Yesterday’s story on the smallest exoplanet yet discovered somewhat obscured work on brown dwarfs released at the same conference. But this year’s meeting of the American Astronomical Society has been filled with interesting items, and I don’t want to neglect the latest news about a type of star that may be as plentiful as any in the cosmos. We don’t know that that is the case, but we have much to learn about brown dwarfs as we compile a census of those in the Sun’s neighborhood, including the question of what kind of planets might circle them.
New observations studied by Michael Liu (University of Hawaii) and team have now been able to determine the masses of a number of brown dwarfs, with findings that suggest the shape of future research. Says Liu:
“Mass is the fundamental parameter that governs the life-history of a free-floating object, and thus after many years of patient measurements, we are delighted to report the first masses of the very faintest, coldest brown dwarfs. After weighing these tiny, dim, cold objects, we have confirmed that the theoretical predictions are mostly correct, but not entirely so.”
It’s understandable that these objects should be tricky to observe. They can be 300,000 times less energetic than the Sun, with temperatures at the surface below 500 degrees Celsius. The image at left shows two such brown dwarfs, orbiting a star not terribly different from Sol. How tempting it would be to a civilization just moving out into its solar system to have a nearby stellar target like this with, perhaps, a planet or two around each brown dwarf. The incentive to develop deep space technologies might well be accelerated with the prospect of exploring such exotic targets. Call it the ‘brown dwarf incentive.’
Image: Infrared image of the dusty brown dwarf binary HD 130948BC. The binary is seen in the upper left and has a total mass about 11 percent the mass of the sun. The binary is in orbit around a young sun-like star, seen to the lower right. This image was obtained with the adaptive optics system on the Keck II Telescope, located on Mauna Kea, Hawaii. The image is 3.75 arc seconds on a side (about 1/500 the size of the moon), and the binary’s separation is about 0.1 arc seconds. Credit: Mr. Trent Dupuy and Dr. Michael Liu (Institute for Astronomy, University of Hawaii).
About fifteen percent of the brown dwarfs within 100 light years of Earth occur in binary systems, and it is these that Dr. Liu’s team has focused on, for the study of their orbits (in size and duration) can help determine the total mass of the system. These pairs are between 45 and 60 light years from Earth, with the two components of each separated by about two AU, a distance somewhat larger than the distance of Mars from the Sun.
Using the Keck II telescope (Mauna Kea) and previous data from the Hubble instrument, precise measurements of the orbits in question became possible. Each brown dwarf in the binary HD 130948BC shows an individual mass of about 5.5 percent the mass of the Sun. The brown dwarfs in the other binary, 2MASS 1534-2952AB, each weigh in with a mass about three percent of that of the Sun. That would make each of the objects the equivalent of about thirty Jupiter masses.
As to that disagreement in theory that Liu refers to above, it emerges when you compare these mass measurements with what you would expect when studying the energy output and temperature of these objects. One binary pair was cooler than theory would predict, the other warmer. The implication is that the model for temperature determination or energy output is off, but it will take more brown dwarf measurements to pin down precisely what is happening. We’ll follow that investigation with interest, and I’ll post links to the two papers, slated for the Astrophysical Journal, as soon as they become available online.
If you ask me, brown dwarfs resemble more like Mega-gas planets than puny stars.
But regardless how you classify them, they would be interesting to observe up close! :-)
There’s no way these two ‘browns’ could be exchanging material is there? Just thinking about the mass anomalies….
Move along, nothing to see here…
P
Question: could brown dwarfs account for a large part of the ‘missing mass’ or dark matter of the universe?
BTW, I think living in a binary system consisting of two sunlike stars, such as (of course) Alpha Centauri, or even a very wide binary system, such as Zeta Reticuli, would be a much greater incentive for space exploration.
Hi Darnell
One contentious issue is how to classify a Big Jovian versus a Brown Dwarf. Some core-accretion scenarios can result in a massive heavy-element core, with a heavy hydrogen/helium layer that actually undergoes deuterium fusion – the usual standard for defining a “brown dwarf”. Is a hot, deuterium burning object with a massive core a “star” or a “planet” or have we hit the limits of our definitions?
Do note another thing. Brown dwarfs (BDs) of around 60 MJup are capable of fusing lithium. Jovians and even Super Jovians can’t.
GD 552: a cataclysmic variable with a brown dwarf companion?
Authors: E. Unda-Sanzana, T. R. Marsh, B. T. Gansicke, P. F. L. Maxted, L. Morales-Rueda, V. S. Dhillon, T. D. Thoroughgood, E. Tremou, C. A. Watson, R. Hinojosa-Goni
(Submitted on 30 Jun 2008)
Abstract: GD 552 is a high proper motion star with the strong, double-peaked emission lines characteristic of the dwarf nova class of cataclysmic variable star, and yet no outburst has been detected during the past 12 years of monitoring.
