A ‘Benchmark’ Brown Dwarf

by Paul Gilster on November 23, 2010

The WISE mission has received a lot of press in terms of discovering nearby brown dwarfs, but it’s clear that finding low-temperature objects is a major investigation at many Earth-bound sites as well. That includes the UKIRT (United Kingdom Infrared Telescope) Deep Sky Survey’s project to find the coolest objects in our galaxy, an effort that has paid off in the form of a unique binary system. One of the stars here is a cool, methane-rich T-dwarf, while the other is a white dwarf, the two low-mass stars orbiting each other though separated by a quarter of a light year.

Understanding Brown Dwarf Atmospheres

We need to put this find in context. In the absence of hydrogen fusion at the core, brown dwarfs depend upon gravitational contraction as their internal energy source. Cooling slowly over time as they shed their energies, brown dwarfs emit most of their radiation in the infrared, with spectra showing absorption bands of water, methane, carbon monoxide and other molecules in the stellar atmosphere, the absorption patterns being dependent on the star’s temperature. And in the study of brown dwarfs, what’s going on in that atmosphere has a lot to say about what we can surmise.

A number of surveys, ranging from the 2-Micron All Sky Survey to the UKIRT attempt and the Sloan Digital Sky Survey have been identifying brown dwarfs and pushing our knowledge down into the range of very low temperature stars. But the paper on the binary find notes the fact that many of the processes going on in brown dwarf atmospheres are not well understood, adding:

…the nature of BD [brown dwarf] evolution means that the mass-luminosity relation depends strongly on age, and in the absence of well constrained atmospheric properties there is no way to accurately determine mass and age… Identifying objects where one can pin down these properties independently can help aid the calibration of models.

But we do know a good deal about white dwarfs, so finding brown dwarfs in association with white dwarfs is helpful. Only a few such binaries have been identified — five, to be precise — and these five pair white dwarfs with the somewhat warmer L-dwarfs. The new binary system is the first discovery of a T-dwarf in association with a white dwarf, and all indications are that it has survived for close to 5 billion years, the wide separation reflecting the loss of mass as the white dwarf expelled its outer layers and thus weakened the gravitational pull between the stars.

The Beauty of a Binary

What we have, then, is a look into the physics of ultra-cool stellar temperatures (temperatures less than 1000 degrees Celsius), with the white dwarf establishing the age of both objects. UKIRT scientists are referring to the find as a ‘Rosetta stone’ for methane dwarfs like the one in this system.The T-dwarf is about Jupiter-size and, like the gas giant, is too cool to power up hydrogen fusion, so that the star becomes cooler and cooler over time. The white dwarf companion is a star that, having used up its nuclear fuel, has expelled its outer layers, leaving a cooling core about the size of the Earth, in a process that will eventually happen to our Sun.

What we have in the new binary is a system in which the so-called ‘planetary’ nebula formed by white dwarf material has fully dissipated over time, leaving us with the two widely spaced stars. Says Avril Day-Jones (Universidad de Chile):

“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 twin objects are now known as LSPM 1459+0857 A and B, a binary that has held together despite the perturbations of the white dwarf’s history and the system’s own passage through the galactic disk. The paper notes that “This system is an example of how wide BD binary companions to white dwarfs make good benchmark objects, which will help test model atmospheres, and may provide independent means to calibrate BD properties of field objects.”

And although the binary is the first candidate system under study by the UKIRT team, the expectation is that many more will be found by combining the brown dwarf search with survey results on white dwarfs from the Sloan Digital Sky Survey. The paper calls for follow-up parallax measurements of the two components and fuller spectral studies of the T-dwarf, which would improve our estimates of the system’s age, peg the radius and mass of the white dwarf, and thus maximize the effectiveness of the benchmark provided by the cool brown dwarf.

The paper is Day-Jones et al., “Discovery of a T dwarf + white dwarf binary system,” accepted by Monthly Notices of the Royal Astronomical Society (preprint).

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{ 17 comments }

Istvan November 23, 2010 at 13:55

This fascinating material seems to me to beg the interesting questions of how the T dwarf would have been affected by the changes to the primary star during its evolution off the main sequence, and by immersion in the planetary nebula for some considerable length of time. Really good stuff, and as always, thanks for sharing it!

Michael November 23, 2010 at 15:35

I thought brown dwarfs were not stars

‘…..with spectra showing absorption bands of water, methane, carbon monoxide and other molecules in the stellar atmosphere, the absorption patterns being dependent on the star’s temperature. And in the study of brown dwarfs, what’s going on in that atmosphere has a lot to say about what we can surmise.’

