R136 observed with WFC3
Pseudo image of R136
Astronomers using the unique ultraviolet
capabilities of the NASA/ESA Hubble Space Telescope have identified nine
monster stars with masses over 100 times the mass of the Sun in the
star cluster R136. This makes it the largest sample of very massive
stars identified to date. The results, which will be published in the
Monthly Notices of the Royal Astronomical Society, raise many new
questions about the formation of massive stars.
An international team of scientists using the NASA/ESA Hubble Space Telescope has combined images taken with the
Wide Field Camera 3 (WFC3) with the unprecedented ultraviolet spatial resolution of the
Space Telescope Imaging Spectrograph (STIS) to successfully dissect the young star cluster
R136 in the ultraviolet for the first time
[1].
R136 is only a few light-years across and is located in the
Tarantula Nebula within the
Large Magellanic Cloud,
about 170 000 light-years away. The young cluster hosts many extremely
massive, hot and luminous stars whose energy is mostly radiated in the
ultraviolet [2]. This is why the scientists probed the ultraviolet emission of the cluster.
As well as finding dozens of stars exceeding 50 solar masses, this
new study was able to reveal a total number of nine very massive stars
in the cluster, all more than 100 times more massive as the Sun.
However, the current record holder R136a1 does keep its place as the
most massive star known in the Universe, at over 250 solar masses. The
detected stars are not only extremely massive, but also extremely
bright. Together these nine stars outshine the Sun by a factor of 30
million.
The scientists were also able to investigate outflows from these
behemoths, which are most readily studied in the ultraviolet. They eject
up to an Earth mass of material per month at a speed approaching one
percent of the speed of light, resulting in extreme weight loss
throughout their brief lives.
“The ability to distinguish ultraviolet light from such an
exceptionally crowded region into its component parts, resolving the
signatures of individual stars, was only made possible with the
instruments aboard Hubble,” explains Paul Crowther from the University of Sheffield, UK, and lead author of the study. “Together
with my colleagues, I would like to acknowledge the invaluable work
done by astronauts during Hubble’s last servicing mission: they restored
STIS and put their own lives at risk for the sake of future science!” [3]
In 2010 Crowther and his collaborators
showed
the existence of four stars within R136, each with over 150 times the
mass of the Sun. At that time the extreme properties of these stars came
as a surprise as they exceeded the upper-mass limit for stars that was
generally accepted at that time. Now, this new census has shown that
there are five more stars with more than 100 solar masses in R136. The
results gathered from R136 and from other clusters also raise many new
questions about the formation of massive stars as the origin of these
behemoths remains unclear
[4].
Saida Caballero-Nieves, a co-author of the study, explains: “There
have been suggestions that these monsters result from the merger of
less extreme stars in close binary systems. From what we know about the
frequency of massive mergers, this scenario can’t account for all the
really massive stars that we see in R136, so it would appear that such
stars can originate from the star formation process.”
In order to find answers about the origin of these stars the team
will continue to analyse the gathered datasets. An analysis of new
optical STIS observations will also allow them to search for close
binary systems in R136, which could produce massive black hole binaries
which would ultimately merge, producing gravitational waves.
“Once again, our work demonstrates that, despite being in orbit
for over 25 years, there are some areas of science for which Hubble is
still uniquely capable,” concludes Crowther.
Notes
[1] R136 was originally listed in a catalogue of the brightest stars
in the Magellanic Clouds compiled at the Radcliffe Observatory in South
Africa. It was separated into three components a, b, c at the European Southern Observatory,
with R136a subsequently resolved into a group of eight stars (a1-a8) at
ESO, and confirmed as a dense star cluster with the NASA/ESA Hubble
Space Telescope after the first servicing mission in 1993.
[2] Very massive stars are exclusive
to the youngest star clusters because their lifetimes are only 2-3
million years. Only a handful of such stars are known in the entire
Milky Way galaxy.
[3] STIS’s capabilities were restored in 2009 by astronauts who successfully comp
leted Serving Mission 4 (
SM4), one of the Hubble’s most challenging and intense servicing missions, involving five spacewalks.
[4] The ultraviolet signatures of even
more very massive stars have also been revealed in other clusters —
examples include star clusters in the dwarf galaxies NGC 3125 and NGC 5253. However, these clusters are too distant for individual stars to be distinguished even with Hubble.
More Information
The Hubble Space Telescope is a project of international cooperation between ESA and NASA.
The results were published in the paper “The R136 star cluster
dissected with Hubble Space Telescope/STIS. I. Far-ultraviolet
spectroscopic census and the origin of Heii λ1640 in young star
clusters” in the Monthly Notices of the Royal Astronomical Society.
The international team of astronomers in this study consists of Paul
A. Crowther (Department of Physics and Astronomy, University of
Sheffield, Sheffield, UK), S.M. Caballero-Nieves(Department of Physics
and Astronomy, University of Sheffield, Sheffield, UK), K.A. Bostroem
(Space Telescope Science Institute, Baltimore MD, USA; Department of
Physics, University of California, Davis CA, USA), J. Maíz Apellániz
(Centro de Astrobiología, CSIC/INTA, Madrid, Spain), F.R.N. Schneider
(Department of Physics, University of Oxford, Oxford, UK;
Argelanger-Institut fur Astronomie der Universität Bonn, Bonn, Germany),
N.R. Walborn(Space Telescope Science Institute, Baltimore MD, USA),
C.R. Angus (Department of Physics and Astronomy, University of
Sheffield, Sheffield, UK; Department of Physics, University of Warwick,
Coventry, UK), I. Brott (Institute for Astrophysics, Vienna, Austria),
A. Bonanos (Institute of Astronomy & Astrophysics, National
Observatory of Athens, P. Penteli, Greece), A. de Koter (Astronomical
Institute Anton Pannekoek, University of Amsterdam, Amsterdam,
Netherlands; Institute of Astronomy, Leuven, Belgium), S.E. de Mink
(Astronomical Institute Anton Pannekoek, University of Amsterdam,
Amsterdam, Netherlands), C.J. Evans (UK Astronomy Technology Centre,
Royal Observatory Edinburgh, Edinburgh, UK), G. Gräfener (Armagh
Observatory, Armagh, UK), A. Herrero (Instituto de Astrofísica de
Canarias, La Laguna, Tenerife, Spain; Departamento de Astrofísica,
Universidad de La Laguna, La Laguna, Tenerife, Spain), I.D. Howarth
(Department of Physics & Astronomy, University College London,
London, UK), N. Langer (Argelanger-Institut fur Astronomie der
Universität Bonn, Bonn, Germany), D.J. Lennon (European Space Astronomy
Centre, ESA, Villanueva de la Cañada, Madrid, Spain), J. Puls
(Universitäts-Sternwarte, Munchen, Germany), H. Sana (Space Telescope
Science Institute, Baltimore MD, USA; Institute of Astronomy, Leuven,
Belgium), J.S. Vink (Armagh Observatory, Armagh, UK).
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