planet directly. It would need to be actively
accreting material to be bright enough to
detect easily in future GPI observations. The
Gemini website has some more information,
and complete results are published in The Astrophysical Journal Letters.
Figure 2.
Limits on separation
and magnitude for a
binary companion to
one of the Y dwarfs
Opitz and colleagues
observed using GeMS/
GSAOI. An equalbrightness companion
is ruled out to within
about 0.5 AU (0.04
arcsecond), and fainter
companions are ruled
out at somewhat
larger radii.
Seeking Companions of the
Coolest Brown Dwarfs
Examples of the coolest and least massive
brown dwarfs, Y dwarfs, were first identified in
2011. Having temperatures just above those
of the gas giant planets (around 250 K), they
help bridge the gap from stellar objects to
planets. The binary nature of any of these objects is linked to their formation process. Previous observations indicate that the frequency
of multiplicity declines from around 65% (for
solar-type stars) to 10–30% (for the slightly
warmer and more massive L and T dwarfs).
Does this trend continue to the Y dwarfs, or
does it indicate only our observational limits?
Also, some Y dwarfs show a spread of luminosity or otherwise seem overluminous. Are
undetected companions the explanation?
Daniela Opitz (University of New South Wales,
Australia) and colleagues used the fine spatial resolution of the Gemini Multi-conjugate
adaptive optics System (GeMS) and the Gemini South Adaptive Optics Imager (GSAOI) to
begin to answer these questions, examining
8
GeminiFocus
a small sample of five Y dwarfs. The delivered
Full-Width at Half-Maximum was ~0.1arcsecond and the limiting angular separation was
around 0.04 arcsecond. Although the observations were sufficiently sensitive to detect
companions of roughly equal mass at separations of 0.5–1.9 astronomical units (AU), they
did not find any evidence for binaries. Figure 2
shows the limits on separation and brightness
for a binary companion to one of the Y dwarfs
studied.
At least one of the sources had previously
been identified as “overluminous.” The presence of clouds in the atmosphere, rather than
a companion, may account for the excess luminosity. The few cases observed here are a
good start, not a definitive determination of
the general trends. They do point to the extreme scenarios (separations less than 1 AU
and extremely faint sources) that may arise in
cases of Y dwarf binaries. This work is featured
on the Gemini website, and complete results
appear in The Astrophysical Journal.
A Supermassive Black Hole
That Wasn’t So Massive
One sign of an extreme supermassive black
hole at a galaxy’s core is a light deficit — the
consequence of stars ejected from the central
region. The brightest cluster galaxy of Abell
85 had been identified as such an example,
claimed to host one of the most massive black
holes ever detected in the Universe at around
1011 MSun.
Juan Madrid, then a Science Fellow at Gemini
South, along with Carlos Donzelli (Observatorio Astronómico de Córdoba, Argentina), used
images obtained with the Gemini Multi-Object Spectrograph (GMOS) on Gemini South
(Figure 3) to probe the galaxy’s center and
demonstrate that the black hole’s mass is not
so extreme. Rather than a deficit, data from
their Director’s Discretionary Time program
show the strong nuclear emission in the cen-
April 2016