[BRIEF NOTE] On the discovery of brown dwarf WISE 0855–0714, 2 parsecs away

[rfmcdonald]

Kevin Luhman, the man who last year discovered the nearby binary brown dwarf pair now known as Luhman 16, has done it again. From the abstract of the discovery paper, "Discovery of a ~250 K Brown Dwarf at 2 pc from the Sun".

Through a previous analysis of multi-epoch astrometry from the Wide-field Infrared Survey Explorer (WISE), I identified WISE J085510.83–071442.5 as a new high proper motion object. By combining astrometry from WISE and the Spitzer Space Telescope, I have measured a proper motion of 8.1 ± 0.1'' yr–1 and a parallax of 0.454 ± 0.045'' ($2.20^{+0.24}_{-0.20}$ pc) for WISE J085510.83–071442.5, giving it the third highest proper motion and the fourth largest parallax of any known star or brown dwarf. It is also the coldest known brown dwarf based on its absolute magnitude at 4.5 μm and its color in [3.6]-[4.5]. By comparing M 4.5 with the values predicted by theoretical evolutionary models, I estimate an effective temperature of 225-260 K and a mass of 3-10 M Jup for the age range of 1-10 Gyr that encompasses most nearby stars.

WISE J085510.83–071442.5, or WISE 0855–0714 as Wikipedia more manageably calls it, is a very cool Y-class brown dwarf. How cool is it? As Niall at we are all in the gutter describes, WISE 0855–0714 is cool by the standards of Earth.

[WISE 0855–0714] was found by looking at images from the WISE satellite which studies the universe in mid-infrared radiation. Nearby stars and brown dwarfs move slowly across the sky compared to background stars due to proper motion. This can be pretty slow, a very nearby star might move at one arcsecond per year, about the apparent angular speed of a tortoise walking at the distance of the Sun. So Luhman looked for objects that had moved a lot between different WISE images and found one which he published last year. This was a pair of cool brown dwarfs with temperatures of about 1100C. Now he’s published another that is moving even faster, about 8 arcseconds per year. Despite this, it is about the same distance as the previously published one, 2.2pc (a bit more than 6 lightyears). This distance was determined by follow-up Spitzer Space Telescope observations using a trick called trigonometric parallax.

So what is this thing? Well, we know it is bright in the mid-infrared, light which it is difficult to observe from Earth and which is way beyond what the human eye can see. And that’s where the observations of it stop, well not really, we can tell a bit about this object from what we don’t see, near-infrared light. Luhman’s new object was observed by the VISTA telescope in Chile a few years back. Well I say observed, it didn’t see it, neither did Luhman’s follow-up observations with Gemini. But from those observations one can set a limit of how bright this object is in the near-infrared and hence constrain its properties. Luhman used these along with his measurement of how bright the object was in the mid-infrared to find that the temperature was -48 to -13C, colder than ice on Earth, you’d even struggle to play at Lambeau Field in those temperatures. Not that this is a solid, icy planet, it’s about 3 to 10 times the mass of Jupiter and about the same size. It’s also a bit warmer than Jupiter which has an effective temperature at the top of its clouds of about -160C.

More to come, I'm sure.

(Thanks to Constantine from the 2300AD Facebook group for linking to Niall's post describing the discovery.)