Red supergiant cauldrons let off steam (Forwarded)

Royal Astronomical Society
London, U.K.

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Robert Massey
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AND
Anita Heward
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CONTACT:

Dr Anita Richards
AstroGrid Astronomer
MERLIN/VLBI National Facility
University of Manchester
Jodrell Bank Observatory
Macclesfield
Cheshire SK11 9DL, U.K.
Tel +44 (0)1477 572683 (direct)
Tel +44 (0)1477 571321 (switchboard)

Dr Richards can be contacted through the NAM press office.

PRESS INFORMATION NOTE: RAS PN 07/15 (NAM 11)

EMBARGOED FOR 00:01 BST, TUESDAY 17 APRIL 2007

RED SUPERGIANT CAULDRONS LET OFF STEAM

Steamy clouds have been observed bubbling away from four massive stars known
as red supergiants. A team from Jodrell Bank, using the MERLIN array and
European and Global VLBI Networks, found that the stars are actually
'steaming' as they enter their final death throes, driving out thick clouds
of water vapour immersed in more tenuous gas.

Dr Anita Richards, who is presenting results at the RAS National Astronomy
Meeting in Preston on 17th April, said, "Red supergiants lose more than half
their mass before ending their lives as supernovae. Our observations show
that this doesn't happen smoothly, like an onion shedding layers. We see
water vapour clouds which are over-dense, over-magnetised and rapidly
accelerated away from the star. They are embedded in a cooler, more diffuse
gas producing distinctive emission from hydroxyl, a break-down product of
water."

The group studied 'maser' emissions from the gas clouds surrounding the
star: molecules in the gas amplify and emit beams of microwave radiation in
much the same way as a laser produces very narrow, bright beams of light.
Water emits at 1.3 cm wavelength, under hot, dense conditions (around 1000
degrees Kelvin). Hydroxyl emission at 18.0 cm can only occur from cooler,
less dense gas and it was very unexpected to detect it as close to two of
the stars as the water masers. The only explanation seems to be that the
water masers come from clumps where the gas density is, typically, 50 times
higher than the rest of the wind from the star. Supporting evidence comes
from measurements of the magnetic field strength associated with the
hydroxyl masers, which is much weaker than that of the adjacent water
masers, as is expected if the hydroxyl environment is more diffuse.

The water vapour clouds appear to be very dusty and are accelerating faster
than the surrounding gas. Only a few of these steam clouds form each stellar
period (several years), filling just a few percent of the volume of the
maser shell around the star, but they contain most of the mass lost by the
star.

In the study, the maser emissions from the water vapour appeared to show
that the clouds had a lifetime of only a few decades, although clouds were
observed at distances that would have taken about a century to reach. The
puzzle was solved by comparing the MERLIN results with longer-term
observations from the Puschino radio telescope in Russia, which revealed
individual clouds winking off and back on again due to the fickle nature of
maser excitation or beaming. Dr Anita Richards said, "These observations are
intriguing because, from the size of the masing shell, we estimate that the
water vapour clouds take about 100 years to bubble away into interstellar
space, but we can only actually 'see' any particular cloud for a few years."

The group hope to follow this up by using the e-MERLIN, eVLBI and ALMA
networks of radio telescopes to trace the mass loss process back to the star
to discover whether star-spots, convection cells, dust formation or some
other mechanism gives birth to the clumps.

FURTHER INFORMATION

The team

The research was conducted by Anita Richards, Jim Cohen, Phil Diamond,
Sandra Etoka, Malcolm Gray and Wouter Vlemmings from Manchester University;
Mike Masheder from Bristol University; Indra Bains from the University of
Melbourne; Ania Bartkiewicz and Marian Szymczak from the Niclaus Copernicus
University, Torun; Evgueny Lekht and Eduardo Mendoza-Torrez from INAOEP,
Mexico; Koji Murakawa from the Max Planck Institute, Bonn; Huib van
Langevelde from JIVE/Leiden University, The Netherlands and Jeremy Yates
from University College London.

Red Supergiant stars

We studied four Red Supergiants within 7000 light years of the Sun, S
Persei, VX Sagitarii, VY Canis Majoris and NML Cygni. Stars more than about
8 times the mass of the Sun live fast and go out in a blaze of glory as a
supernova. Even before then, these stars contribute up to half of all the
dust and a large proportion of light elements which enrich the interstellar
medium (clouds of gas and dust between the stars which will go into the next
generation of star formation). Earth-like planets cannot form without dust
grains and elements like oxygen, nitrogen, carbon and silicon. Less than one
in 200 stars are high-mass at birth, but they make up for that by their
productivity. They lose most of their mass in the Red Super Giant stage --
when they have swelled up to the size of the orbit of Jupiter or even Saturn
and their cool atmosphere allows dust and molecules to form. Betelgeuse in
Orion is approaching this stage.

MERLIN

MERLIN (http://www.merlin.ac.uk/), operated by Jodrell Bank Observatory, is
the Multi-Element Radio Linked Interferometer Network, an array of radio
telescopes distributed around Great Britain, with separations of up to
217km. It operates at wavelengths ranging from 1.3 to 23 cm. At 1.3 cm, the
resolution of MERLIN is better than 10 milliarcseconds, even greater than
that of the Hubble Space Telescope. MERLIN is operated by the University of
Manchester as a National Facility of the Science and Technology Facilities
Council. It is currently being upgraded (including connecting the telescopes
by optical fibres), which will increase its sensitivity more than tenfold
(the e-MERLIN project, http://www.jb.man.ac.uk/news/e-merlin/)

European Very Long Baseline Interferometry Network/Global VLBI

The European VLBI Network (EVN, http://www.evlbi.org/) is an interferometric
array of radio telescopes spread throughout Europe and beyond, which
conducts unique, high resolution, radio astronomical observations of cosmic
radio sources. It is the most sensitive VLBI array in the world, thanks to
the collection of extremely large telescopes that contribute to the network.
We used it in combination with telescopes from the USA VLBA
(http://www.vlba.nrao.edu/).

Pushchino Radio Astronomy Observatory

The Puschino RT-22 LPI radio telescope
(http://www.prao.psn.ru/English/index.php)
is run by the Lebedev Physical Institute and the Russian Academy of
Sciences. It has been used to monitor water vapour masers since 1979.

IMAGES

Images can be found at:
http://www.nam2007.uclan.ac.uk/press...20070417f1.png (147KB)
http://www.nam2007.uclan.ac.uk/press...20070417f2.png (37KB)

Caption: These images show the water vapour clouds in shades of blue (S
Persei) and red (VX Sagitarii), observed using MERLIN at 1.3 cm wavelength.
The image of S Persei is about 370 AU across and the image of VX Sagitarii
is about 680 AU across, where an AU is the distance between the Earth and
the Sun. The Doppler shifts of the maser spectral lines mean that we can
measure the speed and direction of the wind blown away from the star. The
symbols show the positions of the hydroxyl masers observed using MERLIN and
VLBI at 18.0 cm wavelength (the 'mainline' transition), colour coded
according to velocity with respect to the star (blue is expanding towards
us, red away from us etc.). The high resolution of Very Long Baseline
Interferometry, using radio telescopes in Europe and the USA, confirmed that
the hydroxyl masers are found in the water maser region and the appearance
is not just a projection effect.