Tracing the ISM–PN interaction: a morphokinematic study of Abell 71

ABSTRACT Abell 71 is an intermediate-age planetary nebula (PN) previously classified as spherical, though its actual morphology remains uncertain. In this study, we present a detailed morphokinematic analysis of Abell 71 based on high-resolution spectroscopy and narrow-band imaging obtained at the San Pedro Mártir National Astronomical Observatory (OAN-SPM). Position–velocity (PV) diagrams were constructed from long-slit Echelle spectra of the [N ii]$\lambda$6583 line, revealing complex kinematic features. We determined the main velocities of Abell 71: an expansion velocity of $V_{\rm exp} = 23\pm 2$ km s$^{-1}$ and a systemic velocity of $V_{\rm sys}^{\rm LSR} = 13 \pm 2$ km s$^{-1}$ ($V_{\rm sys}^{\rm HEL}=-7\pm 2\, {\rm km\, s}^{-1}$). Furthermore, we determined that the angular size of the nebula is $156\pm 2$ arcsec. Using the shapex modelling tool, we reconstructed the three-dimensional structure of the nebula, identifying an ellipsoidal main body with a thin shell, an upper arm, and two lower arms. Kinematic age estimates suggest a sequential history of ejections, with the main structure forming $\simeq 17\,400\pm 2800$ yr ago, followed by the lower arms at $\simeq 11\,800\pm 3200$ and $\simeq 3000\pm 300$ yr. The upper arm shows no detectable expansion, suggesting an interaction with pre-existing interstellar medium (ISM) material. The strong [N ii]/H$\alpha$ emission in this region indicates the presence of J-type shocks, reinforcing the hypothesis of an ISM interaction. Furthermore, the nebula exhibits multiple bulges and voids, likely resulting from density or hydrodynamic instabilities and past mass-loss episodes. Our findings confirm that Abell 71 is not purely spherical but rather an ellipsoidal nebula influenced by ISM interactions, shedding new light on its evolutionary history.