First detection of surface magnetic fields in post-AGB stars: the cases of U Monocerotis and R Scuti

While several observational investigations have revealed the presence of magnetic fields in the circumstellar envelopes, jets and outflows of post-asymptotic giant branch stars (PAGBs) and planetary nebulae, none has clearly demonstrated their presence at the stellar surface. The lack of information on the strength of the surface magnetic fields prevents us from performing any thorough assessment of their dynamic capability (i.e. material mixing, envelope shaping, etc). We present new high-resolution spectropolarimetric (Stokes V) observations of a sample of PAGB stars, realized with the instruments ESPaDOnS and Narval, where we searched for the presence of photospheric magnetic fields. Out of the seven targets investigated, the RV Tauri stars U Mon and R Sct display a clear Zeeman signature and return a definite detection after performing a least squares deconvolution analysis. The remaining five PAGBs show no significant detection. We derived longitudinal magnetic fields of 10.2 +/- 1.7 G for U Mon and 0.6 +/- 0.6 G for R Sct. In both cases, the Stokes profiles point towards an interaction of the magnetic field with the atmosphere dynamics. This first discovery of weak magnetic fields (e.g. similar to 10 G level) at the stellar surface of PAGB stars opens the door to a better understanding of magnetism in evolved stars.