Is a Brown Dwarf a Planet or a Star?
So many possibilities exist for describing a star. You can go by its color, whether it be blue, red, yellow, or white. Size is also an important contributor, for it could be a main sequence, a giant, a supergiant, or even a dwarf. But how many know about an odd member of the star family known as brown dwarfs? Many don’t, and that is because at face value they seem to have more in common with Jupiter-like planets than a star and so are passed by frequently. Curious? Read on.
From Theory to Fact
Brown dwarfs were first postulated by Shiv Kumar in the 1960’s when exploring the fusion of matter inside a star. He wondered what would happen if the center of a star were degenerate (or in a state where electrons are confined to their orbitals) but the overall star was not massive enough to fuse the material located there. They would be slightly larger than a gas giant and would radiate heat still but at first glance it would visibly look similar to those planets. In fact, because of the degenerate matter and the limiting radius of the object, only a certain amount of thermal heat can be gained before flattening out. You see, stars form when a cloud of molecular gas collapses under gravitational potential energy until the density and heat are sufficient for hydrogen to start fusing. However, stars need to obtain a density larger than this to initiate the fusion in the first place, for once it is obtained then some energy is lost through partial degeneracy and contraction (Emspak 25-6, Burgasser 70).
But that degeneracy pressure requires a certain mass to overcome it. Kumar determined that 0.07 solar masses was the lowest possible mass for hydrogen to have sufficient pressure to fuse for Population I stars and 0.09 solar masses for Population II stars. Anything below that allows electrons to combat degenerate pressure and avoid compaction. Kumar wanted to name these objects black dwarfs, but that title belongs to a white dwarf which has cooled off. It would not be until 1975 that Jill Tarter came up with the brown dwarf term used today. But then all was quiet for 20 years, with none being known to exist. Then in 1995 Teide 1 was found, and scientists were able to start finding more and more. The reason for the large delay between idea and observation was that the wavelength brown dwarfs emit light at 1-5 micrometers, near the limits of the IR spectrum. Technology needed to catch up with this range and so it was years before those first observations. Currently, 1000’s are known to exist (Emspak 25-6, Kumar 1122-4 Burgasser 70).
Mechanics of a Brown Dwarf
To discuss how a brown dwarf star does work is slightly complicated. Because of their low mass, they do not follow typical H-R diagram trends that most stars do. After all, they cool faster than a typical star because of a lack of fusion creating heat, with bigger dwarfs cooling slower than smaller ones. To help make some distinctions, brown dwarfs are broken into M, L, T, and Y classes, with M being the hottest and Y being the coolest. If any method exists for using these to help figure out the age of the dwarf, it remains unknown at this time. No one is really sure how to age them! They may follow standard temperature laws of stars (hotter meaning younger) but no one is 100% sure, especially the ones that are near planet-level temperatures. In fact, despite different spectrums, most brown dwarfs that are cool are at nearly the same temperature. Again, no one is sure why but hopefully by studying gas giant planet atmospheric physics (their closet kin), scientists hope to solve some of these riddles (Emspak 26, Ferron "What").
And good luck finding their mass. Why? Most are alone out there, and without a companion object to apply orbital mechanics to, it is nearly impossible to accurately measure the mass. But scientists are clever, and by looking at the spectrum from them it may be possible to determine the mass. Some elements have a known spectral line that can be moved and stretched/compressed based on volume and pressure changes, which can then be related back to mass. By comparing the measured spectrums with known changes, scientists can perhaps find out how much material would be needed to impact the spectrum (Emspak 26).
But now the distinction between the planet-like nature and star-like nature becomes murky. For brown dwarfs have weather! Not like anything here on Earth though. This weather is solely based on temperature differentials, with them reaching heights of 3000 Kelvin. And as the temperature starts to drop, materials start to condense. First it is clouds of silicon and iron, and as you get to lower and lower temps those clouds become methane and water, making brown dwarfs the only other known place outside the solar system with water in the clouds. Evidence for this was uncovered when WISE 0855-0714 was found by Jackie Fakerty of the Carnegie Institution of Washington. It is a relatively cold brown dwarf, clocking in at about 250 kelvin with a mass of 6-10 Jupiters and a distance of 7 light years from Earth (26-7).
