June2006
by Keivan G. Stassun
Phoenix Measures a Brown-Dwarf Binary O
bservations of a newly discovered
periodic diminutions of flux. The I-band (0.8-
brown-dwarf eclipsing binary using the
micron) light curve (Figure 1) clearly shows the
high-resolution near-infrared Phoenix
eclipsing nature of the system and provides a
spectrograph on Gemini South provide the first
precise measure of the orbital period (about 9.8
direct, accurate measurements of the fundamental
days).
physical properties of two young brown dwarfs. These measurements have important implications
Phoenix/Gemini Observations
for theories about how such objects form and In order to measure the masses of the 2M0535-05
evolve.
components, we required accurate radial-velocity Brown dwarfs are “failed stars” that span the
measurements of the system from time-series
divide between stars and planets. They serve as
observations of its double-lined spectrum. Given
a critical link between theories of star and planet
the extreme faintness of the pair in visible light
formation. Yet, even the most fundamental physical
(at about 22nd magnitude) and the need for high
properties of brown dwarfs—their masses and
resolving power (R ~ 30,000), we sought high-
radii—have so far eluded direct measurement. Prior
resolution spectroscopy in the near-infrared, a
to our observations, the mass of only one brown
capability provided to the U.S. community only
dwarf had been measured with sufficient accuracy
through Phoenix on Gemini South. The Gemini
to demonstrate that the object was, indeed, a
Time Allocation Committee generously awarded
brown-dwarf. In no case had a brown dwarf ’s
40 hours of Scientific Band 1 queue time for this
1
radius been measured directly. We have discovered the object 2M0535-05 to be a brown-dwarf eclipsing binary—the first of its kind—in the young (about a million years old) Orion Nebula Cluster. This discovery was made as part of an ongoing long-term program to identify and study new eclipsing binaries in nearby starforming regions by repeatedly imaging thousands of young stars and searching for those that exhibit
www.gemini.edu
Figure 1.
A light curve of 2M0535-05 at 0.8 microns, folded on a period of 9.78 days and phased relative to periastron passage. A model fit incorporating the orbital and physical parameters of the binary system is shown (solid red curve). Surprisingly, in this system, the deeper eclipse corresponds to the eclipse of the lower-mass component, implying that it is hotter than the higher-mass component.
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