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PN G135.9+55.9 is a compact, high excitation nebula that has been identifiedrecently as the most oxygen-poor halo planetary nebula. Given its very peculiarcharacteristics and potential implications in the realms of stellar andGalactic evolution, additional data are needed to firmly establish its truenature and evolutionary history. Here we present the first long-slit, highspectral resolution observations of this object in the lines of H$\alpha$ andHe II 4686. The position-velocity data are shown to be compatible with theinterpretation of PN G135.9+55.9 being a halo planetary nebula. In bothemission lines, we find the same two velocity components that characterize thekinematics as that of an expanding elliptical envelope. The kinematics isconsistent with a prolate ellipsoidal model with axis ratio about 2:1, aradially decreasing emissivity distribution, a velocity distribution that isradial, and an expansion velocity of 30 km/s for the bulk of the material. Tofit the observed line profiles, this model requires an asymmetric matterdistribution, with the blue-shifted emission considerably stronger than thered-shifted emission. We find that the widths of the two velocity componentsare substantially wider than those expected due to thermal motions, butkinematic structure in the projected area covered by the slit appears to besufficient to explain the line widths. The present data also rule out thepossible presence of an accretion disk in the system that could have beenresponsible for a fraction of the H$\alpha$ flux, further supporting theplanetary nebula nature of PN G135.9+55.9.Comment: accepted by Astronomy & Astrophysic

The kinematics of the most oxygen-poor planetary nebula PN G 135.9+55.9