Pluto's atmosphere from stellar occultations in 2012 and 2013

We analyze two multi-chord stellar occultations by Pluto observed on July 18th, 2012 and May 4th, 2013, and monitored respectively from five and six sites. They provide a total of fifteen light-curves, twelve of them being used for a simultaneous fit that uses a unique temperature profile, assuming a clear (no-haze) and pure N_2 atmosphere, but allowing for a possible pressure variation between the two dates. We find a solution that fits satisfactorily (i.e. within the noise level) all the twelve light-curves, providing atmospheric constraints between ~1,190 km (pressure ~ 11 µbar) and ~ 1,450 km (pressure ~0.1 µbar) from Pluto's center. Our main results are: (1) the best-fitting temperature profile shows a stratosphere with strong positive gradient between 1,190 km (at 36 K, 11 µbar) and r = 1,215 km (6.0 µbar), where a temperature maximum of 110 K is reached; above it is a mesosphere with negative thermal gradient of -0.2 K/km up to ~ 1,390 km (0.25 µbar), where, the mesosphere connects itself to a more isothermal upper branch around 81 K; (2) the pressure shows a small (6%) but significant increase (6σ level) between the two dates; (3) without a troposphere, Pluto's radius is found to be ${R}_{{\rm{P}}}=1190\;\pm $ 5 km. Allowing for a troposphere, RP is constrained to lie between 1168 and 1195 km; and (4) the currently measured CO abundance is too small to explain the mesospheric negative thermal gradient. Cooling by HCN is possible, but only if this species is largely saturated. Alternative explanations like zonal winds or vertical compositional variations of the atmosphere are unable to explain the observed mesospheric negative thermal gradient.