Abstract:
The perihelion of the trans-Neptunian object (TNO) 2009 MS9 brings it close to the distance where some longperiod comets are seen to become active. Knowing this, and the fact that this object appears to brighten in excess of
its predicted nucleus brightness, suggests that 2009 MS9 has a delayed onset of activity brought on by the
sublimation of a species more volatile than water. In this paper, we characterize 2009 MS9ʼs physical properties
and investigate potential outgassing through composite images, sublimation models, and measurements of spectral
reflectivity. We find that deep composite images of the object at various epochs along its orbit show no evidence of
dust yet place sensitive limits to the dust production. We estimate the nucleus radius to be 11.5 ± 3.5 km using
thermal IR modeling from NEOWISE data and use this and data pre-perihelion to estimate a geometric albedo of
0.25. We compare a CO sublimation activity model to its post-perihelion heliocentric light curve and find that these
data support an active fractional area of 5 × 10−6 assuming 2 μm–sized grains and other typical comet parameters.
The spectral reflectivity of the surface materials obtained with the Gemini Observatory and CFHT at different
epochs shows a reddening spectral slope. We compare the physical properties of 2009 MS9 to both TNO and
comet populations and speculate that 2009 MS9ʼs reddening may be due to the buildup of a dust mantle on the
surface and could be an explanation of why TNOs exhibit a color bimodality
