Limits on quaoar's atmosphere

Here we present high cadence photometry taken by the Acquisition Camera on Gemini South, of a close passage by the 540 km radius Kuiper belt object, (50000) Quaoar, of a r′ = 20.2 background star. Observations before and after the event show that the apparent impact parameter of the event was 0.″019 ± 0.″004, corresponding to a close approach of 580 ± 120 km to the center of Quaoar. No signatures of occultation by either Quaoar's limb or its potential atmosphere are detectable in the relative photometry of Quaoar and the target star, which were unresolved during closest approach. From this photometry we are able to put constraints on any potential atmosphere Quaoar might have. Using a Markov chain Monte Carlo and likelihood approach, we place pressure upper limits on sublimation supported, isothermal atmospheres of pure N₂, CO, and CH₄. For N₂ and CO, the upper limit surface pressures are 1 and 0.7 μbar, respectively. The surface temperature required for such low sublimation pressures is 33 K, much lower than Quaoar's mean temperature of 44 K measured by others. We conclude that Quaoar cannot have an isothermal N₂ or CO atmosphere. We cannot eliminate the possibility of a CH₄ atmosphere, but place upper surface pressure and mean temperature limits of 138 nbar and 44 K, respectively.