Saturday, April 26, 2014

A Cold Brown Dwarf in the Sun’s Neighbourhood

By analysing data from NASA’s Wide-field Infrared Survey Explorer (WISE) and Spitzer Space Telescope, Kevin Luhman, an astronomer at Pennsylvania State University recently announced the discovery of the coldest brown dwarf found to date in a paper published on 21 April 2014 in the Astrophysical Journal Letters. The brown dwarf, identified as WISE J085510.83-071442.5 (hereafter WISE J0855-0714) is reported to have an estimated temperature of 225 to 260 K (-48 to -13 °C) and a mass of 3 to 10 times Jupiter’s mass. This makes WISE J0855-0714 literally as cold as ice, about as chilly as summer at the South Pole. Before this discovery, the coldest known brown dwarfs, also found by WISE and Spitzer, were about room temperature.

Figure 1: Artist’s impression of a cold brown dwarf. Image credit: NASA/JPL-Caltech.

Brown dwarfs are objects that form as stars do, but they lack the mass to sustain nuclear fusion in their cores to shine like stars. They cool with time and emit nearly all of their energy in the form of infrared radiation. WISE was able to detect WISE J0855-0714 because it surveyed the entire sky in infrared twice, with some areas up to three times. The area of sky where WISE J0855-0714 is situated was imaged by WISE on 4 May 2010 and 11 November 2010. Between these two epochs of WISE images, WISE J0855-0714 was found to have moved by an amount which indicates a proper motion that is unusually high among the known stars. In fact, the proper motion of WISE J0855-0714 is the 3rd highest for any known object outside the Solar System, behind only Barnard’s star and Kapteyn’s star. “This object appeared to move really fast in the WISE data,” said Luhman. “That told us it was something special.”

The high proper motion of WISE J0855-0714 indicates that it must be a relatively nearby object. An analogy for this is the view looking out the window of a moving train where nearby foreground objects would appear to move by much more rapidly than distant mountains. Combined with data from Spitzer, WISE J0855-0714 is estimated to lie at a distance of only 7.2 light-years, making it the 4th closest system to the Sun. The three closest systems are Alpha Centauri AB (including Proxima Centauri), Barnard’s star and Luhman 16AB. The discovery of WISE J0855-0714 demonstrates just how little is known about the population of objects in the Sun’s neighbourhood. “It is remarkable that even after many decades of studying the sky, we still do not have a complete inventory of the Sun’s nearest neighbours,” said Michael Werner, the project scientist for Spitzer at NASA’s Jet Propulsion Laboratory in Pasadena, California. In fact, the discovery of Luhman 16AB - a pair of warmer brown dwarfs at a distance of 6.5 light-years, was made by Luhman only in March of 2013.

Figure 2: This diagram illustrates the locations of the star systems closest to the Sun. The year when the distance to each system was determined is listed after the system’s name. Data from WISE led to the discovery of two of the four closest systems: the binary brown dwarf Luhman 16AB and the brown dwarf WISE J0855-0714. Image credit: NASA/Penn State University.

WISE J0855-0714 could either be a brown dwarf or a gas giant planet that was ejected from its planetary system. The latter is probably unlikely since planetary-mass brown dwarfs are known to exist, while the frequency of ejected gas giant planets is still unknown. Without the overwhelming glare from a nearby star, WISE 0855-0714 presents a good opportunity to study atmospheric models in an unexplored temperature regime, offering more insights about the atmospheres of planets. In depth observations of the atmosphere of WISE 0855-0714 can be done with the deployment of the James Webb Space Telescope (JWST). WISE J0855-0714 is most probably a Y-dwarf and its discovery means that the 4 closest known systems to the Sun consist of at least one object of each spectral type from G through Y. This lettering is a classification scheme for stars and brown dwarfs using the letters O, B, A, F, G, K, M, L, T and Y - a sequence from the hottest (O-type) to the coolest (Y-type).

K. L. Luhman (2014), “Discovery of a ~250 K Brown Dwarf at 2 pc from the Sun”, ApJ 786 L18