The Nature and Origin of Molecular Knots in Planetary Nebulae

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The Nature and Originof Molecular Knots
inPlanetary Nebulae
Sarah Eyermann U. of Missouri Angela Speck U.
of Missouri Margaret Meixner STScI Peter
McCullough STScI Joe Hora CfA
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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
• Cant observe PPNe as well, so we observe PNe and
  attempt to trace their development back

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Classical Model of a PN
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NGC 7293 Helix Nebula
David Malin, Anglo-Australian Telescope
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NGC 7293 Helix Nebula
Speck, et al.
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Helix Nebula
Meixner, et. al.
Speck, et. al.
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Hubble Space Telescope Helix
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Revised Model
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ODells 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

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M57 - Ring Nebula
Bond, et. al.
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Ring NebulaClump Correspondence in visible and
H2
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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 H2 emission behind the ionized gas as seen
  from the star

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Huggins work on single Helix knot (2002)

• Strongest H2 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
• H2 distribution meets with expectations of H2
  excitation in a thin PDR at surface of the
  molecular gas
• Unlikely to be caused by shocks

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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

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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?

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

• 2-40 in previous H2 studies
• 0.2 in this study
• 0.01 in optical studies

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Serendipitous viewing of Helix
Meixner, et. al.
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Results

• Radial Distribution of Knots
• 162 knots/arcmin2 in denser regions
• 18 knots/arcmin2 in lower density outer regions
• More Knots Detected
• Estimated 23,000 total knots in Helix
• Factor of 6.5 larger than previous estimates

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Good candidates for future work

• Ring (NGC 6720)
• New IRAC image (resolution 2)
• Better understanding of radial distribution in
  outer regions

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New H2 Ring image
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Petal structure seen previously in ionized gas
Tony and Daphne Hallas
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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

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Dumbbell
Hora
Jacoby et. al.
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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

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PNe in Archives
NOAO/AURA/NSF
Bond
BD30 3639
NGC 7027
Sahai
Heyer, et. al.
Hb 12
NGC 2346
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Questions?
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IRC 10216
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Egg Nebula
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Helix Nebula
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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.