
at Paranal, observing in the near- and mid-infrared, and the NRAO-operated
Very Long Baseline Array (VLBA), that takes measurements in the radio wave
domain.
Because the star's luminosity changes periodically, the astronomers
observed it simultaneously with both instruments, at several different
epochs. The first epoch occurred close to the stellar minimum luminosity
and the last just after the maximum on the next cycle.
The astronomers found the star's diameter to vary between 7.9
milliarcseconds and 9.7 milliarcseconds. At the distance of S Ori, this
corresponds to a change of the radius from about 1.9 to 2.3 times the
distance between the Earth and the Sun, or between 400 and 500 solar
radii!
As if such sizes were not enough, the inner dust shell is found to be
about twice as big. The maser spots, which also form at about twice the
radius of the star, show the typical structure of partial to full rings
with a clumpy distribution. Their velocities indicate that the gas is
expanding radially, moving away at a speed of about 10 km/s.
The multi-wavelength analysis indicates that near the minimum there is
more dust production and mass ejection: in these phases indeed the amount
of dust is significantly higher than in the others. After this intense
matter production and ejection the star continues its pulsation and when
it reaches the maximum luminosity, it displays a much more expanded dust
shell. This clearly supports a strong connection between the Mira
pulsation and the dust production and expulsion.
Furthermore, the astronomers found that grains of aluminum oxide -- also
called corundum -- constitute most of S Ori's dust shell: the grain size
is estimated to be of the order of 10 millionths of a centimetre, that is
one thousand times smaller than the diameter of a human hair.
"We know one chapter of the secret life of a Mira star, but much more can
be learned in the near future, when we add near-infrared interferometry
with the AMBER instrument on the VLTI to our (already broad) observational
approach," said Wittkowski.
More Information
The research presented here is reported in a paper in press in the journal
Astronomy and Astrophysics ("The Mira variable S Ori: Relationships
between the photosphere, molecular layer, dust shell, and SiO maser shell
at 4 epochs", by M. Wittkowski et al.). It is available in PDF format from
the publisher's web site.
The team consists of Markus Wittkowski (ESO), David A. Boboltz (U.S. Naval
Observatory, USA), Keiichi Ohnaka and Thomas Driebe (MPIfR Bonn, Germany),
and Michael Scholz (University of Heidelberg, Germany and University of
Sydney, Australia).
Notes:
A maser is the microwave equivalent to a laser, which emits visible light.
A maser emits powerful microwave radiation instead and its study requires
radio telescopes. An astrophysical maser is a naturally occurring source
of stimulated emission that may arise in molecular clouds, comets,
planetary atmospheres, stellar atmospheres, or from various conditions in
interstellar space.
ESO operates the Very Large Telescope Interferometer at Paranal
Observatory, Chile, with four fixed 8.2-m telescopes and four relocatable
1.8-m telescopes, working at optical/infrared wavelengths. NRAO operates
the Very Long Baseline Array with 10 stations across the U.S. working at
radio wavelengths between 3 mm and 90 cm (0.3-90 GHz). ESO, NRAO and other
partners will operate the Atacama Large Millimeter/submillimeter Array
(ALMA) in Chile, working at millimetre wavelengths between 0.3 and 10 mm
(30-950 GHz).
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France: Dr. Daniel Kunth, +33-1-44 32 80 85
Germany: Dr. Jakob Staude, +49-6221-528229
Italy: Dr. Leopoldo Benacchio, +39-347-230 26 51
The Netherlands: Ms. Marieke Baan, +31-20-525 74 80
Portugal: Prof. Teresa Lago, +351-22-089 833
Spain: Dr. Miguel Mas-Hesse, +34918131196
Sweden: Dr. Jesper Sollerman, +46-8-55 37 85 54
Switzerland: Dr. Martin Steinacher, +41-31-324 23 82
United Kingdom: Mr. Peter Barratt, +44-1793-44 20 25
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