TIME-VARIABLE NON-THERMAL EMISSION in the PLANETARY NEBULA IRAS 15103-5754

The beginning of photoionization marks the transition between the post-AGB and planetary nebula (PN) phases of stars with masses ≲ 8 M ⊙ . This critical phase is difficult to observe, as it lasts only a few decades. The combination of jets and magnetic fields, the key agents of PN?shaping, could give rise to synchrotron emission, but this has never been observed before in any PNe, since freefree emission from the ionized gas is expected to dominate its radio spectrum. In this paper we report radio continuum observations taken with the ATCA between 1 and 46 GHz of the young PN IRAS 151035754. Our observations in 20102011 show non-thermal emission compatible with synchrotron emission from electrons accelerated at a shock with spectral index a α -0.54. However, in 2012, the spectral index a ≃ -0.28 is no longer compatible with synchrotron emission in these types of processes. Several hypotheses are discussed to explain this change. The more plausible ones are related to the presence of the newly photoionized region in this young PN: either energy loss of electrons due to Coulomb collisions with the plasma, or selective suppression of synchrotron radiation due to the Razin effect. We postulate that the observed flattening of non-thermal radio spectra could be a hallmark identifying the beginning of the PN phase. © 2015. The American Astronomical Society. All rights reserved.