A really, really nearby brown dwarf binaryPosted: March 14, 2013
OK I’m going to break my blogging silence and my aversion to blogging in my subject area to post about a really cool result that came out earlier this week.
Looking up at the night sky you see a hodge-podge collection of stars, perhaps a few thousand of the hundreds of billions of stars in the Galaxy. Some are extremely bright types of star that are really far away. Others like the Sun’s nearest neighbouring system Alpha Centauri are fairly run of the mill but appear bright because they are so close. But not all stars close to the Sun can be seen with the naked eye. Take Barnard’s Star, the second closest system to the Sun, it’s situated roughly twice as far away from us as Alpha Centauri but because it’s a red, faint type of star, it’s over 6,000 times fainter and 25 times too faint to see with the naked eye. This means that even though some stars are very close to us, they are so faint that we need to use a few tricks to pick them out from bright background stars.
One of the best tricks to use is to take a picture of the sky and look back a few years later and compare the positions of stars. Stars move around the Galaxy with different orbits and hence every star has a velocity with respect to the Sun. Due to their closeness, nearby stars appear to move more compared to background stars (their proper motion). This is simply a perspective effect, they aren’t actually moving through space faster. Hence if you look for stars moving quickly across the sky, chances are a lot of them will be near the Solar System. This isn’t simply a matter of cartography, if you want to pick out a population of faint objects, your best bet is to look close-by.
And that’s exactly what Kevin Luhman did. By taking the positions of objects observed by the WISE satellite, he found one which stuck out. It moved across the sky pretty fast and was very bright in infrared light. Looking back at images taken by other surveys he also found it detected there. This often happens in astronomy, sometimes you find an object nobody had noticed was interesting before but which may have been first detected 50 or even 100 years ago. Anyway, the object Luhman found was moving across the sky pretty fast. Well actually it wasn’t, nearby stars tend to have their motions measured in arcseconds per year. One arcsecond per year is the same angular speed as seeing the average tortoise walking at the distance of the Sun from Earth. The newly discovered high proper motion object was moving at about 2.8 Solar Tortoises, which is pretty big for stars. Well I say star, but it isn’t, it’s a brown dwarf, well actually not “a” brown dwarf.
Stars are fuelled by nuclear reactions in their core. These work because of the huge temperatures in their cores caused by all the mass above pushing down. It’s like the atomic nuclei in the core are caught at the bottom of a really big rugby ruck*. Anyway, they get so hot that they can sometimes overcome their mutual repulsion and fuse together. However some objects, with masses below about 8% of the Sun can’t reach the appropriate minimum temperature to begin stable fusion and hence are “failed” stars or brown dwarfs. When Luhman took a spectrum of his object, he found it was a brown dwarf, well actually while taking the observation he found that it was actually two brown dwarfs in orbit around one and other. Finally, using the data from the WISE satellite and other surveys he was able to work out its distance from an effect known as trigonometric parallax. This showed the two brown dwarfs are about 6.5 lightyears away, slightly more distant than Barnard’s Star.
My reaction to this was probably like others in my field, “how did we miss this?” Well the answer is simple, the system lies close to the Milky Way. The density of stars on the sky increases sharply as you go close to the plane of the Milky Way, meaning searches of nearby stars are often flooded with spurious candidates. Additionally the gas and dust in the plane make background stars appear redder and faint in the optical but still bright in the infrared. This can mimic the colour of brown dwarfs, again contaminating searches. Brown dwarf searches therefor often avoid the region around the Milky Way to make sure they can have clean samples without wading through a load of junk. Hence the extremely nearby, bright brown dwarf lay undiscovered for decades after it had first been detected.
And that brings me to my last point, this is a really cool discovery yet it hasn’t got the attention it deserves. The third closest system to the Sun was just found, that should at least be on the BBC News front page.
*There are no known instances of a rugby ruck leading to nuclear fusion