But it got even better when scientists announced that brown dwarfs have storms! According to a January 7, 2014 meeting of the American Astronomical Society, when 44 brown dwarfs were examined for 20 hour durations each by Spitzer, half exhibited surface turbulence consistent with a storm pattern. And in a January 30, 2014 issue of Nature, Ian Crossfield (Max Planck Institute) and his team looked at WISE J104 915.57-531906.AB, otherwise known as Luhman 16A and B. They are a pair of close brown dwarfs 6.5 light-years away which offer great views of their surfaces to scientists. When the spectrograph on the VLT soaked in light from both for a 5 hour duration each, the CO portion was examined. Bight and dark regions appeared on maps of the dwarfs that appear to track storms. That's right, the first extra-solar weather map was created from another object's atmosphere! (Kruesi "Weather").
Amazingly, scientists can actually look at light that has passed through the atmosphere of a brown dwarf to learn details about it. Kay Hiranaka, at the time a grad student at Hunter College, began a study on this. Looking at models of brown dwarf growth, it was found that as a brown dwarf ages more material falls into it, making them less opaque due to lack of cloud cover. Therefore, the amount of light one lets through could be an indicator of age (27).
But Kelle Cruz, Hiranaka’s advisor, found a few interesting deviations from the simulations which may hint at new behavior. When looking at low mass brown dwarfs, many of their absorption spectrums lack sharp peaks and was either shifted slightly to the blue portion or the red portion of the spectrums. Sodium, cesium, rubidium, potassium, iron hydrides, and titanium oxides spectral lines were weaker than expected but vanadium oxides were higher than anticipated. And on top of this, lithium levels were off. As in non-existent. Why is this weird? Because the only way for lithium to not be there is if it fuses with hydrogen into helium, something a brown dwarf is not massive enough to do. So what could have caused this? Some wonder if a low initial gravity caused the heavier element to be lost in the past. Also, it is possible for the cloud composition of the brown dwarf to scatter the lithium waves, for the dust size may be small enough to block it (Ibid).
Stanimir Metchev, from the University of Western Ontario in London, decided a different aspect needed looking at: temperature. Using brightness levels recorded over years, a map was made to show how brown dwarf surfaces change. They typically range from 1300 to 1500 Kelvin with younger brown dwarfs not only having a higher temperature overall but a higher differential between the low and the high when compared to colder, older brown dwarfs. But while looking at the surface maps, Metchev found that the spin rate of these objects do not match models, with many spinning slower than expected. The spin should be dictated by the conservation of angular momentum, and with much of the mass close to the core of the object it should spin fast. Yet most complete a revolution in 10 hours. And with no other known forces to slow them down, what could have? Possibly a magnetic field interaction with the interstellar medium, although most models show brown dwarfs not having enough mass for a substantial magnetic field (27-8).
Those models got a huge upgrade when some new trends on brown dwarfs were revealed by a study led by Todd Henry (Georgia State University). In his report, Todd references how the Research Consortium on Nearby Stars (RECONS) looked at 63 brown dwarfs that were at that 2100 K boundary point (as seen in the graph above) in an effort to better understand the defining moment when a brown dwarf wouldn't be a planet. Unlike gas giants, where diameter is directly proportional to mass and temperature, brown dwarfs have temperatures that go up as diameter and mass decrease. Scientists found that the conditions for the smallest brown dwarf possible should be a temperature of 2100 K, a diameter of 8.7% that of the Sun's, and a luminosity that is 0.000125% that of the Sun's (Ferron "Defining")
Burgasser, Adam J. “Brown Dwarfs – Failed Stars, Super Jupiters.” Physics Today June 2008: 70. Print.
Emspak, Jesse. “The Little Stars That Couldn’t.” Astronomy May 2015: 25-9. Print.
Ferron, Karri. "Defining the Boundary Between Stars and Brown Dwarfs." Astronomy Apr. 2014: 15. Print.
---. "What are We Learning About the Coldest Brown Dwarfs?" Astronomy Mar.2014: 14. Print.
Kruesi, Liz. "Weather Forecasts on Brown Dwarfs." Astronomy Apr. 2014: 15. Print.
Kumar, Shiv S. “The Structure of Stars of Very Low Mass.” American Astronomical Society 27 Nov. 1962: 1122-5. Print.
© 2016 Leonard Kelley
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