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

TL;DR

This study reports the first detection of weak surface magnetic fields in post-AGB stars U Monocerotis and R Scuti, using high-resolution spectropolarimetry, revealing magnetic influence on stellar atmospheres.

Contribution

It provides the first direct evidence of magnetic fields at the stellar surface of post-AGB stars, advancing understanding of magnetism in evolved stellar objects.

Findings

Detected magnetic fields of ~10 G in U Mon and R Sct.

Magnetic fields interact with atmospheric dynamics.

No significant magnetic detection in five other PAGB stars.

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.