GeminiFocus July 2017 | Page 7

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. 5