Photoionization modelling based on HST images of Magellanic Cloud planetary nebulae - I. SMCN2 and SMCN5

Abstract

We construct fully self-consistent, detailed photoionization models for two planetary nebulae (PNe) in the Small Magellanic Cloud (SMC), namely SMC N 2 and SMC N 5, to fit optical and UV spectrophotometric observations as well as HST Faint Object Camera (FOC) narrow-band images taken in the light of Hβ. The derived density structure shows that both PNe have a central cavity surrounded by a shell of decreasing density described by a parabolic curve. For both nebulae, our models fail to reproduce the HST images taken in the light of the [OIII]lambda5007 line, in the sense that the observed [OIII]lambda5007 surface brightness decreases more slowly outside the peak emission than predicted. An effective temperature of T_eff=111500K, a stellar surface gravity of logg=5.45 and a luminosity of L_*=8430L_solar are derived for the central star of SMC N2; similarly T_eff=137500K, logg=6.0 and L_*=5850L_solar are derived for SMC N 5. SMC N 2 is optically thin and has a total nebular mass (H plus He) of 0.180 M_solar, while SMC N 5 is optically thick and has an ionized gas mass of 0.194 M_solar. Using the H-burning SMC metal abundance (Z=0.004) evolutionary tracks calculated by Vassiliadis & Wood, core masses of 0.674M_solar and 0.649M_solar are derived for SMC N 2 and SMC N 5, respectively. Similarly, from the He-burning evolutionary tracks of Vassiliadis & Wood for progenitor stars of mean LMC heavy-element abundance (Z=0.008), we find M_c=0.695 and 0.675M_solar for SMC N 2 and SMC N 5, respectively. We find that Hβ images are needed if one is to derive accurate stellar luminosities directly from photoionization modelling. However, in the absence of an Hβ image, photoionization models based on [OIII] images (and nebular line intensities) yield accurate values of T_eff and logg, which in turn allow reliable stellar masses and luminosities to be derived from a comparison with theoretical evolutionary tracks. We show that the correct nebular ionized mass can be deduced from the nebular Hβ flux, provided the mean nebular density given by the CIII]lambda1909/lambda1907 ratio is also known.