Stassun - 2006 Nature Publishing Group Discovery of two...

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Unformatted text preview: 2006 Nature Publishing Group Discovery of two young brown dwarfs in an eclipsing binary system Keivan G. Stassun 1 , Robert D. Mathieu 2 & Jeff A. Valenti 3 Brown dwarfs are considered to be failed stars in the sense that they are born with masses between the least massive stars (0.072 solar masses, M ( ) 1 and the most massive planets ( , 0.013 M ( ) 2 ; they therefore serve as a critical link in our understanding of the formation of both stars and planets 3 . Even the most fundamental physical properties of brown dwarfs remain, however, largely unconstrained by direct measurement. Here we report the dis- covery of a brown-dwarf eclipsing binary system, in the Orion Nebula star-forming region, from which we obtain direct measurements of mass and radius for these newly formed brown dwarfs. Our mass measurements establish both objects as brown dwarfs, with masses of 0.054 6 0.005 M ( and 0.034 6 0.003 M ( . At the same time, with radii relative to the Suns of 0.669 6 0.034 R ( and 0.511 6 0.026 R ( , these brown dwarfs are more akin to low-mass stars in size. Such large radii are generally consistent with theoretical predictions for young brown dwarfs in the earliest stages of gravitational contraction 4,5 . Surprisingly, however, we find that the less-massive brown dwarf is the hotter of the pair; this result is contrary to the predictions of all current theoretical models of coeval brown dwarfs. Mass is the most fundamental property of a brown dwarf, as it determines all other physical characteristics and governs how brown dwarfs evolve with time. Theoretical brown-dwarf evolution models 2,4,5 make specific, testable predictions of the relationship between a brown dwarfs mass and its other physical properties, such as radius and temperature. Unfortunately, these predictions have yet to be empirically tested because the basic physical properties of brown dwarfs are extremely difficult to measure; the mass of only one brown dwarf has been measured with sufficient accuracy to make the brown-dwarf nature of the object definitive 6 , and in no case has a brown dwarfs radius been measured directly. Astronomers have traditionally used stellar eclipsing binaries to measure the masses and radii of stars 7 , but until now no brown-dwarf eclipsing binaries were known. Here we have discovered the 2-Micron All-Sky Survey (2MASS) object J053521840546085 to be a previously unknown eclipsing binary in the Orion Nebula, and the first example of an LETTERS Figure 1 | Light curve of 2MASS J053521840546085 at 0.8 m m. We repeatedly imaged 2MASS J053521840546085 with the 0.9-m telescope at Kitt Peak National Observatory and with the 0.9-m, 1.0-m and 1.3-m telescopes at the Cerro Tololo Inter-American Observatory, from December 1994 to March 2005. In total, 1,590 flux measurements were obtained on 280 separate nights and with an average cadence of 56 measurements per night. Error bars show ^ 1 s.d.; the typical uncertainty is 2%. A time-series1 s....
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This note was uploaded on 11/29/2011 for the course AST 371 taught by Professor Bonamente during the Fall '09 term at University of Alabama - Huntsville.

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Stassun - 2006 Nature Publishing Group Discovery of two...

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