2012A&A...537A...1A

HII regions and planetary nebulae (PNe) both emit at radio and infrared (IR) wavelengths, and angularly small HII regions can be mistaken for PNe. This problem of classification is most severe for HII regions in an early evolutionary stage, those that are extremely distant, or those that are both young and distant. Previous work has shown that HII regions and PNe can be separated based on their infrared colors. Using data from the Herschel Hi-GAL survey, as well as WISE and the Spitzer MIPSGAL and GLIMPSE surveys, we wish to establish characteristic IR colors that can be used to distinguish between HII regions and PNe. We perform aperture photometry measurements for a sample of 126 HII regions and 43 PNe at wavelengths from 8.0µm to 500µm. We find that HII regions and PNe have distinct IR colors. The most robust discriminating color criteria are [F₁₂/F₈]<0.3, [F₁₆₀/F₁₂]>1.3, and [F₁₆₀/F₂₄]>0.8 (or alternately [F₁₆₀/F₂₂]>0.8), where the brackets indicate the log of the flux ratio. All three of these criteria are individually satisfied by over 98% of our sample of HII regions and by ∼10% of our sample of PNe. Combinations of these colors are more robust in separating the two populations; for example all HII regions and no PNe satisfy [F₁₂/F₈]<0.4 and [F₁₆₀/F₂₂]>0.8. When applied to objects of unknown classification, these criteria prove useful in separating the two populations. The dispersion in color is relatively small for HII regions; this suggests that any evolution in these colors with time for HII regions must be relatively modest. The spectral energy distributions (SEDs) of HII regions can be separated into ``warm'' and ``cold'' components. The ``cold'' component is well-fit by a grey-body of temperature 25K. The SEDs of nearly two-thirds of our sample of HII regions peak at 160µm and one third peak at 70µm. For PNe, 67% of the SEDs peak at 70µm, 23% peak at either 22µm or 24µm, and 9% (two sources) peak at 160µm.