Co-authors on this work include Drs. I. Neill Reid, Roc M. Cutri, and Charles A. Beichman also of the California Institute of Technology, Dr. James Liebert of the University of Arizona in Tucson, AZ, and Dr. Michael F. Skrutskie of the University of Massachusetts in Amherst, MA. These "stars" appear to be more numerous than any other type of star in our galaxy, the Milky Way.
These objects, referred to as "L" dwarfs, have been discovered only recently because they are exceedingly dim at visible wavelengths. Their surface temperatures are only 1500-2000 degrees Celsius, less than one-third the solar value, and they are believed to be similar to Jupiter in size, with masses less than one-tenth that of the Sun. Because they are so small and cool, the feeble light they emit is more easily detectable at infrared, rather than optical, wavelengths. The "L" dwarfs announced here -- twenty in all -- were discovered in data acquired by the Two-Micron All-Sky Survey (2MASS), which is currently mapping the entire heavens at those near-infrared wavelengths.
Follow-up observations with the Low Resolution Spectrograph (LRIS) on the 10-meter (400-inch) Keck II Telescope on Mauna Kea, Hawaii, were used to study the details of each object's spectrum, the color distribution of the emitted light. Each chemical produces a series of dark bands or lines at specific positions in the spectrum, and each spectral class has its own characteristic features which depend mainly on its temperature and mass. Thus, describing the spectral class of a star is a shorthand method of telling an astronomer that star's vital statistics. The Keck spectra of the 2MASS objects showed strong features due to atoms such as sodium, potassium, rubidium, and cesium (and sometimes lithium) and of molecules such as chromium hydride, iron hydride, and water. These are radically different from any 'normal' star, and demand separate classification.
The current spectral classification system of letter designations was first introduced by Williamina Fleming and Edward Pickering in 1890. Annie Jump Cannon, in 1901, was the first to arrange these classes into a sequence ordered by descending temperature -- OBAFGKM. Our "L" class should now be added to the end of this sequence because these objects are even cooler than "M" stars. Many introductory astronomy textbooks use the mnemonic "Oh Be A Fine Girl/Guy, Kiss Me" to aid students in remembering the correct sequence of spectral types for stars. This now needs to be changed, perhaps to "Oh Be A Fine Girl/Guy, Kiss My Lips", to reflect the new OBAFGKML sequence of letters.
At least six of the twenty "L" dwarfs have lithium in their atmospheres, which identifies them as brown dwarfs - objects with such low masses that they fail to ignite the hydrogen fusion process which sustains energy production in the Sun. As a result, rather than shining steadily for billions of years, they fade from view in a matter of tens or hundreds of millions of years - a rapid timescale in astronomical terms. These six "L" dwarfs have less than 6% of the Sun's mass and are thus only a few tens of times more massive than the planet Jupiter. "Since the dawn of astronomy, there has been a giant void in our knowledge between stars and planets," says Dr. Kirkpatrick, a Staff Scientist at the Infrared Processing and Analysis Center at Caltech, "We are now beginning to fill that void."
Unlike many other astronomical discoveries which lie at the edge of the visible Universe, these objects are believed to be located within only a few tens of light years of the Sun. The displacements in position of these "L" dwarfs relative to stars in the background can be measured over the course of a year as our vantage point in space changes, that is, as the Earth orbits the Sun. This positional shift, known as "trigonometric parallax", is currently being measured to confirm the proximity of the "L" dwarfs. "They are so commonplace," says Dr. Kirkpatrick, "that there is a very good chance we will discover one which lies closer to the Sun than Proxima Centauri," the closest of the known stars. "As such, it may be the first place we visit after we leave the confines of our own solar system."
This work was supported by the National Aeronautics and Space Administration, the National Science Foundation, the United States Naval Observatory, and the University of Massachusetts. Travel to observing facilities was supported by 2MASS Core Project funds administered by the Jet Propulsion Laboratory.