Missing star looks "heavy"

Although mass is the most important property of stars, it has proved very hard to measure for the lowest-mass objects in the universe. Thanks to a powerful new camera, however, a very rare low-mass companion star finally has been photographed--and measured by an international team of astronomers, including Americans Eric Nielsen, Beth Biller, and Eric Mamajek.

This object, more than 100 times fainter than its close primary star, is 93 times as massive as Jupiter and almost twice as heavy as theory predicts it should be. The findings challenge current ideas about the astronomical brown dwarf population and the existence of widely publicized free-floating extrasolar planets.

Brown dwarfs are objects 75 times more massive than Jupiter but not massive enough to burn as stars. If young objects identified as brown dwarfs are twice as massive as has been thought, many actually are low-mass stars. Objects recently identified as "free-floating" planets are, in turn, likely to be just low-mass brown dwarfs.

Astronomers searching for very low-mass objects look at young nearby stars because low-mass companion objects will be brightest when young, before they contract and cool. Scientists had suspected since the early 1990s that the well-known AB Dor A--a star 48 light-years (14.9 parsecs) from Earth and only 50,000,000 years old--has a low-mass companion because its position "wobbles" as it is pulled by an unseen (but nearby) mass. Yet, even the Hubble Space Telescope tried and failed to detect the companion because it was too faint or too close to the glare of the primary star.

Astronomers succeeded in photographing the elusive companion in February using a novel high-contrast camera on the European Southern Observatory's 8.2-meter Very Large Telescope in Chile. Researchers developed the new high-contrast adaptive optics camera--the NACO Simultaneous Differential Imager, or NACO SDI--for hunting extrasolar planets. The SDI camera enhances the ability of the powerful VLT and its existing adaptive optics systems to detect faint companions that normally would be lost in the glare of the primary star.

This is the first image of a companion so faint--120 times fainter than its star--and yet so near its star. The tiny distance between the star and the companion (0.156 arcseconds) is the same as the width of a dime seen eight miles away. Once the companion was located, it was observed at near infrared wavelengths to measure its temperature and luminosity.

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