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Astronomers find 'Rosetta Stone' for T-dwarf stars

Last Updated on Monday, 22 November 2010 12:55
Published on Tuesday, 23 November 2010 00:01

An international team of astronomers have discovered a unique and exotic star system with a very cool methane-rich (or T-) dwarf star and a 'dying' white dwarf stellar remnant in orbit around each other. The system is a 'Rosetta Stone' for T-dwarf stars, giving scientists the first good handle on their mass and age.

Methane_dwarf
An artist's impression of the binary as it might appear from a point in space near the methane dwarf. From here the distant white dwarf would appear as a bright star, only gently illuminating its cooling companion. Credit: Andrew McDonagh
The team, led by Dr Avril Day-Jones of the Universidad de Chile and including Dr David Pinfield of the University of Hertfordshire as well as astronomers from the University of Montreal, publish their results in the journal Monthly Notices of the Royal Astronomical Society.

The system is the first of its type to be found. The two stars are low in mass and have a weak mutual gravitational attraction as they are separated by about a quarter of a light year or 2.5 trillion km (to put this in context Neptune is only 4.5 billion km from the Sun). Despite the frailty of the system it has stayed together for billions of years, but its stars are cooling down to a dark demise.

Methane dwarfs are on the star / planet boundary and are about the size of the giant planet Jupiter. They have temperatures of less than 1000 degrees Celsius (in comparison the Sun's surface is at 5500 degrees Celsius). Methane is a fragile molecule destroyed at warmer temperatures, so is only seen in very cool stars and objects like Jupiter. Neither giant planets nor T-dwarf stars are hot enough for the hydrogen fusion that powers the Sun to take place, meaning that they simply cool and fade over time.

White dwarfs are the end state of stars similar to and including the Sun. Once such stars have exhausted the available nuclear fuel in their cores, they expel most of their outer layers into space forming a remnant planetary nebula and leaving behind a hot, but cooling core or white dwarf about the size of the Earth. For our Sun this process will begin about 5 billion years in the future.

In the newly-discovered binary, the remnant nebula has long since dissipated and all that is left is the cooling white dwarf and methane dwarf pair.

Dr Day-Jones puts this in context, commenting, "In about 6 billion years' time, when our Sun 'dies' and becomes a white dwarf itself, the stars in the newly-discovered system will have changed dramatically. The methane dwarf will have cooled to around room temperature, and the white dwarf will have cooled to 2700 Celsius or the temperature of the methane dwarf at the start of its life".

This binary is providing a crucial test of the physics of ultra-cool (temperatures less than 1000 degrees Celsius) stellar atmospheres because the white dwarf lets us establish the age of both objects. It calibrates properties of the methane dwarf such as its mass, making it a kind of 'Rosetta Stone' for similar stars with complex, hazy ultra-cool atmospheres.

The methane dwarf was identified in the UKIRT Infrared Deep Sky Survey (UKIDSS) as part of a project to identify the coolest objects in the galaxy. Its temperature and spectrum were measured by the Gemini North Telescope in Hawaii.

The team then found that the methane dwarf shares its motion across the sky with a nearby blue object catalogued as LSPM 1459+0857. They studied the blue object using the world's largest optical telescope, the European Southern Observatory's Very Large Telescope (VLT) in Chile. The new VLT observations revealed the blue object to be a cool white dwarf and companion to the methane dwarf. The objects were thus re-christened LSPM 1459+0857 A and B.

The two stars are today separated by at least 2.5 trillion km, but would have been closer in the past before the white dwarf was formed. Once the star that formed the white dwarf reached the end of its life and expelled its outer layers, the loss of mass weakened the gravitational pull between the stars, causing the methane dwarf to spiral outwards to create the gravitationally fragile system that we see today. But the current age of the white dwarf indicates that this system has survived for several billion years. So the new discovery shows that despite their fragility, such binaries are able to remain united even as they move through the maelstrom of the disc of our Galaxy.

"Binary systems like this provide vital information and allow us to better understand ultra-cool atmospheres and the very low-mass dwarfs and planets they enshroud" says Dr Pinfield. "The fact that these binaries survive intact for billions of years means that we could find many more lurking out there in the future."

Contacts

 

Dr David Pinfield
University of Hertfordshire
Tel: +44 (0)1707 284 171
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Dr Avril Day-Jones
Universidad de Chile
Tel: +56 (0)2 977 1139
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Joana Gomes
University of Hertfordshire
Mob: +44 (0)756 427 1176
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Dr Robert Massey
Royal Astronomical Society
Tel: +44 (0)20 7734 3307 x 214
Mob: +44 (0)794 124 8035
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Inge Heyer
Public Information Officer
Joint Astronomy Centre
Hawaii
Tel: +1 808 969 6524
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Julia Maddock
Senior Press Officer
Science and Technology Facilities Council
Tel: +44 (0)1793 442094
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Further information

 

The discovery is due to be published in the Monthly notices of the Royal Astronomical Society. A preprint of the paper can be seen at http://arxiv.org/abs/1008.2960

Images and associated captions are available on the UKIRT website at http://outreach.jach.hawaii.edu/pressroom/2010_ukirt_methane_bd/index.html

Notes for editors

 

Light year

One light year is about 10 million million (10 trillion) kilometres. This is the distance light travels in a year.

Infrared Light

Infrared radiation is emitted at longer wavelengths than visible light waves. They are typically measured in microns, also called micrometres. One micron is one millionth of a metre, one 10000th of a centimetre. Visible light has wavelengths around half a micron, while the observations reported here were at wavelengths of about 2 microns. Human eyes are not sensitive to infrared light. We need specially designed cameras with detectors sensitive to infrared radiation to detect them.

