My first Magazine Sky & Telescope - 01.2019 | Page 39

The mystery behind this dearth of nearby
brown dwarfs deeply puzzles Sergio
Dieterich. “Trying to understand why this
happens is something I lose sleep on.”
5 components
4 components
3 components
2 components
1 component
versity of St. Andrews, UK), at the forefront
of these searches. SONYC’s studies of the
star-forming regions NGC 1333 in Perseus
and RCW 38 in Vela, as well as of the Rho
Ophiuchus cloud complex, have turned up
a brown dwarf-to-star ratio of between 0.2
and 0.5. In other words, for every brown
dwarf in this region, there are two to five
stars. The number of brown dwarfs could be
higher if some of the stars host brown dwarf
companions that have yet to be detected.
Extrapolating from this, Scholz’s group
calculates that there could be as many as
100 billion brown dwarfs inhabiting the
Milky Way Galaxy. Yet these numbers fail to
add up in the Sun’s neighborhood, where the
ratio of brown dwarfs to stars is just 0.13, or
about 1 to 8.
The mystery behind this dearth of nearby
brown dwarfs deeply puzzles Sergio Dieterich
(Carnegie Institution for Science), who is
also a member of the RECONS consortium.
“Trying to understand why this happens is
something I lose sleep on,” he says.
In an effort to reconcile the observations,
Dieterich is modeling the cooling rates of
brown dwarfs to figure out if there could be
more brown dwarfs in the Sun’s neighbor-
hood that remain undetected because they
are too cold to emit at the observed wave-
lengths. Brown dwarfs spend their entire
lives cooling after they form, and those that
RECONS and SONYC see have temperatures
in the 1500K to 3000K range, which means
they must be fairly young.
However, NASA’s Wide-field Infrared
Survey Explorer (WISE) was optimized to
find cool objects. The coldest brown dwarf
yet discovered is WISE 0855−0714, which has
a surface temperature of some 250K (−23°C).
Yet WISE’s 2012 survey may support the
RECONS findings, by detecting on average
just one brown dwarf for every six stars in
the solar neighborhood.
If there are many, much older brown
dwarfs that have cooled to become too faint
for even WISE to see, maybe Scholz is correct
and there are more brown dwarfs out there.
p NGC 1333 This composite image combines X-ray (pink), infrared (red),
and optical data (red, green, blue) of the cluster NGC 1333, populated
with stars that are less than 2 million years old. The X-ray data reveal
95 young stars, 41 of which had not been identii ed in infrared because
there was no glow from a surrounding disk.
L.,
ASA
= 1 system
SIRIUS At apparent
magnitude −1.46,
Alpha Canis Majoris
is the brightest star
in the sky and lies 8.6
light-years from Earth.
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