Sorry just being picky

Paul Gilster November 23, 2010 at 16:09

Well, true enough — I should have said ‘dependent on the brown dwarf’s temperature’ etc. Brown dwarfs challenge us all over the place, including linguistically!

andy November 23, 2010 at 16:28

@Michael: that depends on what you mean by “star” really. Certainly the process that produces stars also appears to produce objects with masses below the hydrogen-fusion threshold (brown dwarfs). It is not so unreasonable to call such objects “stars”.

Mike November 23, 2010 at 16:45

I’m astonished that there are binaries with a separation of that distance.
Still, the paper makes a strong case for the WD and BD being a pair.
A learning experience for me!
Perhaps someone with more astronomical education could tackle this question. Isn’t there still some debate over whether Proxima Centauri, at a similar large separation distance from Alpha Centauri A and B, is really part of that system?
I wonder how distantly stars can be separated and still be gravitationally linked in a stable orbit?

kurt9 November 23, 2010 at 17:22

Does anyone know the results of the WISE mission? How many brown dwarfs did it find and where are they?

Michael November 23, 2010 at 17:24

I wonder if the brown dwarf was capable of stealing material from the dying central star thereby increasing it’s mass and temperature.

Paul Gilster November 23, 2010 at 18:12

kurt9 writes:

Does anyone know the results of the WISE mission? How many brown dwarfs did it find and where are they?

My understanding is that the first major data release from WISE won’t be until spring.

Tarmen November 23, 2010 at 21:11

Small stars wondering 1/4 Light year apart must sometime be stripped from their companions. Over the history of Milky Way, this must have happened often. So , I suppose many single stars must have originally been companion stars. And/Or else, the far wanderer stars might not be native to that system. They were captured by chance. Both scenarios must happen . Each system is the end of a long story.

(And I wonder if the same things happen to moons everywhere? Maybe one or more of ‘our’ moons. Maybe Triton. Long story??)

Michael Spencer November 24, 2010 at 8:11

Paul,

I am sure you know that your lack of progress, and subsequent report, on the Panek book is most dismaying…especially to those of us out here waiting for publication!

Michael

Paul Gilster November 24, 2010 at 8:38

Hey Michael, I’m a third of the way in! But I can’t do a review until publication date, by agreement with the publisher, so be patient ;-)

kzb November 24, 2010 at 8:51

Tarmen, stars (or should that be “stellar objects”, to include BDs), are born in clusters at low average separation distance from each other. Most current formation models show quite chaotic “early years” for stellar systems in clusters, with the majority of planets formed being stolen or ejected from their natal systems. That being the case, I’d think confidence is low that these two bodies actually started life together from the same accretion disc. Probably beed together for a long time now though.

Phil November 25, 2010 at 22:33

Speaking of brown dwarves, by the look of this very interesting paper:

http://arxiv.org/pdf/1011.5405

Our artists’ perceptions of what brown dwarves look like is going to need some revision. Think giant granulations and dark dusty clouds!

P

bigdan201 November 26, 2010 at 0:44

@kzb – that brings up the possibility of interstellar planets floating through the galaxy. I’m certain that some exist, but their numbers could range from few to many. I’m sure that new detection methods will turn up several of these worlds. It would be useful to know if there are any such planets within the vicinity of our solar system – there would be colonization potential, and they’d be something to watch out for on an interstellar voyage.

This is still well within the realm of science fiction, but I wonder if brown dwarfs could be useful for space colonization. You could orbit them very closely with their low temperature, and perhaps even mine them for energy.

andy November 26, 2010 at 14:29

@Phil: that’s pretty neat actually, was looking for some resources on what the surfaces of red dwarf stars would look like.

kzb November 29, 2010 at 8:52

bigdan201: the free floating planets question is an interesting one. The stellar system formation paper I was reading said that many Jupiter-masses worth of planets are lost in the early days, in fact more planets are lost than retained, on average.

In other words, there ought to be more free planets than stars floating around, IF these models are anywhere near correct.

However, against this theory are the microlensing surveys and also the observations of young stellar clusters, where detected planetary mass objects are fewer in number than say M-class stars. We are also told that existing IR surveys would have detected anything sizable in our neck of the woods. So it’s all up in the air really.

Tobias Holbrook December 7, 2010 at 8:24

Can we get any numbers for the probability of, say, there being a Neptune mass rogue within 1ly of sol? I’m thinking it could prove quite useful as a stepping stone; a craft capable of 10% of c would *only* have to spend a decade in transit, compared to 44 years for a mission to a lit star…

Is there enough mass unaccounted for for such a possibility?

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