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. Full size image (JPG, 762 KB)An international team of astronomers using the United Kingdom Infrared Telescope (UKIRT) and the Gemini Observatory on Mauna Kea have discovered a unique and exotic star system with a very cool methane-rich brown dwarf (T dwarf) and a dying white dwarf stellar remnant in orbit around each other. The system is a 'Rosetta Stone' for T dwarfs, giving scientists the first good handle on their masses and ages.
This 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 light year or 2.5 trillion km, which is about one quarter of a light year. Despite the frailty of this system, it has stayed together for billions of years, but its stars are cooling down to a dark demise. The system is about 5 billion years old and about 160 light years away from us in the constellation of Virgo.
Methane dwarfs are on the boundary between stars and planets with temperatures typically 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 giant planets like Jupiter. Neither giant planets nor T dwarfs are hot enough for the hydrogen fusion that powers the Sun to take place, so that they simply cool and fade over time. This new T dwarf has a temperature of about 1300 Kelvin (= 1030 Celsius = 1900 Fahrenheit) and a mass of about 70 Jupiters.
White dwarfs are the end state of stars similar to and including the Sun. Once such stars have exhausted the available hydrogen fuel in their cores, they expel most of their outer layers into space forming a remnant planetary nebula and leaving behind a small, dense, hot, but cooling core or white dwarf. For our Sun this process will begin about 5 billion years in the future.
White dwarfs are the end state of stars similar to and including the Sun. Once such stars have exhausted the available hydrogen fuel in their cores, they expel most of their outer layers into space forming a remnant planetary nebula and leaving behind a small, dense, hot, but cooling core or white dwarf. For our Sun this process will begin about 5 billion years in the future.
"By the time our Sun 'dies' and becomes a white dwarf itself, the methane dwarf will have cooled to around room temperature, and the white dwarf will be as cool as the methane dwarf was at the start of its life", comments team leader Dr Avril Day-Jones from the Universidad de Chile.
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. This binary system is providing a crucial test of the physics of ultra-cool stellar atmospheres because the white dwarf lets us establish the age of both objects. By comparison it determines 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 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 we know from the current age of the white dwarf 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 in the maelstrom of the galactic disk.
This system was discovered by an international team led by Dr Avril Day-Jones from the Universidad de Chile, with astronomers from the University of Hertfordshire (UK), and the University of Montreal (Canada). The methane dwarf was identified in the UKIRT Infrared Deep Sky Survey (UKIDSS) as part of the Large Area Survey's T-Dwarf Programme to identify the coolest objects in the galaxy. Its temperature and spectrum were measured by the Gemini North Telescope's NIRI Spectrometer in Hawaii.
The team 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 renamed LSPM 1459+0857 A and B.
"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" said Dr David Pinfield of the University of Hertfordshire. "The fact that these binaries survive intact for billions of years means that we could find many more lurking out there in the future."
The team, led by Dr Avril Day-Jones of the Universidad de Chile and including Dr David Pinfield of the University of Hertfordshire, will publish their results in the journal Monthly Notices of the Royal Astronomical Society.
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