Space Telescope Imaging Spectrograph Slitless Observations of Small Magellanic Cloud Planetary Nebulae: A Study on Morphology, Emission‐Line Intensity, and Evolution

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 ofthe ratios of bright emission lines. The morphologies of SMC PNs are similar tothose of LMC and Galactic PNs. However, only a third of the resolved SMC PNsare asymmetric, compared to half in the LMC. The low metallicity environment ofthe SMC seems to discourage the onset of bipolarity in PNs. We measured theline intensity, average surface brightness (SB), and photometric radius of eachnebula in halpha, hbeta, [O III] lambda4959 and 5007, [NII] 6548 and 6584, [SII] lambda6716 and 5731, He I 6678, and [OI] 6300 and 6363. We show that thesurface brightness to radius relationship is the same as in LMC PNs, indicatingits possible use as a distance scale indicator for Galactic PNs. We determinethe electron densities and the ionized masses of the nebulae where the [S II]lines were measured accurately, and we find that the SMC PNs are denser thanthe LMC PNs by a factor of 1.5. The average ionized mass of the SMC PNs is 0.3Msun. We also found that the median [O III]/hbeta intensity ratio in the SMC isabout half than the corresponding LMC median. We use Cloudy to model thedependence of the [O III]/hbeta ratio on the oxygen abundance. Our modelsencompass very well the average observed physical quantities. We suggest thatthe SMC PNs are principally cooled by the carbon lines, making it hard to studytheir excitation based on the optical lines at our disposal.