The Nature and Origin of Molecular Knots in Planetary Nebulae Sarah Eyermann – U. of Missouri Angela Speck – U. of Missouri Margaret Meixner – STScI Peter McCullough – STScI Joe Hora – CfA

Why study morphology of PN? ● Very important contributors to the interstellar medium ● Distribution of gas and dust dispersed impacts how it is processed in ISM ● Can’t observe PPNe as well, so we observe PNe and attempt to trace their development back

Classical Model of a PN H2H2 H+H+ H+H+ H0H0 CO He 2+ He + He 0 Dust

NGC 7293 – Helix Nebula David Malin, Anglo-Australian Telescope

NGC 7293 – Helix Nebula Speck, et al.

Helix Nebula Meixner, et. al. Speck, et. al.

Hubble Space Telescope Helix

Revised Model He 2+ He + He 0 H2H2 O 2+ O+O+ O IO I CO H I star light dust H 2 CO shadow H0 H0 H+H+ Molecular Clumps O 3+

O’Dell’s work (2002) ● Examined the distribution of clumps in ionized gas in several nebulae ● Showed an evolution in clumps that seems to correspond with age of the nebula, with clumps in older nebulae being smaller and well-formed, and clumps in younger nebulae being larger and less sculpted

M57 - Ring Nebula Bond, et. al.

Ring Nebula: Clump Correspondence in visible and H 2

Possible Methods of Excitation ● Shock Excitation – Shocked-gas regions result from interaction of the fast central star wind with the slowly expanding nebula ● Photo-Dissociation Regions (PDR) – Excited by far-UV and X-ray radiation of central star – Expect a thin layer of ionized gas emission on the surface of the clump facing the central star, with H 2 emission behind the ionized gas as seen from the star

Huggins work on single Helix knot (2002) ● Strongest H 2 emission occurs at the face of the globule toward the central star ● Little emission directly behind the globule ● Observable emission extends large distances (≥24”) ● Closely follows ionized gas emission ● H 2 distribution meets with expectations of H 2 excitation in a thin PDR at surface of the molecular gas ● Unlikely to be caused by shocks

When do the knots form? ● Before PNe stage – Already clumping when material ejected during AGB phase enters the ISM ● As a result of the PNe stage – Material entering ISM during AGB phase is relatively smooth

What do we look for? ● Do we see small scale structures (knots) in both the ionized and molecular gas? ● Compare structure and determine whether knots and filaments seen in optical images of PNe are spatially coincident with the molecular clumps – so far only shown for Helix and Ring Nebulae ● Is there a pattern to the way that the distributions appear to change with the age of the nebula? ● How does the structure of the knots change with distance from the center?

Serendipitous viewing of Helix by Hubble during 2002 Leonids meteor shower Meixner, et. al.

Greater resolution on Knots ● 2”-40” in previous H 2 studies ● ~0.2” in this study ● ~0.01” in optical studies

Serendipitous viewing of Helix Meixner, et. al.

Results ● Radial Distribution of Knots – 162 knots/arcmin 2 in denser regions – 18 knots/arcmin 2 in lower density outer regions ● More Knots Detected – Estimated ~23,000 total knots in Helix – Factor of 6.5 larger than previous estimates

Good candidates for future work ● Ring (NGC 6720) – New IRAC image (resolution 2”) – Better understanding of radial distribution in outer regions

New H 2 Ring image

Petal structure seen previously in ionized gas Tony and Daphne Hallas

Good candidates for future work ● Ring (NGC 6720) – New IRAC image (resolution 2”) – Better understanding of radial distribution in outer regions ● Dumbbell (NGC 6853) – New IRAC image shown at AAS meeting – Shows need for higher resolution image and can guide future observations

Dumbbell Jacoby et. al. Hora

Good candidates for future work ● Ring (NGC 6720) – New IRAC image (resolution 2”) – Better understanding of radial distribution in outer regions ● Dumbbell (NGC 6853) – New IRAC image seen at AAS meeting – Shows need for higher resolution image and can guide future observations ● Other nebulae already imaged in HST archives – Study a range of ages to determine evolution of knots

BD NGC 7027 Hb 12 PNe in Archives: NGC 2346 Heyer, et. al. Bond Sahai NOAO/AURA/NSF

Questions?

IRC 10°216

Egg Nebula

Helix Nebula

Abstract Planetary Nebulae (PNe) are major contributors to the enrichment of the interstellar medium (ISM). Knots and filaments in the ionized gas images of PNe are common, if not ubiquitous. Additionally, it has been shown that molecular gas exists inside dense condensations within the ionized regions. The origins of these clumps are not known, though the suggested formation mechanisms fall into two main scenarios: (1) they form during the AGB phase; (2) they form as a result of the onset of the PN phase as the fast wind ploughs into the slower moving AGB wind. The currently favored model is that the knots are formed by the onset of the PN phase and then sculpted as the ionizing radiation penetrates deeper into the circumstellar envelope. We have studied the morphologies of molecular and ionized gas for five PNe, which cover a range of ages, and which have been imaged by HST using both WFPC2 and NICMOS (at the 2.12um H2 line). The structure and appearance of the knots in ionized and molecular gas for each PNe has been compared to assess the evolutionary status of the molecular clumps and how it is affected by the evolutionary status of the whole PN. We also compare our results with the ground-based studies of the molecular knots in Ring and Helix Nebulae and to a detailed HST study of the knots in the Helix Nebula as imaged by NICMOS. This will aid our understanding of the origin of the molecular knots, and the enrichment of the ISM by dying intermediate mass stars.