Abstract
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 (PNe), 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, realised 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 (LSD) 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 (i.e. ~10 gauss level) at the stellar surface of PAGB stars opens the door to a better understanding of magnetism in evolved stars.
Highlights
The magnetic field strength at the surface or photosphere of evolved intermediate mass stars such as Asymptotic Giant Branch (AGB) stars, Post Asymptotic Giant Branch (PAGB) stars and Planetary Nebulae (PNe) is a poorly if not totally unknown parameter to this date
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 (PNe), none has clearly demonstrated their presence at the stellar surface
We present new high resolution spectropolarimetric (Stokes V ) observations of a sample of PAGB stars, realised with the instruments ESPaDOnS and Narval, where we searched for the presence of photospheric magnetic fields
Summary
The magnetic field strength at the surface or photosphere of evolved intermediate mass stars (with initial masses in the range ∼ 0.8–8 M ) such as Asymptotic Giant Branch (AGB) stars, Post Asymptotic Giant Branch (PAGB) stars and Planetary Nebulae (PNe) is a poorly if not totally unknown parameter to this date. This knowledge is of great importance for the understanding of the role played by magnetic fields in the dynamics as well as in the chemistry of these stellar objects.
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