Space Telescope Imaging Spectrograph slitless observations of Small Magellanic Cloud Planetary Nebulae: a study on morphology, emission line intensity, and evolution. 1

A sample of 27 Planetary Nebulae (PNs) in the Small Magellanic Clouds (SMC) have been observed with the Hubble Space Telescope Imaging Spectrograph (HST/STIS) to determine their morphology, size, and the spatial variation of the ratios of bright emission lines. The morphologies of SMC PNs are similar to those of LMC and Galactic PNs. However, only a third of the resolved SMC PNs are asymmetric, compared to half in the LMC. The low metallicity environment of the SMC seems to discourage the onset of bipolarity in PNs. We measured the line intensity, average surface brightness (SB), and photometric radius of each nebula in Hα , Hβ , [Oiii] λ4959 and 5007, [Nii] λ6548 and 6584, [Sii] λ6716 and 5731, He I λ6678, and [Oi] λ6300 and 6363. We show that the surface brightness to radius relationship is the same as in LMC PNs, indicating its possible use as a distance scale indicator for Galactic PNs. We determine the electron densities and the ionized masses of the nebulae where the [Sii] lines were measured accurately, and we find that the SMC PNs are denser than the LMC PNs by a factor of 1.5. The average ionized mass of the SMC PNs is 0.3 M⊙. We also found that the median [Oiii] /Hβ intensity ratio in the SMC is about half than the corresponding LMC median. We use Cloudy to model the dependence Affiliated with the ESA Space Telescope Division; on leave from INAFOsservatorio Astronomico di Bologna