Astronomers discover ‘Rosetta Stone’ for T-Dwarf stars

Tuesday, November 23, 2010

WASHINGTON - 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.

The team, led by 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).

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.

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.

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.

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”.

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. T

The two stars are 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.

The findings were published in the journal Royal Astronomical Society. (ANI)

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