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Closest Low-Mass Star And Orbiting Brown Dwarf Imaged

Astronomers using adaptive optics technology on the Gemini North Telescope have observed a brown dwarf orbiting a low-mass star at just three times the distance between Earth and the sun.

This is the closest distance between a star and its companion in this type of binary system ever found by direct imaging.

A "brown dwarf" is an object too massive and hot to be classified as a planet, but too small and cool to burn like a star. These so-called "failed stars" are best viewed in the infrared because they release surface heat as they slowly contract.

"By using Gemini's advanced imaging capabilities, we were able to clearly resolve this binary pair where the distance between the brown dwarf and its parent star is only about twice the distance of Mars from the sun," said team member Melanie Freed, a graduate student at the University of Arizona in Tucson.

The record-breaking find is just one of a dozen lightweight binary systems observed in the study, led by Laird Close, assistant professor of astronomy at the University of Arizona in Tucson.

The survey suggests that stellar systems and smaller bodies, including large planets, may form differently than previously thought.

The newly identified brown dwarf is estimated to be between 38 and 69 times more massive than Jupiter. It orbits a star called LHS 2397a, which is 46 light-years from Earth.

Finding a brown dwarf companion within 3 Astronomical Units of its parent star is an important step toward imaging massive planets around other stars.

An Astronomical Unit (AU), the distance between the Earth and sun, is 150 million kilometers, or 93 million miles. The previous record image for the closest distance between a brown dwarf and its parent star, which is a much brighter, sun-like star, was almost five times greater at 14 AU.

Close and his team used the Gemini North Telescope to detect eleven other low mass companions. That many low-mass pairs is striking. It suggests that these binary pairs may be common. It contradicts the idea that most very low-mass stars and brown dwarfs are solo objects that have been wandering through space alone after being ejected from their stellar nurseries during star formation.

"We have completed the first adaptive optics-based survey of stars with about one-tenth of the sun's mass, and we found nature does not discriminate against low-mass stars when it comes to making tight binary pairs," Close said. He is the lead author on a paper presented Tuesday (May 21) at an International Astronomical Union symposium on brown dwarfs in Kona, Hawaii.

The team looked at 64 low-mass stars that appeared to be solo stars in lower-resolution infrared images produced by the Two Micron All-Sky Survey (2MASS). Once the team used adaptive optics on Gemini to make images that were ten times sharper, they found twelve of these stars have companions. That is, almost one out of every five low-mass stars in their survey has a companion, just as about one out of every five more massive, sun-like stars have companions, at distances of 3 to 200 AU.

How close the companions orbit their stars was an extra surprise.

"We find companions to low-mass stars are typically only 4 AU from their primary stars," said team member Nick Siegler, a UA graduate student. "That's surprisingly close together. More massive binaries have typical separations closer to 30 AU, and many binaries are much wider than this." The new Gemini observations "imply strongly that low-mass stars do not have companions that are far from their primaries," Close said.

Taken as a whole, their new results suggest that contrary to theory, low-mass binaries and more massive binaries may form in a similar process. The findings add to growing evidence that the same ratio of stars occur in binary systems, whether stars are one solar mass or one-twentieth solar mass.

Astronomers led by Neil Reid of the Space Telescope Science Institute and the University of Pennsylvania, for example, reached a similar conclusion after they surveyed 20 even lower-mass stars and brown dwarfs with the Hubble Space Telescope.

The fact that low-mass stars have brown dwarf companions inside 5 AU is also striking because the exact opposite is true around sun-like stars. Very few sun-like stars have brown dwarf companions inside this distance, according to radial velocity studies.

"This lack of brown dwarf companions within 5 AU of sun-like stars has been called the 'brown dwarf desert,'" Close noted. "However, we see there is likely no brown dwarf desert around low-mass stars."

These results are important input for theorists working to understand how the mass of a star affects the mass of the companion that forms with it and the distance between the pair. "Any accurate model of star and planet formation must reproduce these observations," Close said.

These observations were possible only because of the combination of the University of Hawaii's uniquely sensitive Hokupa'a adaptive optics imaging system and the technical performance of the Gemini telescopes.

Adaptive optics is an increasingly crucial technology that eliminates most of the "blurring" caused by the turbulence in the Earth's atmosphere (i.e., the twinkling of the stars). It does this by rapidly adjusting the shape of a special flexible telescope mirror to match local turbulence, based on real-time feedback to the mirror's support system from observations of the low-mass star. Hokupa'a can count individual photons (particles of light) and so can sharpen accurately even very faint, low-mass stars.

The near-infrared adaptive optics images made by the 8-meter Gemini telescope in this survey were twice as sharp as those that can be made at the same wavelengths by the Earth-orbiting, 2.4-meter Hubble Space Telescope. The only ground-based survey of its kind, this work required five nights over one year with the Hokupa'a system at Gemini North.

Others on the observing team include James Liebert (Steward Observatory, University of Arizona), Wolfgang Brandner (European Southern Observatory, Garching, Germany), and Eduardo Martin and Dan Potter (Institute for Astronomy, University of Hawaii).

This ongoing survey is supported in part by the U.S. Air Force Office of Scientific Research and the University of Arizona's Steward Observatory. Hokupa'a is supported by the University of Hawaii Adaptive Optics Group and the National Science Foundation. The International Gemini Observatory is a multi-national collaboration that has built two nearly identical 8-meter telescopes on Mauna Kea, Hawaii, (Gemini North) and Cerro Pachón in central Chile (Gemini South). - By Lori Stiles

(Editor's Note: Images and illustrations related to this story are available at this URL.)

[Contact: Laird Close, Lori Stiles]






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