LIU, XW; BARLOW, MJ; BLADES, JC; OSMER, S; CLEGG, RES; (1995) Photoionization modelling based on HST images of Magellanic Cloud planetary nebulae – I. SMC N 2 and SMC N 5. Monthly Notices of the Royal Astronomical Society , 276 (1) pp. 167-181. 10.1093/mnras/276.1.167.

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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 [O III] λ 5007 line, in the sense that the observed [O III] λ 5007 surface brightness decreases more slowly outside the peak emission than predicted. An effective temperature of Teff = 111 500 K, a stellar surface gravity of log g = 5.45 and a luminosity of L* = 8430 Lʘ are derived for the central star of SMC N2; similarly Teff = 137 500 K, log g = 6.0 and L* = 5850 Lʘ are derived for SMC N 5. SMC N 2 is optically thin and has a total nebular mass (H plus He) of 0.180 Mʘ, while SMC N 5 is optically thick and has an ionized gas mass of 0.194 Mʘ. Using the H-burning SMC metal abundance (Z = 0.004) evolutionary tracks calculated by Vassiliadis & Wood, core masses of 0.674 Mʘ and 0.649Mʘ 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 Mc = 0.695 and 0.675 Mʘ 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 [O III] images (and nebular line intensities) yield accurate values of Teff and log g, 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 C III] λ 1909/λ 1907 ratio is also known.

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