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PhD student Poshih Chiang and his supervisor Professor Wen-Ping Chen at the Graduate Institute of Astronomy (IoA) of NCU reported the discovery of two cool, and youngest brown dwarfs known so far. At a distance of 400 light years from us, these objects have surface temperatures less than 600 degree Celsius and ages no more than a couple million years. In comparison, our Sun is almost ten times as hot, with an estimated age of 5 billion years. With a mass between that of a star and a planet, a brown dwarf is not hot enough at the core to sustain thermonuclear reactions. It is an unsettled issue whether a brown dwarf is formed like a star, i.e., by gravitational collapse out of interstellar clouds, or like a planet, i.e., as a byproduct condensing out of gas and dust surrounding a star. A sample of new born brown dwarfs is the critical first step to offer an answer. All some hundreds of brown dwarfs known so far, however, are within tens of light years and with ages more than 100 million years old. The nearest places in space where celestial objects are being formed lie about 400 light years away, where brown dwarfs are very faint to detect and easily confused with distant background objects. The NCU group led an international team to use the 4-meter Canada-France-Hawaii Telescope located atop the volcanic Mauna Kea in Hawaii to obtain images with thousands of objects to select candidates showing methane absorptions, a characteristic molecular species in cool atmospheres. The researchers then used the 8-meter Gemini Telescope in Chile to acquire the confirmation spectroscopic data. Two methane-bearing brown dwarfs were found. The region they studied, the Rho Ophiuchi dark cloud, is teeming with stars which are well dated to be no more than a couple million years old. Spectral analysis suggests the two objects, Oph-T3 and Oph-T17, to have surface temperatures cooler than 600 degrees. The research article was published in Volume 811 of The Astrophysical Journal Letters in October 2015. “This is an important discovery,” said Chen, “to bridge our knowledge of the birth and early evolution of stars of exoplanets.” Chiang is now a postdoctoral fellow at IoA, continuing on scrutiny into these elusive celestial bodies.
