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).