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 Post subject: WD 0806-661 B
PostPosted: Thu Oct 20, 2011 10:26 pm 
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Posts: 168
So, they have spotted the "coolest" known brown dwarf. (It must have some very big sunglasses on). http://arxiv.org/PS_cache/arxiv/pdf/110 ... 5411v1.pdf
It is the companion to a white dwarf. It is separated from its companion by 2500 AU. Companion stars are generally coeval. White dwarfs are generally pretty old/advanced in their evolution. So that would mean that the Brown dwarf is also quite old. Brown dwarfs by definition do not have enough mass to have nuclear fusion going on.

My question is this, what is keeping it so warm after such a long time? Its temperature is 300k.


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 Post subject: Re: WD 0806-661 B
PostPosted: Thu Oct 20, 2011 11:22 pm 
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JustinInOz wrote:
Brown dwarfs by definition do not have enough mass to have nuclear fusion going on.

Not exactly, brown dwarfs do not have enough mass for the
normal hydrogen fusion, but they may have deuterium or li-
thium fusion:

http://en.wikipedia.org/wiki/Brown_dwarf


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 Post subject: Re: WD 0806-661 B
PostPosted: Fri Oct 21, 2011 1:02 am 
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The age of the system is listed as 1.5 Ga, which isn't too old - a brown dwarf can still be quite warm after that amount of time. After a couple of billion years though it'll cool off more. They stay warm through gravitational contraction, and radiation of residual heat from their fusion-burning days.

(white dwarfs are evolved, but not necessarily old. They could easily originate from a massive star with a total lifespan of only one or two billion years, and then could be sitting around as WDs for any amount of time up to the age of the universe).

If you subscribe to JTAS Online, I wrote a JTAS article about Brown Dwarfs in 2003 (I'm trying to see if SJG will allow me to release it outside of JTAS).

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 Post subject: Re: WD 0806-661 B
PostPosted: Fri Oct 21, 2011 4:01 pm 
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Jupiter is still warm enough to give off more energy than it absorbs and it is almost 4 billion years old; so brown dwarfs are going to be putting out Infrared radiation for a long time.

Also, 300K is not all that hot; it is below the boiling point of water.

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 Post subject: Re: WD 0806-661 B
PostPosted: Sun Oct 23, 2011 11:14 pm 
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Thanks for the replies guys. I really consider this board a "go to" for the low down on sci fi gaming.

I understand what you mean that it is "not that hot". Sure it won't boil water or compare to the coolest of hydrogen burning stars but, in terms of the back gound temp of 2.7 kelvin, I consider 300k to be searing. It is 2500 AU from the nearest star, which happens to be a white dwarf. The WD won't be heating it much if at all at that range. So it is effectively sitting in deep space. That is what made me think that it might be due to some internal processes, such as the deuteriumn or lithium fusing mentioned.


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