Water-maser emission from a planetary nebula with a magnetized torus

A star like the Sun becomes a planetary nebula towards the end of its life, when the envelope ejected during the earlier giant phase becomes photoioni zed as the surface of the remnant star reaches a temperature of similar to 30,000 K. The spherical symmetry of the giant phase is lost in the transiti on to a planetary nebula, when nonspherical shells and powerful jets develo p. Molecules that were present in the giant envelope are progressively dest royed by the radiation(1). The water-vapour masers that are typical of the giant envelopes(2,3) therefore are not expected to persist in planetary neb ulae(1,4). Here we report the detection of water-maser emission from the pl anetary nebula K3-35. The masers are in a magnetized torus with a radius of about 85 astronomical units and are also found at the surprisingly large d istance of about 5,000 astronomical units from the star, in the tips of bip olar lobes of gas. The precessing jets from K3-35 are probably involved in the excitation of the distant masers, although their existence is neverthel ess puzzling. We infer that K3-35 is being observed at the very moment of i ts transformation from a giant star to a planetary nebula.