The determination of the mass of a Magellanic Cloud planetary nebula by speckle interferometry

We have resolved a Magellanic Cloud planetary nebula by speckle interferometry for the first time. Our observations of SMC N2 show it to have a double-ring structure, the rings having angular radii of 0.22 and 0.38 arcsec, corresponding to absolute radii of 0.06 and 0.10 pc. Our speckle observations of the galactic planetary nebula Vy 2–2 show a ring of angular diameter 0.4 arcsec, in agreement with previous VLA radio data. The radial hydrogen density profile for SMC N2 is determined and masses of 0.09$M_\odot$ and 0.27$M_\odot$ for the inner and outer shells respectively are derived, so that the total mass of this optically thin nebula is 0.36$M_\odot$. The nebular expansion velocity of SMC N2 has been derived from a high-resolution spectrum of the [O III] 5007 Å line and expansion ages of between 5000 and 12 000 yr have been derived for the shells. A reanalysis of the available nebular data leads us to derive a central star effective temperature of 110 000 K and a luminosity of 4340$L_\odot$, corresponding to a stellar mass of 0.59$M_\odot$, from a comparison with published evolutionary tracks. The stellar evolutionary age since leaving the AGB is about 8000 yr, consistent with the nebublar expansion age.