A synchrotron jet from a post-asymptotic giant branch star

Abstract

The evolution of low- and intermediate-initial-mass stars beyond the asymptotic giant branch (AGB) remains poorly understood. High-velocity outflows launched shortly after the AGB phase are thought to be the primary shaping mechanism of bipolar and multipolar planetary nebulae. However, little is known about the launching and driving mechanism for these jets, whose momentum and energy often far exceed the energy that can be provided by radiation pressure alone. Here, we report on the direct evidence of a magnetically collimated jet shaping the bipolar morphology of the circumstellar envelope of a post-AGB star. We present radio continuum observations of the post-AGB star IRAS 15445-5449 (OH 326.5-0.4) which has water masers tracing a fast bipolar outflow. Our observations confirm the earlier observed steep negative spectral index of the spectral energy distribution (SED) above similar to 3 GHz, and resolve, for the first time, the emission to originate from a radio jet, proving the existence of such jets around a post-AGB star. The SED is consistent with a synchrotron jet embedded in a sheath of thermal electrons. We find a close correspondence between the extent and direction of the synchrotron jet and the bipolar shape of the object observed at other wavelengths, suggesting that the jet is responsible for the source morphology. The jet is collimated by a magnetic field of the order of mG at almost 7000 au from the central star. We recover observations from the Australia Telescope Compact Array archive that indicate that the emission measure of the thermal component has increased by a factor of 3 between 1998 and 2005 after which it has remained constant. The short time-scale evolution of the radio emission suggests a short lifetime for the jet. The observations of a synchrotron jet from a post-AGB star with characteristics similar to those from protostars and young stellar objects, for instance, suggest that magnetic launching and collimation is a common feature of astrophysical jets