2019ApJ...878...90S

Jets and outflows are ubiquitous phenomena in astrophysics, found in our Galaxy in diverse environments, from the formation of stars to late-type stellar objects. We present observations conducted with the 305 m Arecibo Telescope of the preplanetary nebula CRL 618 (Westbrook Nebula)-a well-studied late-type star that has developed bipolar jets. The observations resulted in the first detection of 4765 MHz OH in a late-type stellar object. The line was narrow (FWHM ∼ 0.6 km s^–1) and ∼40 km s^–1 blueshifted with respect to the systemic velocity, which suggests association with the expanding jets/bullets in CRL 618. We also report nondetection at Arecibo of any other OH transition between 1 and 9 GHz. The nondetections were obtained during the observations in 2008, when the 4765 MHz OH line was first discovered, and also in 2015 when the 4765 MHz OH line was not detected. Our data indicate that the 4765 MHz OH line was a variable maser. Modeling of the 4765 MHz OH detection and nondetection of the other transitions is consistent with the physical conditions expected in CRL 618. The 4765 MHz OH maser could originate from dissociation of H₂O by shocks after sublimation of icy objects in this dying carbon-rich stellar system, although other alternatives such as OH in an oxygen-rich circumstellar region associated with a binary companion are also possible.