The star cluster Westerlund 1 and the positions of the magnetar and its probable former companion star
Wide-field view of the sky around the star cluster Westerlund 1
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Magnetars are the bizarre super-dense
remnants of supernova explosions. They are the strongest magnets known
in the Universe — millions of times more powerful than the strongest
magnets on Earth. A team of European astronomers using ESO’s Very Large
Telescope (VLT) now believe they’ve found the partner star of a magnetar
for the first time. This discovery helps to explain how magnetars form —
a conundrum dating back 35 years — and why this particular star didn’t
collapse into a black hole as astronomers would expect.
When a massive star collapses under its own gravity during a supernova explosion it forms either a neutron star or black hole. Magnetars
are an unusual and very exotic form of neutron star. Like all of these
strange objects they are tiny and extraordinarily dense — a teaspoon of
neutron star material would have a mass of about a billion tonnes — but
they also have extremely powerful magnetic fields. Magnetar surfaces
release vast quantities of gamma rays when they undergo a sudden
adjustment known as a starquake as a result of the huge stresses in their crusts.
The Westerlund 1 star cluster [1],
located 16 000 light-years away in the southern constellation of Ara
(the Altar), hosts one of the two dozen magnetars known in the Milky
Way. It is called CXOU J164710.2-455216 and it has greatly puzzled
astronomers.
“In our earlier work (eso1034) we showed that the magnetar in the cluster Westerlund 1 (eso0510)
must have been born in the explosive death of a star about 40 times as
massive as the Sun. But this presents its own problem, since stars this
massive are expected to collapse to form black holes after their deaths,
not neutron stars. We did not understand how it could have become a
magnetar,” says Simon Clark, lead author of the paper reporting these results.
Astronomers proposed a solution to this mystery. They suggested that
the magnetar formed through the interactions of two very massive stars
orbiting one another in a binary system so compact that it would fit
within the orbit of the Earth around the Sun. But, up to now, no
companion star was detected at the location of the magnetar in
Westerlund 1, so astronomers used the VLT to search for it in other
parts of the cluster. They hunted for runaway stars
— objects escaping the cluster at high velocities — that might have
been kicked out of orbit by the supernova explosion that formed the
magnetar. One star, known as Westerlund 1-5 [2], was found to be doing just that.
“Not only does this star have the high velocity expected if it is
recoiling from a supernova explosion, but the combination of its low
mass, high luminosity and carbon-rich composition appear impossible to
replicate in a single star — a smoking gun that shows it must have
originally formed with a binary companion,” adds Ben Ritchie (Open University), a co-author on the new paper.
This discovery allowed the astronomers to reconstruct the stellar
life story that permitted the magnetar to form, in place of the expected
black hole [3].
In the first stage of this process, the more massive star of the pair
begins to run out of fuel, transferring its outer layers to its less
massive companion — which is destined to become the magnetar — causing
it to rotate more and more quickly. This rapid rotation appears to be
the essential ingredient in the formation of the magnetar’s ultra-strong
magnetic field.
In the second stage, as a result of this mass transfer, the companion
itself becomes so massive that it in turn sheds a large amount of its
recently gained mass. Much of this mass is lost but some is passed back
to the original star that we still see shining today as Westerlund 1-5.
“It is this process of swapping material that has imparted the
unique chemical signature to Westerlund 1-5 and allowed the mass of its
companion to shrink to low enough levels that a magnetar was born
instead of a black hole — a game of stellar pass-the-parcel with cosmic
consequences!” concludes team member Francisco Najarro (Centro de Astrobiología, Spain).
It seems that being a component of a double star may therefore be an
essential ingredient in the recipe for forming a magnetar. The rapid
rotation created by mass transfer between the two stars appears
necessary to generate the ultra-strong magnetic field and then a second
mass transfer phase allows the magnetar-to-be to slim down sufficiently
so that it does not collapse into a black hole at the moment of its
death.
Notes
[1] The open cluster Westerlund 1 was
discovered in 1961 from Australia by Swedish astronomer Bengt
Westerlund, who later moved from there to become ESO Director in Chile
(1970–74). This cluster is behind a huge interstellar cloud of gas and
dust, which blocks most of its visible light. The dimming factor is more
than 100 000, and this is why it has taken so long to uncover the true
nature of this particular cluster.
Westerlund 1 is a unique natural laboratory for the study of extreme
stellar physics, helping astronomers to find out how the most massive
stars in the Milky Way live and die. From their observations, the
astronomers conclude that this extreme cluster most probably contains no
less than 100 000 times the mass of the Sun, and all of its stars are
located within a region less than 6 light-years across. Westerlund 1
thus appears to be the most massive compact young cluster yet identified
in the Milky Way galaxy.
All the stars so far analysed in Westerlund 1 have masses at least
30–40 times that of the Sun. Because such stars have a rather short life
— astronomically speaking — Westerlund 1 must be very young. The
astronomers determine an age somewhere between 3.5 and 5 million years.
So, Westerlund 1 is clearly a newborn cluster in our galaxy.
[2] The full designation for this star is Cl* Westerlund 1 W 5.
[3] As stars age, their nuclear reactions change
their chemical make-up — elements that fuel the reactions are depleted
and the products of the reactions accumulate. This stellar chemical
fingerprint is first rich in hydrogen and nitrogen but poor in carbon
and it is only very late in the lives of stars that carbon increases, by
which point hydrogen and nitrogen will be severely reduced — it is
thought to be impossible for single stars to be simultaneously rich in
hydrogen, nitrogen and carbon, as Westerlund 1-5 is.
More information
The research presented in this ESO Press Release will soon appear in the research journal Astronomy and Astrophysics (“A
VLT/FLAMES survey for massive binaries in Westerlund 1: IV.Wd1-5 binary
product and a pre-supernova companion for the magnetar CXOU J1647-45” by J. S. Clark et al.). The same team published a first study of this object in 2006 (“A Neutron Star with a Massive Progenitor in Westerlund 1” by M. P. Muno et al., Astrophysical Journal, 636, L41).
The team is composed of Simon Clark and Ben Ritchie (The Open
University, UK), Francisco Najarro (Centro de Astrobiología, Spain),
Norbert Langer (Universität Bonn, Germany, and Universiteit Utrecht, the
Netherlands) and Ignacio Negueruela (Universidad de Alicante, Spain).
The astronomers used the FLAMES instrument on ESO’s Very Large
Telescope at Paranal, Chile to study the stars in the Westerlund 1
cluster.
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Links
Contacts
Simon ClarkThe Open University
Milton Keynes, United Kingdom
Tel: +44 207 679 4372
Email: jsc@star.ucl.ac.uk
Richard Hook
ESO, La Silla, Paranal and E-ELT Press Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Email: rhook@eso.org
Source: ESO
