Using the Gemini Multi-Object Spectrograph (GMOS) on Gemini South, a
team led by Jay Farihi (University College London) found, for the first
time, a dust and debris disk surrounding a binary star with a white
dwarf as a substellar companion. To date, almost all of the known
planetary systems which include a white dwarf are single stars. Using
GMOS spectra Farihi et al. identified critical metal features in
the spectrum as well as the higher Balmer lines. From the Gemini data
the team estimated a surface temperature of 21,800 Kelvin (about 3.5
times hotter than the Sun) and a mass of ~0.4 solar masses for the white
dwarf star and a mass of ~0.063 solar masses for the companion.
The following text is provided verbatim from the University College London press release:
Evidence of planetary debris surrounding a double sun, ‘Tatooine-like’
system has been found for the first time by a UCL-led team of
researchers.
Published today in Nature Astronomy and funded by the Science and
Technology Facilities Council and the European Research Council, the
study finds the remains of shattered asteroids orbiting a double sun
consisting of a white dwarf and a brown dwarf roughly 1000 light-years
away in a system called SDSS 1557.
The discovery is remarkable because the debris appears to be rocky and
suggests that terrestrial planets like Tatooine – Luke Skywalker’s home
world in Star Wars – might exist in the system. To date, all exoplanets
discovered in orbit around double stars are gas giants, similar to
Jupiter, and are thought to form in the icy regions of their systems.
In contrast to the carbon-rich icy material found in other double star
systems, the planetary material identified in the SDSS 1557 system has a
high metal content, including silicon and magnesium. These elements
were identified as the debris flowed from its orbit onto the surface of
the star, polluting it temporarily with at least 1017 g (or 1.1 trillion
US tons) of matter, equating it to an asteroid at least 4 km in size.
Lead author, Dr Jay Farihi (UCL Physics & Astronomy), said:
“Building rocky planets around two suns is a challenge because the
gravity of both stars can push and pull tremendously, preventing bits of
rock and dust from sticking together and growing into full-fledged
planets. With the discovery of asteroid debris in the SDSS 1557 system,
we see clear signatures of rocky planet assembly via large asteroids
that formed, helping us understand how rocky exoplanets are made in
double star systems."
In the Solar System, the asteroid belt contains the leftover building
blocks for the terrestrial planets Mercury, Venus, Earth, and Mars, so
planetary scientists study the asteroids to gain a better understanding
of how rocky, and potentially habitable planets are formed. The same
approach was used by the team to study the SDSS 1557 system as any
planets within it cannot yet be detected directly but the debris is
spread in a large belt around the double stars, which is a much larger
target for analysis.
The discovery came as a complete surprise, as the team assumed the dusty
white dwarf was a single star but co-author Dr Steven Parsons
(University of Valparaíso and University of Sheffield), an expert in
double star (or binary) systems noticed the tell-tale signs. "We know
of thousands of binaries similar to SDSS 1557 but this is the first time
we've seen asteroid debris and pollution. The brown dwarf was
effectively hidden by the dust until we looked with the right
instrument", added Parsons, "but when we observed SDSS 1557 in detail we
recognised the brown dwarf's subtle gravitational pull on the white
dwarf."
The team studied the binary system and the chemical composition of the
debris by measuring the absorption of different wavelengths of light or
‘spectra’, using the Gemini Observatory South telescope and the European
Southern Observatory Very Large Telescope, both located in Chile.
Co-author Professor Boris Gänsicke (University of Warwick) analysed
these data and found they all told a consistent and compelling story.
"Any metals we see in the white dwarf will disappear within a few weeks,
and sink down into the interior, unless the debris is continuously
flowing onto the star. We'll be looking at SDSS 1557 next with Hubble,
to conclusively show the dust is made of rock rather than ice."
Notes to Editors
For more information or to speak to the researchers involved, please
contact Dr Rebecca Caygill, UCL press office. T: +44 (0)20 3108 3846 /
+44 (0)7733 307 596, E: r.caygill@ucl.ac.uk
J. Farihi, S. G. Parsons, B. T. Gansicke, ‘A circumbinary debris disk in
a polluted white dwarf system’ will be published by Nature Astronomy at
1600 London time / 1100 US Eastern Time on 27 February 2017 and is
under a strict embargo until then. DOI: 10.1038/s41550-016-0032.
About UCL (University College London)
UCL was founded in 1826. We were the first English university
established after Oxford and Cambridge, the first to open up university
education to those previously excluded from it, and the first to provide
systematic teaching of law, architecture and medicine. We are among the
world's top universities, as reflected by performance in a range of
international rankings and tables. UCL currently has over 38,000
students from 150 countries and over 12,000 staff. Our annual income is
more than £1 billion.
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About the University of Warwick
The University of Warwick is consistently ranked in the top 10
universities in the UK and top 100 in the world. It is one of the UK's
leading universities, with an acknowledged reputation for excellence in
research, teaching and innovation, alongside pioneering links with
business and industry.
About the University of Sheffield
With almost 27,000 of the brightest students from over 140 countries,
learning alongside over 1,200 of the best academics from across the
globe, the University of Sheffield is one of the world’s leading
universities.
A member of the UK’s prestigious Russell Group of leading research-led
institutions, Sheffield offers world-class teaching and research
excellence across a wide range of disciplines.
Unified by the power of discovery and understanding, staff and students
at the university are committed to finding new ways to transform the
world we live in.
Sheffield is the only university to feature in The Sunday Times 100 Best
Not-For-Profit Organisations to Work For 2016 and was voted number one
university in the UK for Student Satisfaction by Times Higher Education
in 2014. In the last decade it has won four Queen’s Anniversary Prizes
in recognition of the outstanding contribution to the United Kingdom’s
intellectual, economic, cultural and social life.
Sheffield has six Nobel Prize winners among former staff and students
and its alumni go on to hold positions of great responsibility and
influence all over the world, making significant contributions in their
chosen fields.
Global research partners and clients include Boeing, Rolls-Royce,
Unilever, AstraZeneca, Glaxo SmithKline, Siemens and Airbus, as well as
many UK and overseas government agencies and charitable foundations.
About the Science and Technology Facilities Council (STFC)
The Science and Technology Facilities Council is keeping the UK at the
forefront of international science and tackling some of the most
significant challenges facing society such as meeting our future energy
needs, monitoring and understanding climate change, and global security.
The Council has a broad science portfolio including supporting UK work
in space and ground-based astronomy technologies and research. http://www.stfc.ac.uk/
Source: Gemini Observatory
