Tracing the ISM–PN Interaction: A Morphokinematic Study of Abell 71

Abstract Abell 71 is an intermediate-age 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 narrowband 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 λ6583 line, revealing complex kinematic features. We determined the main velocities of Abell 71: an expansion velocity of Vexp = 23 ± 2 km s−1 and a systemic velocity of V ₒₘₒ^ LSR = 13 2 km s−1 (V ₒₘₒ^ HEL=-7 2\, km\, s^-1). Furthermore, we determined that the angular size of the nebula is 156 ± 2 arcsec. Using the ShapeX modeling 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 ≃ 17, 400 ± 2800 years ago, followed by the lower arms at ≃ 11, 800 ± 3, 200 and ≃ 3, 000 ± 300 years. The upper arm shows no detectable expansion, suggesting an interaction with pre-existing ISM material. The strong N ii/Hα 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.