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 \sim 1,190Â km ( pressure \sim 11Â \mu bar ) and \sim 1,450Â km ( pressure \sim 0.1Â \mu bar ) from Pluto ’ s center .
Our main results are : ( 1 ) the best-fitting temperature profile shows a stratosphere with strong positive gradient between 1,190Â km ( at 36Â K , 11Â \mu bar ) and r = 1 , 215 Â km ( 6.0Â \mu bar ) , where a temperature maximum of 110Â K is reached ; above it is a mesosphere with negative thermal gradient of -0.2Â KÂ km ^ { -1 } up to \sim 1,390Â km ( 0.25Â \mu bar ) , where , the mesosphere connects itself to a more isothermal upper branch around 81Â K ; ( 2 ) the pressure shows a small ( 6 % ) but significant increase ( 6- \sigma level ) between the two dates ; ( 3 ) without a troposphere , Pluto ’ s radius is found to be R _ { P } = 1 , 190 \pm 5Â km .
Allowing for a troposphere , R _ { P } is constrained to lie between 1,168 and 1,195Â km ; ( 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 trend .