Limits on Quaoar's Atmosphere

TL;DR

This study uses high-cadence photometry to constrain the presence of Quaoar's atmosphere, setting upper limits on atmospheric pressure and temperature, and concluding it cannot sustain an isothermal N2 or CO atmosphere.

Contribution

The paper provides the first observational constraints on Quaoar's atmosphere using occultation data and MCMC analysis, establishing upper limits on atmospheric pressure and temperature.

Findings

No detectable occultation signatures were observed.

Upper limits on N2 and CO surface pressures are 1 and 0.7 μbar.

Quaoar cannot have an isothermal N2 or CO atmosphere.

Abstract

Here we present high cadence photometry taken by the Acquisition Camera on Gemini South, of a close passage by the $\sim540$ 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 $0.019\pm0.004$", corresponding to a close approach of $580\pm120$ km to the centre 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 N$_2$, CO, and CH$_4$. For N$_2$ and CO, the upper limit surface pressures are 1 and 0.7 $\mu{bar}$ respectively. The surface temperature required for such low sublimation pressures is $\sim33$ K, much lower than Quaoar's mean temperature of $\sim44$ K measured by others. We conclude that Quaoar cannot have an isothermal N$_2$ or CO atmosphere. We cannot eliminate the possibility of a CH$_4$ atmosphere, but place upper surface pressure and mean temperature limits of $\sim138$ nbar and $\sim44$ K respectively.