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 (PNe). 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 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 ∼ 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 wavelenghts, 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 ATCA archive that indicate that the emission measure of the thermal component has increased by a factor of three between 1998 and 2005 after which it has remained constant. The short timescale 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.

Deep into the Water Fountains

“…Among them, the SED measured towards WF IRAS 15445−5449 was found to display a steep negative spectral index between ≈ 5 GHz and ≈ 8.7 GHz. Pérez-Sánchez et al (2013) confirmed that the non-thermal emission is arising from a synchrotron jet, spatially resolved at 22 GHz.…”

“…This scenario implies the presence of a strong magnetic field which could be related to the collimation of the high-velocity outflow traced by the H 2 O maser emission (e.g. Pérez-Sánchez et al 2013). Nonetheless, new observations of radio continuum emission above 10 GHz are needed in order to probe the source of the non-thermal component.…”

“…Given the velocity of the spectral features detected, the dynamics of these layers might be a consequence of the passage of a highvelocity outflow throughout the slower and relic AGB wind. Observational tools such as molecular H 2 emission at near infrared wavelengths, radio continuum emission, and both Hydroxyl (OH) and H 2 O maser emission have been used in order to trace the actions of shocks, magnetic fields, and ionized shells on large-scale bipolar structures observed towards post-AGB stars (Sahai et al 2005a;Cerrigone et al 2008;Walsh et al 2009;Bains et al 2009;Lagadec et al 2011;Cerrigone et al 2011;Pérez-Sánchez et al 2013;Tafoya et al 2014;Gonidakis et al 2014;Gledhill & Forde 2015). In the context of the evolution of the CSEs of late-type stars, multi-frequency observational studies of Water Fountain nebulae provide important information regarding the physics behind the shapes of the bipolar structures observed towards more evolved counterparts.…”

The jet feedback mechanism (JFM) in stars, galaxies and clusters

“…Many new observations and theoretical studies since the turn of the Millennium further consolidated the jet-shaping scenario of many PNe (e.g., Parthasarathy et al 2000;Bujarrabal et al 2001;Miranda et al 2001;Corradi et al 2001b;Guerrero et al 2001;Imai et al 2002;Riera et al 2003;Vinković et al 2004;Huggins et al 2004;Peña et al 2004;Balick & Hajian 2004;Arrieta et al 2005;Oppenheimer et al 2005;Sahai et al 2005;Vlemmings et al 2006;Miranda et al 2010;Nakashima et al 2010;Phillips et al 2010;Sahai et al 2011;Szyszka et al 2011;López et al 2012;Clark et al 2013;Pérez-Sánchez et al 2013;Vlemmings et al 2014;Fang et al 2015 Dennis et al 2008Dennis et al , 2009Lee et al 2009;Huarte-Espinosa et al 2012;Velázquez et al 2012;Balick et al 2013;Velázquez et al 2014 for theoretical studies; these papers composed a small fraction of the papers studying jets in PNe). X-ray observations seem also to support the presence of jet-inflated bubbles and of jets Sahai et al 2003), similar to X-ray jets in symbiotic systems (e.g., Kellogg et al 2001;Galloway & Sokoloski 2004).…”

Simplified models of stellar wind anatomy for interpreting high-resolution data

“…Decin et al 2012), magnetic fields (e.g. Pérez-Sánchez et al 2013;van Marle et al 2014), or binarity (Soker 1997;Huggins 2007). The latter need not be surprising as the multiplicity frequency of the progenitors of AGB stars is above 50 percent Article published by EDP Sciences A118, page 1 of 26 (Raghavan et al 2010;Duchêne et al 2013).…”