Co-moving Y dwarf companion to white dwarf WD 0806-661


Image credits

This image shows just how difficult it is to pick out faint brown dwarf companions against the stellar background, in this case by measurement of proper motion to confirm the association of the companion with the white dwarf, over a five year baseline. Lead author Kevin Luhman on this 2011 discovery:

“This planet-like companion is the coldest object ever directly photographed outside our solar system,” said Luhman, who led the discovery team. “Its mass is about the same as many of the known extra-solar planets — about six to nine times the mass of Jupiter — but in other ways it is more like a star. Essentially, what we have found is a very small star with an atmospheric temperature about cool as the Earth’s.”

Two further papers further clarify the nature of the companion, which is also known as GJ 3483b. It has an effective temperature in the range 300 – 345K, and a mass less than 10 – 13 Jupiter masses. It is also the reddest brown dwarf known to date.
Luhman-BrownDwarf-JanellaWilliams

Today a new, similar object, WISE 1828+2650, has been reported, a Y2 dwarf with an estimated temperature of 250 – 400K. The two objects are compared in the paper. Unlike the companion to WD 0806-661, where the age is known from the age of the white dwarf (about 1.5 Gyr) there is no age constraint, so the mass could lie anywhere between 0.5 and 20 Jupiter masses for ages 0.1 – 10 Gyr. Comparison with theoretical models gives a mass 3 – 6 Jupiter masses for a plausible age between 2 and 4 Gyr. This estimate is backed by the high tangential motion of the object. Whether WISE 1828+2650 sits at the low mass end of the brown dwarf population or is the first example of a large number of “free-floating” planets is not yet known. If such a population exists, it may yet be discoverable at lower signal-to-noise ratios in the WISE data. I caution that the status of these objects is still uncertain. From the new paper:

The absolute magnitude of WISE 1828+2650 is brighter by several magnitudes (depending on wavelength) than extrapolated from other Y dwarfs making the true nature of this source somewhat of a mystery. WISE 1828+2650 is similar in color and absolute magnitude to the cool object orbiting the nearby white dwarf WD 0806-661 B. The exact nature and evolutionary state, including its mass and age, of WISE 1828+2650 will require further observation and theoretical investigation.

On the subject of a population of isolated planetary mass objects, the young (3 Myr) sigma Orionis cluster was one of the first hunting grounds for such objects, which are brighter at young ages for a given mass. As reported in another new paper today, the number of planetary mass objects (4 – 12 Jupiter masses) known in the region has increased by 23 to 37, together with 69 brown dwarfs.

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One Response to “Co-moving Y dwarf companion to white dwarf WD 0806-661”

  1. […] dwarfs are also few in number. Prior to the discovery of the Y dwarf companion to WD 0806-661 which I drew attention to a few days ago, one wide T dwarf companion is also in the literature, the companion to the high proper motion […]

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