Constraints on the Orbit of the Moon of Mid-Size TNO 2013 FY27

The Trans-Neptunian Object (TNO) 2013 FY27 has an ALMA-estimated diameter of 765+80-85 km, placing it in the transition between small low-density TNOs and large high-density TNOs. It is similar to “mid-size” TNOs 2002 UX25 and 2007 UK126 (Gǃkúnǁ'hòmdímà and Gǃò'é ǃHú). Since the cosmochemical abundance is expected to be reasonably well-mixed among TNOs, a leading hypothesis for the density transition is that mid-sized objects are smaller and able to retain a significant porosity (50-90%), suggesting they were never heated. Better understanding this transition and its implication for TNO formation requires expanding the number of mid-size TNOs with well-measured densities. Although 2013 FY27 has a known moon and multiple epochs of HST data, its orbit has remained elusive. The moon is faint (delta magnitude of 3.0) and never seen more than 0.25 arcseconds away and highly blended in most epochs suggesting an edge-on orbit. A common approximation for non-detections is to assume relative astrometric position of 0 with uncertainties roughly encompassing the area where the satellite could have been seen. With ~half of the relatively few epochs showing non-detections, we have developed a new technique within our PSF-fitting code “nPSF” to return a “likelihood map”, marginalized over the expected brightness of the moon, that more precisely represents the result of weak or non-detections. Combining this with our TNO binary orbit fitting tool MultiMoon, we will provide updated orbital constraints on the density of FY27 with implications for mid-size TNOs.