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

¹ Observatorio Astronómico Nacional, Instituto de Astronomía, UNAM, PO Box 439027, San Diego, CA 92143-9027, USA e-mail: {jal, wsteffen, gag}@astrosen.unam.mx
² LUTH, Observatoire de Meudon, 5 place Jules Janssen, 92195 Meudon Cedex, France e-mail: grazyna.stasinska@obspm.fr
³ Instituto de Astronomía, UNAM, Apartado Postal 70-264, 04510 México, D.F. México e-mail: jer@astroscu.unam.mx

Corresponding author: M. Richer, richer@astrosen.unam.mx

Received: 16 December 2002
Accepted: 2 August 2003

Abstract

PN G 135.9+55.9 is a compact, high excitation nebula that has been identified recently as the most oxygen-poor halo planetary nebula. Given its very peculiar characteristics and potential implications in the realms of stellar and Galactic evolution, additional data are needed to firmly establish its true nature and evolutionary history. Here we present the first long-slit, high spectral resolution observations of this object in the lines of Hα and λ4686. The position-velocity data are shown to be compatible with the interpretation of PN G 135.9+55.9 being a halo planetary nebula. In both emission lines, we find the same two velocity components that characterize the kinematics as that of an expanding elliptical envelope. The kinematics is consistent with a prolate ellipsoidal model with axis ratio about 2:1, a radially decreasing emissivity distribution, a velocity distribution that is radial, and an expansion velocity of 30 km s^-1 for the bulk of the material. To fit the observed line profiles, this model requires an asymmetric matter distribution, with the blue-shifted emission considerably stronger than the red-shifted emission. We find that the widths of the two velocity components are substantially wider than those expected due to thermal motions, but kinematic structure in the projected area covered by the slit appears to be sufficient to explain the line widths. The present data also rule out the possible presence of an accretion disk in the system that could have been responsible for a fraction of the Hα flux, further supporting the planetary nebula nature of PN G 135.9+55.9.