Near-IR survey of low ionization structures in PNe

Denise R. Gonçalves^1,2

(1) IAC (Spain) (2) IAG/USP

Low-Ionization Structures (LIS) in Planetary Nebulae (PNe) are important for understanding (post-)AGB mass loss mechanisms, and PNe formation. The nature of LIS is still poorly known. We have studied the morphology and kinematics of LIS in the optical (Gonçalves et al. 2001), showing that they may be dense, neutral fossil clumps left over from the AGB phase. We therefore expect LIS to contain H2 and propose to investigate -for the first time- the morphology and content of molecular (H2) gas of LIS, to test the hypothesis of their origin in AGB wind events preceding PNe formation. Since most of the material lost by the central star of a PN during its AGB phase remains neutral in the form of dust grains, molecules and atoms, and since this material is most easily detected at NIR wavelengths, we propose to search for molecular hydrogen associated with LIS, as a key diagnostic of their origin in the fossil DENSE AND NEUTRAL clumps of the AGB wind. In this manner we hope to shed light on the formation mechanism of PNe and on the detailed behaviour of the winds in (post-)AGB stars. Note that there are no previous observations of H2 associated with LIS in PNe. LIS are faint and small features, and data obtained in general PNe surveys (i.e. Latter et al. 1995; Kastner et al. 1996) are not useful for our proposes because the images are not deep enough and lack the high spatial resolution needed to map in detail the emission associated with LIS. It has also been shown that among proto- and bipolar PNe the detection rate of H2 is nearly 100% as opposed to that in other PNe, indicating a higher molecular hydrogen content (Kastner et al. 2000; Guerrero et al. 2000; García-Hernández et al. 2002). These observations have shown that the molecular hydrogen distribution in PNe traces the optical [NII] emission; and, in particular, that the bright H2 condensations observed in the rings of bipolar PNe are clearly identified in the [NII] images. Because of this empirical H2-[NII] relationship, we expect to detect H2 associated with the [NII]-enhanced LIS. This fact allows us to "see" the history of the (post-)AGB wind and try to understand PN formation in more detail.