A survey for very faint planetary nebulae in the SMC. I. Identification, confirmation, and preliminary analysis

Using the ESO 2.2 m telescope with the 8K x 8K mosaic CCD, we have surveyed 2.8 deg(2) (similar to1.degrees6 x 1.degrees7) of the Small Magellanic Cloud (SMC) to search for faint planetary nebulae (PNs); 34 PNs were previously known in this central region. We identified 25 new PNs, all faint and spectroscopically confirmed. Most of these are spatially extended with typical diameters of 1 pc, but a few are as large as similar to3 pc. Based on the total number of PNs previously known (similar to80), we can now estimate that there should be similar to139 PNs in the SMC to the limits of a survey such as this one, which is complete to 6 mag down the planetary nebulae luminosity function (PNLF). For a complete survey (8 mag down the PNLF), there should be about 216 PNs. Because no new bright PNs were found in this survey, the bright end of the PNLF remains unchanged from that reported by Jacoby, Walker, & Ciardullo. Consequently, the distance modulus to the SMC, derived using the PNLF technique, is still similar to19.1. However, a strong new feature is now evident in the PN brightness distribution that may be attributed to central stars evolving from a relatively young population. This feature may serve as an indicator of the ages of the PN progenitors. The survey spectra that were used to con firm the candidates as PNs also provide a clue to the nature of the very faint PNs. Statistically, the fainter PNs of our survey exhibit a high incidence (similar to28%) of strong [N II] emission [where I([N II])/I(Halpha) > 1] relative to the bright Sanduleak et al. sample (similar to6%) reported by Meatheringham & Dopita, and comparable to the intermediate-brightness Jacoby sample (26%) reported by Boroson & Liebert. This incidence of strong [N II] is higher than in the Kingsburgh & Barlow sample (similar to17%) of Galactic PNs, despite the similar to3 times higher abundance of nitrogen in the Galaxy. We propose that the very faint SMC PNs are selectively biased toward the chemically enriched Type I objects derived from younger, more massive progenitors and are partially obscured by their own dust. This brightness-dependent population change is also seen in the Large Magellanic Cloud.