The neighborhood watch is on patrol...for brown dwarfs, that is. Our sun resides within a spiral arm of our Milky Way galaxy about two-thirds of the way out from the center. This area is fairly calm and suburban-like, with an average number of stars. Recently, NASA's Wide-field Infrared Survey Explorer (WISE) has been turning up a new crowd of stars close to home: the coldest of the brown dwarf family.
To refresh your memory, brown dwarfs are sub-stellar objects which are too low in mass to sustain hydrogen-1 fusion reactions in their cores, which is characteristic of stars on the main sequence. Brown dwarfs occupy the mass range between that of large gas giant planets and the lowest-mass stars; this upper limit is between 75 and 80 Jupiter masses. Currently there is some debate concerning what criterion to use for defining the separation between a brown dwarf and a giant planet at very low brown dwarf masses (~13), and whether brown dwarfs are required to have experienced fusion at some point in their history. In any event, brown dwarfs heavier than 13 Jupiter masses do fuse deuterium and those above about 65 Jupiter masses also fuse lithium. Some planets are known to orbit brown dwarfs (2M1207b, MOA-2007-BLG-192Lb, and 2MASS J044144b). Enough of the refresh. Let's push on...
The interesting thing is that WISE found there are far fewer brown dwarfs in our stellar neighborhood than astronomers had predicted. Previous estimates were as many brown dwarfs as typical stars, but the initial tally from WISE shows just one brown dwarf for every six stars, or 1:6. It's the cosmic equivalent to finally being able to see into a mysterious gated community and finding only a few homes.
Nonetheless, the observations are providing crucial information about how these exotic worlds form, and hinting at what their population densities might be like in our galaxy and beyond. Astronomers think these brown dwarfs formed by several different mechanisms, including having their growth stunted by a variety of factors that prevent them from becoming full-blown stars. Even so, they are not exactly certain of the process.
WISE was launched in 2009 and surveyed the entire sky in infrared light in 2010. One of the mission's main science goals was to survey the sky for the elusive brown dwarfs. These small bodies start their lives like stars, but lack the bulk required to burn nuclear fuel. With time, they cool and fade, making them difficult to find.
Improvements in WISE's infrared vision over past missions have allowed it to pick up the faint glow of many of these hidden objects. In August 2011, the mission announced the discovery of the coolest brown dwarfs spotted yet, a new class of stars called Y dwarfs. One of the Y dwarfs is less than 80 degrees Fahrenheit (25 degrees Celsius), or about room temperature, making it the coldest star-like body known. Since then, the WISE science team has surveyed the entire landscape around our sun and discovered 200 brown dwarfs, including 13 Y dwarfs.
Determining the distances to these objects is a key factor in knowing their population density in our solar neighborhood. After carefully measuring the distance to several of the coldest brown dwarfs via a method called parallax, the scientists were able to estimate the distances to all the newfound brown dwarfs. They concluded that about 33 brown dwarfs reside within 26 light-years of sun. There are 211 stars within this same volume of space, so that means there are about six stars for every brown dwarf.
Mission team members emphasize that the WISE survey results are still preliminary. The suggest it is highly likely that WISE will discover additional Y dwarfs, but not in vast numbers, and probably not closer than the closest known star, Proxima Centauri. Those discoveries could bring the ratio of brown dwarfs to stars up a bit, to about 1:5 or 1:4, but not to the 1:1 level previously anticipated.
The new observations still allow the possibility of free-floating planets up to a few times the mass of Jupiter beyond a few light-years from the sun, which other surveys have predicted might exist. Those bodies would be too faint for WISE to see in the processed data in hand.
The new results are due to appear in the July 10 issue of The Astrophysical Journal. The Journal issues may be read online here: http://iopscience.iop.org/0004-637X .
And now, the mission particulars...
JPL manages, and operated, WISE for NASA's Science Mission Directorate. The spacecraft was put into hibernation mode after it scanned the entire sky twice, completing its main objectives. Edward Wright is the principal investigator and is at UCLA. The mission was selected competitively under NASA's Explorers Program managed by the agency's Goddard Space Flight Center in Greenbelt, Maryland. The science instrument was built by the Space Dynamics Laboratory in Logan, Utah. The spacecraft was built by Ball Aerospace & Technologies Corp. in Boulder, Colorado. Science operations and data processing take place at the Infrared Processing and Analysis Center at the California Institute of Technology in Pasadena. Caltech manages JPL for NASA.
More information is online at www.nasa.gov/wise and wise.astro.ucla.edu and jpl.nasa.gov/wise .
The Two-Micron All-Sky Survey (2MASS) mission was a joint effort between Caltech, the University of Massachusetts and NASA/JPL. Data are archived at the Infrared Processing and Analysis Center at Caltech: www.ipac.caltech.edu/ .
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