Star on the Run (Forwarded)

ESO Education and Public Relations Dept.

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Contacts:

Ralf Napiwotzki
Centre for Astrophysics Research
University of Hertfordshire, UK
Phone: +44 1707 286071

Uli Heber
Dr. Remeis-Sternwarte
Astronomisches Institut der Universität Erlangen-Nürnberg
Bamberg, Germany
Phone: +49 951 9522214

For immediate release: 9 November 2005

ESO Press Release 27/05

Star on the Run

Speeding Star Observed with VLT Hints at Massive Black Hole

Observations with Kueyen, one of the 8.2m telescopes composing
the ESO Very Large Telescope (VLT), have led to the discovery
of a short-lived massive star that is moving at a very high
speed through the outer halo of the Milky Way galaxy and into
intergalactic space. This finding could provide evidence for
a previously unknown massive black hole in the heart of the
Milky Way's closest neighbour, the Large Magellanic Cloud.

The star, named HE 0437-5439, was discovered by the Hamburg/ESO
sky survey [1] , a project aimed at detecting quasars but which
discovered many faint blue stars as well. Scientists [2] at the
Dr. Remeis-Sternwarte (University of Erlangen-Nürnberg, Germany)
and the Centre for Astrophysics Research (University of
Hertfordshire, UK) found what is likely to be a hot massive
main-sequence star, far out in the halo.

This came as a great surprise. Massive stars have lifetimes of
only some tens or hundreds of million years, short lived for
astronomical standards, but the halo does not usually host
stars as young as this. In fact, it contains the oldest stars
in the Milky Way that are more than ten billion years old.
Massive stars are usually found in or near star forming
regions in the Galactic disc such as the famous Orion nebula:
HE 0437-5439 is indeed similar to the trapezium stars that
make the Orion nebula shine.

Data were obtained with the ESO VLT and its high resolution
UVES spectrograph. This allowed the chemical composition to
be measured which turned out to be similar to that of the Sun,
confirming that HE0437-5439 is a young star. Its mass is eight
times larger than that of the Sun and the star is only 30
million years old. It is almost 200,000 light years away from
us in the direction of the Doradus Constellation ("the
Swordfish").

Even more exciting was the fact that the data indicated the
star to be receding at a velocity of 723 km/s, or 2.6 million
kilometres per hour. HE0437-5439 moves so fast that the
gravitational attraction of the Milky Way is too small to
keep it bound to the Galaxy. Hence the hyper-velocity star
will escape into intergalactic space.

As the star is moving so fast, it must have been born far away
from its present position and accelerated to where we observe
it today. What accelerated the star to such a high speed?
Calculations carried out already in the late 1980s showed that
a so-called massive black hole (SMBH), i.e. a black hole a
million times as massive as the Sun, or larger, could provide
the enormous acceleration. If a binary star approaches the SMBH,
one star falls towards the SMBH while its companion is ejected.
The Galactic Centre of the Milky Way hosts such a black hole of
about 2.5 million solar masses, and this might have accelerated
HE0437-5439.

But the necessary travel time was found to be more than three
times the age of the star. Hence the star is too young to have
travelled all the way from the Galactic centre to its present
location. Either the star is older than it appears or it was
born and accelerated elsewhere.

ESO PR Photo 35/05
Star Ejected from the Large Magellanic Cloud (Artist's View)

A different clue to the origin of HE0457-5439 comes from its
position in the sky. HE0437-5439 is 16 degrees away from the
Large Magellanic Cloud (LMC), one of the nearest neighbouring
galaxies to the Milky Way. This galaxy lies at a distance of
156,000 light years. HE0457-5439 is even more distant than the LMC and is
much closer to the LMC than to the galaxy. The
astronomers showed that the star could have reached its present
position within its lifetime if it were ejected from the centre
of the LMC. This, in turn, would provide evidence for the
existence of a SMBH in the LMC.

Another explanation would require the star to be the result
of the merging of two stars, belonging to so-called blue
stragglers class of stars, which are older than standard
evolution models predict them to be. Indeed, its age could then
be as much as the lifetime of a 4 solar mass star which is more
than 6 times the lifetime of an 8 solar mass star.

The astronomers propose two additional observations to
distinguish between the two options. The abundance of certain
elements in stars belonging to the LMC is only half that of the
Sun. A more precise measurement with UVES would indicate whether
the star has a metal abundance appropriate to LMC stars or not.
The second is to measure how much the star moves in the
transverse direction on the sky, using astrometric measurements.

The research presented here is detailed in a paper to be
published in Astrophysical Journal Letters.

Notes

[1]: The Hamburg/ESO sky survey is a collaborative project of
the Hamburger Sternwarte and ESO to provide spectral information
for half of the southern sky using photographic plates taken
with the now retired ESO-Schmidt telescope. These plates were
digitized at Hamburger Sternwarte.

[2]: The astronomers are Heinz Edelmann (Dr. Remeis-Sternwarte
of the University of Erlangen-Nürnberg, Germany, now at
University of Texas, Austin, USA), Ralf Napiwotzki (Centre for
Astrophysics Research, University of Hertfordshire, UK), Uli
Heber (Dr. Remeis-Sternwarte of the University of Erlangen-
Nürnberg, Germany), Norbert Christlieb and Dieter Reimers
(Hamburger Sternwarte, Germany).

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