An analysis of 2MASS near-infrared photometry for galactic planetary nebulae

The recently published 2MASS all-sky infrared survey includes JHK S photometry of over 300 million sources. It also provides sizes and aspect ratios for the ∼0.55 per cent of objects that appear in the extended-source catalogue. Although most of these detections correspond to galaxies, stars, H II regions, young stellar objects, reflection nebulae and so forth, a number of them also appear to correspond to galactic planetary nebulae (PNe). We have used this survey to determine the near-infrared properties of some 325 known galactic PNe. We shall show that most of the extended PNe possess a highly restrictive range of indices (J - H) o and (H - K S ) 0 ; much smaller, in fact, than has been determined in previous surveys of such outflows. There are also a large number of sources where emission is dominated by the central-star continua, as well as perhaps cases where H 2 S(1) emission is detectable within the K S band. Symbiotic type sources stand out as a separate category of outflow, as has been noted on several previous occasions. However, the correlation between the He + /H + ratio and (J - H) 0 appears weaker than determined before. We note that K S -band dimensions θ KS are often in close accord with their radio and/or visual counterparts. However, we also point out that certain sources appear to be significantly larger or smaller at NIR wavelengths. Similarly, we find that the aspect ratios r of larger sources are often in conflict with values measured at radio and/or visual wavelengths. We discuss possible reasons for the disparities in r and θ KS . Finally, it is pointed out that the 2MASS fluxes are sometimes in conflict with prior measurements at these wavelengths. Most of these discrepancies are likely to arise from photometric errors, as well as, possibly, from the effects of field-star contamination.