We present spectroscopy taken with the aim of detecting emission from the mass donor in this system. We fail to do so at a level which allows us to rule out the presence of a near-main-sequence star donor. Given GD 552’s orbital period of 103 minutes, this suggests that it is either a system that has evolved through the ~80-minute orbital period minimum of cataclysmic variable stars and now has a brown dwarf mass donor, or that has formed with a brown dwarf donor in the first place.
This model explains the low observed orbital velocity of the white dwarf and GD 552’s low luminosity. It is also consistent with the absence of outbursts from the system.
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0806.4892v1 [astro-ph]
Submission history
From: Eduardo Unda-Sanzana [view email]
[v1] Mon, 30 Jun 2008 13:38:42 GMT (613kb)
http://arxiv.org/abs/0806.4892
Gravitational fragmentation and the formation of brown dwarfs in stellar clusters
Authors: Ian A. Bonnell (St Andrews), Paul C. Clark (Heidelberg), Matthew R. Bate (Exeter)
(Submitted on 2 Jul 2008)
Abstract: We investigate the formation of brown dwarfs and very low-mass stars through the gravitational fragmentation of infalling gas into stellar clusters. The gravitational potential of a forming stellar cluster provides the focus that attracts gas from the surrounding molecular cloud. Structures present in the gas grow, forming filaments flowing into the cluster centre.
These filaments attain high gas densities due to the combination of the cluster potential and local self-gravity. The resultant Jeans masses are low, allowing the formation of very low-mass fragments. The tidal shear and high velocity dispersion present in the cluster preclude any subsequent accretion thus resulting in the formation of brown dwarfs or very low-mass stars. Ejections are not required as the brown dwarfs enter the cluster with high relative velocities, suggesting that their disc and binary properties should be similar to that of low-mass stars.
This mechanism requires the presence of a strong gravitational potential due to the stellar cluster implying that brown dwarf formation should be more frequent in stellar clusters than in distributed populations of young stars. Brown dwarfs formed in isolation would require another formation mechanism such as due to turbulent fragmentation.
Comments: 8 pages, 7 figures. MNRAS, in press
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0807.0460v1 [astro-ph]
Submission history
From: Ian Bonnell [view email]
[v1] Wed, 2 Jul 2008 21:58:22 GMT (571kb)
http://arxiv.org/abs/0807.0460
Disks around Brown Dwarfs in the Sigma Orionis Cluster
Authors: K. L. Luhman, J. Hernandez, J. J. Downes, L. Hartmann, C. Briceno
(Submitted on 4 Aug 2008)
Abstract: We have performed a census of disks around brown dwarfs in the Sigma Ori cluster using all available images from IRAC onboard the Spitzer Space Telescope. To search for new low-mass cluster members with disks, we have measured photometry for all sources in the Spitzer images and have identified the ones that have red colors that are indicative of disks.
We present 5 promising candidates, which may consist of 2 brown dwarfs, 2 stars with edge-on disks, and a low-mass protostar if they are bona fide members. Spectroscopy is needed to verify the nature of these sources. We have also used the Spitzer data to determine which of the previously known probable members of Sigma Ori are likely to have disks. By doing so, we measure disk fractions of ~40% and ~60% for low-mass stars and brown dwarfs, respectively. These results are similar to previous estimates of disk fractions in IC 348 and Cha I, which have roughly the same median ages as Sigma Ori (3 Myr).
Finally, we note that our photometric measurements and the sources that we identify as having disks differ significantly from those of other recent studies that analyzed the same Spitzer images. For instance, previous work has suggested that the T dwarf S Ori 70 is redder than typical field dwarfs, which has been cited as possible evidence of youth and cluster membership. However, we find that this object is only slightly redder than the reddest field dwarfs in [3.6]-[4.5] (1.56+/-0.07 vs. 0.93-1.46). We measure a larger excess in [3.6]-[5.8] (1.75+/-0.21 vs. 0.87-1.19), but the flux at 5.8um may be overestimated because of the low signal-to-noise ratio of the detection. Thus, the Spitzer data do not offer strong evidence of youth and membership for this object, which is the faintest and coolest candidate member of Sigma Ori that has been identified to date.
Comments: Astrophysical Journal, in press
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0808.0471v1 [astro-ph]
Submission history
From: Kevin Luhman [view email]
[v1] Mon, 4 Aug 2008 17:54:54 GMT (1021kb)
http://arxiv.org/abs/0808.0471