Possible Activity in 468861 (2013 LU28)

Manx comets are objects on long-period comet orbits that are inactive as they approach perihelion. They are of particular interest because they may help constrain solar system formation models. 2013 LU28 was discovered as an inactive asteroidal object on 2013 June 8 at a heliocentric distance of 21.8 au. Images and photometric data were obtained of 2013 LU28 from multiple telescopes from pre-discovery data in 2010 until the present. Its spectral reflectivity is consistent with typical organic-rich comet surfaces with colors of g ′ − r ′ = 0.97 ± 0.02, r ′ − i ′ = 0.43 ± 0.02, and r ′ − z ′ = 0.65 ± 0.03, corresponding to a spectral reflectivity slope of 30 ± 3%/100 nm. There is no obvious indication of dust coma in deep stacked images. We estimate the nucleus radius to be ∼55.7 ± 0.3 km assuming an albedo of 4%. This is much smaller than the 1 σ upper limits on the nucleus size of 79.9 km from the NEOWISE survey assuming the same albedo, since the NEOWISE survey is not very sensitive to objects this small at this distance. The heliocentric light curve suggests possible activity between r ∼ 17 and 13 au where 2013 LU28 is brighter than expected. This is consistent with outgassing from CO or CO 2 . Using surface brightness profiles, we estimate an upper limit of ∼0.01 kg s −1 for micron-sized dust that can be produced without us detecting it for the inactive portion of the light curve, and upper limits of ∼1 kg s −1 for CO and ∼1.5 kg s −1 for CO 2 between 20 and 14.7 au.