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.