The use of Ks band photometric excesses to investigate H 2 emission in planetary nebulae

Abstract

We have determined the distribution of H2 emission in 14 planetary nebulae (PNe), using imaging and photometry published by the 2MASS infrared survey. This technique is only applicable under certain stringent conditions, and requires precise broad band photometry, and accurate spatial registration between the Ks and H band images. It is, in addition, only applicable to certain sources, and excludes outflows in which central star and grain thermal excesses are appreciable. Our results for NGC 3132, NGC 6720, IC 4406 and M 2-9 are closely similar to those of previous narrow band imaging, and confirm that H2 emission is confined to narrow, highly fragmented shells. Similar results are obtained for M 1-7, .M 1-8, and M 3-5. Our spatial profiles also confirm that the emission extends outside of the primary ionised shells. Where envelopes are large, and the PNe are more evolved, then the fractional extensions ??/? appear to be at their smallest. They are also similar to the radial widths predicted for H 2 abundance profiles, and to the values ?R/R determined through magnetohydrodynamic modelling of shocks. There appears, finally, to be evidence for an evolution in this parameter, such that ??/? varies with increasing envelope size d(H) as ??/? ? d(H)2.2.