Planetary Nebulae and How to Find Them: Color Identification in Big Broadband Surveys

Abstract

Planetary nebulae (PNe) provide tests of stellar evolution, can serve as tracers of chemical evolution in the Milky Way and other galaxies, and are also used as a calibrator of the cosmological distance ladder. Current and upcoming large scale photometric surveys have the potential to complete the census of PNe in our galaxy and beyond, but it is a challenge to disambiguate partially or fully unresolved PNe from the myriad other sources observed in these surveys. Here we carry out synthetic observations of nebular models to determine $ugrizy$ color-color spaces that can successfully identify PNe among billions of other sources. As a primary result we present a grid of synthetic absolute magnitudes for PNe at various stages of their evolution, and we make comparisons with real PNe colors from the Sloan Digital Sky Survey. We find that the $r-i$ versus $g-r$, and the $r-i$ versus $u-g$, color-color diagrams show the greatest promise for cleanly separating PNe from stars, background galaxies, and quasars. Finally, we consider the potential harvest of PNe from upcoming large surveys. For example, for typical progenitor host star masses of $\sim$3 M$_\odot$, we find that the Large Synoptic Survey Telescope (LSST) should be sensitive to virtually all PNe in the Magellanic Clouds with extinction up to $A_{\rm V}$ of $\sim$5 mag; out to the distance of Andromeda, LSST would be sensitive to the youngest PNe (age less than $\sim$6800 yr) and with $A_{\rm V}$ up to 1 mag.