Brown Dwarf (T Dwarf)

A brown dwarf is a small, faint, cool object (often called a 'failed' star) that, unlike the Sun and other stars, does not have sufficient mass to achieve hydrogen fusion in its core. With mostly slow gravitational contraction as an internal energy source, a brown dwarf gradually cools down as it radiates energy away into space over billions of years. Brown dwarfs exist in the mass range between about ten times that of Jupiter and one-twelfth the Sun's mass (which marks the boundary between these dwarfs and hydrogen-burning stars). The low temperatures and small sizes of brown dwarfs combine to make them both very faint and red in colour. Most of their radiation is in the infrared, and therefore is not detectable to either the human eye or conventional optical detectors. Detectors sensitive to longer infrared wavelengths, such as those used at UKIRT, are capable of observing these objects in unique ways. The spectrum of a brown dwarf is characterized by large wavelength regions from which almost no light is seen because it is being absorbed by water, methane and other molecules in the object's atmosphere. The details of these absorption patterns depend sensitively on the star's temperature. The T dwarfs are brown dwarfs with the lowest temperatures.

Royal Astronomical Society

The Royal Astronomical Society (RAS: www.ras.org.uk), founded in 1820, encourages and promotes the study of astronomy, solar-system science, geophysics and closely related branches of science. The RAS organizes scientific meetings, publishes international research and review journals, recognizes outstanding achievements by the award of medals and prizes, maintains an extensive library, supports education through grants and outreach activities and represents UK astronomy nationally and internationally. Its more than 3500 members (Fellows), a third based overseas, include scientific researchers in universities, observatories and laboratories as well as historians of astronomy and others.

United Kingdom Infrared Telescope (UKIRT)

One of the world's largest telescopes dedicated solely to infrared astronomy, the 3.8-metre United Kingdom Infrared Telescope (UKIRT) is sited near the summit of Mauna Kea, Hawaii, at an altitude of 4194 metres above sea level. It is operated by the Joint Astronomy Centre in Hilo, Hawaii, on behalf of the UK Science and Technology Facilities Council. UKIRT's technical innovation and privileged position on the high, dry Mauna Kea site have placed it at the forefront of infrared astronomy since its opening in 1979. UKIRT is currently engaged in a world-leading infrared sky survey as well as the type of innovative individual programmes described in this press release.

More about the UK Infrared Telescope: http://outreach.jach.hawaii.edu/articles/aboutukirt/

Wide-Field Camera (WFCAM)

The Wide-Field Camera (WFCAM) was delivered to UKIRT in late 2004 and has been in active operation since spring 2005. In two years of operation WFCAM has taken 30 times the amount of data taken in the entire 25-year history of the telescope before its arrival.

Gemini Observatory

The Gemini Observatory is an international collaboration with two identical 8-meter telescopes. The Frederick C. Gillett Gemini Telescope is located at Mauna Kea, Hawai'i (Gemini North) and the other telescope at Cerro Pachon in central Chile (Gemini South) and hence together they provide full coverage of both hemispheres of the sky. Both telescopes incorporate new technologies that allow large, relatively thin mirrors under active control to collect and focus both optical and infrared radiation from space. More about Gemini Observatory: www.gemini.edu

The Gemini Observatory provides the astronomical communities in each partner country with state-of-the-art astronomical facilities that allocate observing time in proportion to each country's contribution. In addition to financial support, each country also contributes significant scientific and technical resources. The national research agencies that form the Gemini partnership include: the US National Science Foundation (NSF), the UK Science and Technology Facilities Council, the Canadian National Research Council (NRC), the Chilean Comision Nacional de Investigacion Cientifica y Tecnologica (CONICYT), the Australian Research Council (ARC), the Argentinean Consejo Nacional de Investigaciones Cientificas y Tecnicas (CONICET) and the Brazilian Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq). The Observatory is managed by the Association of Universities for Research in Astronomy, Inc. (AURA) under a cooperative agreement with the NSF. The NSF also serves as the executive agency for the international partnership.

Science and Technology Facilities Council

The Science and Technology Facilities Council (www.stfc.ac.uk) is an independent, non-departmental public body of the Office of Science and Innovation which itself is part of the Department of Business, Innovation and Skills. It was formed as a new Research Council on 1 April 2007 through a merger of the Council for the Central Laboratory of the Research Councils (CCLRC) and the Particle Physics and Astronomy Research Council (PPARC) and the transfer of responsibility for nuclear physics from the Engineering and Physical Sciences Research Council (EPSRC). We are one of seven national research councils in the UK. The Science and Technology Facilities Council is government funded and provides research grants and studentships to scientists in British universities, gives researchers access to world-class facilities and funds the UK membership of international bodies such as the European Organisation for Nuclear Research, CERN, the European Space Agency and the European Southern Observatory. It also contributes money for the UK telescopes overseas on La Palma, Hawaii, Australia and in Chile, the UK Astronomy Technology Centre at the Royal Observatory, Edinburgh and the MERLIN/VLBI National Facility.

European Southern Observatory (ESO)

ESO is the pre-eminent intergovernmental science and technology organisation in astronomy. It carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities for astronomy to enable important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. The ESO headquarters are located in Garching, near Munich, Germany. This is the scientific, technical and administrative centre of ESO where technical development programmes are carried out to provide the observatories with the most advanced instruments. ESO also hosts the European Coordinating Facility for the Hubble Space Telescope, a collaboration between ESA and NASA. Read more about ESO at http://www.eso.org