* Astronomy

As part of an ongoing search for Earth's nearest stellar neighbours, astronomers have determined the distance to a stellar-like body known as DEN 0255-477 and discovered that it is the nearest-known L dwarf. This body is now also the faintest object outside our solar system for which its intrinsic visual brightness has been measured.

The new record holder is nearly 100 million times fainter than the Sun. It is located only 16.2 light-years (4.97 parsecs) from Earth, making DEN 0255-4700 the 48th nearest known system of stars or brown dwarfs. This very reddish object is a third closer to Earth than the next known L dwarf, which is 24 light-years away. The discovery by a team led by Edgardo Costa and Rene Mendez of the Universidad de Chile in Santiago will be published in the September 2006 issue of the Astronomical Journal.
The Sun's new neighbour has a formal spectral type of L7.5 V, indicative of a cool object with a temperature of only 2,600 degrees F (1700 K). This is rather cooler than the least massive stars, which have temperatures of 4,000 F (2500 K). A spectrum of DEN 0255-4700 shows that it has exotic elements such as potassium, rubidium, and caesium in its outer atmosphere, mixed with superheated water.
L dwarfs are among the coolest and lowest-mass stellar-like objects known. DEN 0255-4700 is probably not a star at all, but rather a brown dwarf. The primary attribute that separates stars from brown dwarfs is mass, and the mass of DEN 0255-4700 is likely to be below the limit of 80 Jupiter masses required to fuse hydrogen into helium.
DEN 0255-477 was originally identified as an interesting object by Martin et al. in 1999, but a measure of its distance and of its intrinsic visual brightness was not available until the present study, which used two telescopes in Chile.
The distance to DEN 0255-4700 was measured via a classic trigonometric parallax technique using the 1.5-metre telescope at the National Science Foundation's Cerro Tololo Inter-American Observatory (CTIO) in the Chilean Andes. The visual (V-band) magnitude of the object was measured with the European Southern Observatory's 3.6-meter telescope in Chile.
The combination of the two pieces of information yields an absolute magnitude for DEN 0255-4700 of 24.4, nearly 20 magnitudes fainter in intrinsic brightness than the Sun.
The parallax technique for measuring the distance to a star takes advantage of the Earth's changing position in the cosmos as it orbits the Sun each year. The apparent back and forth motion of a nearby star during this year reflects the motion of the Earth around the Sun, much like how your finger jumps back and forth in front of your eyes if you blink one eye, then the other.
From Earth, nearby stars appear to make tiny ellipses in the sky because the Earth does not jump from one side of its orbit to another, but slides smoothly around the Sun. The extreme points of the Earth in its orbit are much like the positions of your eyes in your head, and the breadth of the motion in your finger depends on how close you hold it to your eyes – when nearer, it seems to jump more, relative to distant background objects.

The team of astronomers, including Costa, Mendez, and collaborators at Georgia State University in Atlanta and the University of Virginia in Charlottesville, measured the size of the ellipse traced by DEN 0255-4700 to be so large that it must be only 16 light-years away. Over a period of more than three years, the team took pictures of DEN 0255-4700 among a set of distant background stars and monitored its wobbling.
The long-term parallax program, known as the CTIO Parallax Investigation (CTIOPI), has been sponsored by Fondecyt in Chile, and the National Optical Astronomy Observatory (NOAO) in the United States.
The purpose of CTIOPI is to discover and characterise overlooked stars and brown dwarfs in the vicinity the Sun. Objects are scrutinised by measuring their positions (and wobbles), their brightnesses and colours, and by taking spectroscopic fingerprints to examine their atmospheric composition. The estimated "missing" population of solar neighbourhood members is expected to be composed primarily of very low mass stars with spectral type M (known as red dwarfs), and objects of spectral types L (like DEN 0255-4700) and T, many of which are actually brown dwarfs with too little mass to start long-term thermonuclear reactions. Such objects shine feebly, glowing only because of energy leaking out since their gravitational formation, many billions of years ago.

The Chilean members of CTIOPI have been supported by the Fondo Nacional de Investigacion Cientifica y Tecnologica and the Chilean Centro de Astrofisica FONDAP. The team from the United States has been supported by NASA's Space Interferometry Mission, and the National Science Foundation.

Cerro Tololo Inter-American Observatory, based in La Serena, Chile, is part of the National Optical Astronomy Observatory, Tucson, Arizona, which is operated by the Association of Universities for Research in Astronomy Inc. (AURA), under a cooperative agreement with the National Science Foundation.

www.noao.edu