LUMINOUS SUPERSOFT X-RAY SOURCES AND THE PLANETARY-NEBULA LUMINOSITY FUNCTION

Theory suggests that luminous supersoft X-ray sources can be associated with ionization nebulae that have distinctive properties. In this work we point out that such supersoft nebulae can have luminosities in the [O III] lambda 5007 line similar to those associated with the most [O III]-luminous planetary nebulae. The implications of this connection between the well-studied class of planetary nebulae and the nebulae associated with supersoft sources are explored. Of particular interest to this investigation are ways in which the study of each of these classes of nebula may provide insights useful to the study of the other class. For example, we find that it is possible that some of the ''overluminous'' nebulae discovered in surveys for planetary nebulae may be supersoft nebulae. We propose specific observational strategies to test this hypothesis. Should it be verified, then we would have an example of how a search for planetary nebulae may lead to the discovery of supersoft nebulae, and of how the ''contamination'' of the planetary nebula luminosity function (PNLF) by supersoft nebulae may help us to resolve an outstanding puzzle about the PNLF. We also address the possible influence that such ''contamination'' may have on the use of the PNLF to determine distances. Although any such effect (if present at all) would be small, it is likely to be systematic, and is therefore susceptible to elimination, to help further refine the PNLF method of distance determination. Possibly even more interesting than this connection with a particular type of well-studied nebula is the potential importance of the study of supersoft nebulae to the overall study of supersoft sources, especially in extending the distances out to which systematic searches for individual sources can be performed. Our derivation of the supersoft nebulae luminosity function (SNLF, in [O III]) is a result that is independent of the comparison with the PNLF. The comparison with planetary nebulae, which can be studied in distant galaxies, suggests, however, that should a significant fraction of supersoft sources be associated with nebulae, then we may be able to conduct systematic studies of such sources out to distances at least as great as those to the Virgo cluster. Furthermore, for all sources, including those in the Local Group (where X-ray surveys have provided the complete census to date), we find that the selection effects for discovery via a nebula are different from those for discovery in an X-ray survey, in that surveys of nebulae may favor lower temperature sources than X-ray surveys.