Figure 3.
in the cold classicals
would not have sur-
vived outward scat-
tering experienced
by the hot popula-
tion, providing the
strongest evidence
we have that the
cold classicals have
not moved signifi-
cantly since forma-
tion (Parker & Kave-
laars, 2010).
For some time, the Kuiper Belt science com-
munity has recognized an opportunity to
trace out Neptune’s dynamics using the cur-
rent distribution of KBOs — not just their or-
bital distribution, mind you, but their color
distribution as well. The simple version goes
like this: if the cold classicals all formed with-
in the current cold classical belt, then it holds
that by identifying these objects outside that
region by their red colors and high frequency
of binarity, we can place strong constraints
on the possible migration scenarios that
have sculpted the region.
Currently, there are more than 1,700 KBOs
catalogued in the Minor Planet Center. A big
boost has come from the Outer Solar System
Origins Survey (OSSOS; Bannister et al., 2016)
which searched and tracked nearly 1,000
KBOs over a total area of about 170 square
degrees. OSSOS provided the perfect survey
from which to apply the idea of color map-
ping to trace the early dynamics of the Kuip-
er Belt. From this, the Gemini Large and Long
program, Colours of the Outer Solar System
Origins Survey (Col-OSSOS), was launched.
Operating simultaneously on Gemini-North
and the Canada-France-Hawai’i Telescope
(CFHT; Figure 2), Col-OSSOS measured UV-
Optical-NIR colors of 81 objects (to date and
counting) to find identifying surface signa-
tures of unique populations like the cold-
classicals, and then map those populations
throughout the Kuiper Belt region.
July 2017
The first big success of Col-OSSOS came
from the unexpected discovery of a popula-
tion colloquially known as the blue binaries
(Fraser et al., 2017; Figure 3). As their name
suggests, these objects are predominantly
(if not entirely) in widely separated, binary
pairs (Figure 4), and belong to the blue class
of KBOs. What’s strange, however, is that
these blue binaries are only found among
the cold classicals; to first order, their orbital
distribution is indistinguishable from the red
cold classicals.
The six known blue binaries contrast with
most properties of the red cold classicals:
they aren’t the same color; they are entirely
binary compared to the red cold classicals of
which only ~ 30% are binary; and, critically,
they are all in extremely fragile widely sepa-
rated pairs. That last detail was important to
recognize; recall that the fragility of these
binaries has been used as the best evidence
for the hypothesis of in-situ formation for the
cold classical KBOs. It implies then that the
blue binaries also formed in-situ.
The difficulty with this idea, however, is that
no known coloring process could reproduce
the observations: only binary cold classicals
are blue; only some binaries are blue; and
for all binaries observed to date, both com-
ponents are equally colored. For example,
stochastic collisions could dredge up fresh
GeminiFocus
Left Top: Binary
semi-major axis
versus optical
spectral slope,
s, of known
CCKBO binary
objects with
well determined
colors. We
quantify a
target’s color
with spectral
slope, defined
as percent
increase in reflectance
per 100 nm change in
wavelength normalized to
550 nm. Points in red are
new binaries presented
here. Round points
indicate systems for which
the binary semi-major
axis has been determined.
Triangles are lower limits
on semi-major axis.
Bottom: Cumulative
spectral slope distribution
of single (58 objects,
solid line) and binary
cold classical objects (29
objects, dashed line). The
vertical dotted line is the
spectral slope that divides
the blue and red classes
of the dynamically excited
KBOs.
Right: Images of the four
new binaries, scaled to
the same relative distance
scale. Black lines show the
fitted distances of the two
components. The points
are roughly 5x larger
than the true sizes of the
objects. Clockwise from
top-left, 2002 VD131,
2016 BP81, 2014 UD255,
and 2013 SQ99. The Earth,
with mean diameter
12,742 km, is shown for
scale.
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