Planetary Nebulae

1
Planetary Nebulae

• Morphological and Spectral Characteristics

Jon Voisey University of Kansas December 6, 2006
2
(No Transcript)
3
(No Transcript)
4
(No Transcript)
5
(No Transcript)
6
What is a planetary nebula?

• Term planetary nebula coined by Herschel to
  describe faint, disk appearance

7
What is a planetary nebula?

• Term planetary nebula coined by Herschel to
  describe faint, disk appearance
• This was the extent of the definition until the
  early 1900s

8
What is a planetary nebula?

• Once thought that PNe were young star systems.

9
What is a planetary nebula?

• Once thought that PNe were young star systems.
• 1956 Shklovsky suggests PNe come from RGB stars

10
What is a planetary nebula?

• Once thought that PNe were young star systems.
• 1956 Shklovsky suggests PNe come from RGB stars
• Abell and Goldreich
• of PNe number of low mass stars leaving main
  sequence
• PNe expansion velocity red giant escape
  velocity

11
Things that arent PNe (but look like them)
Supernova Remnants
12
Things that arent PNe (but look like them)
Supernova Remnants
Symbiotic Stars
13
Things that arent PNe(but look like them)
Supernova Remnants
Symbiotic Stars
HII Regions
14
How to get from RGB to PNe?

• Atmospheric ejection mechanisms not well
  understood
• Simple release cannot account for complex
  structure

15
Classification Attempts

• Many based on apparent shape (ring, elliptical,
  irregular, )
• Suffer from inability to describe 3D shape

16
Classification Attempts

• Many based on apparent shape (ring, elliptical,
  irregular, )
• Suffer from inability to describe 3D shape
• Greig (1971) 2 main groups, B C
• B filamentary structure, bipolar, brightness
  falls off quickly, prominent forbidden lines
  relative to Balmer, and at low galactic latitudes
• C high galactic latitudes, smoother appearance

17
New Model

• 1978 It was realized RGB stars have high mass
  loss ? PNe do not expand into vacuum
• Kwok et al. propose Interacting Stellar Winds
  model

18
ISWM

• Red giant releases atmosphere which cools out of
  visual
• As outer layers are expanding, central star
  evolves towards blue side of HR diagram
• Escape velocity decreases as mass is lost
• New fast wind escapes central star
• This wind sweeps up earlier ejected material,
  compressing it on both sides leading to familiar
  shell

19
ISWM
20
Strengths of ISWM

• Predicts fast winds which are observed from
  centers of PNe
• Predicts faint halo outside main shell

21
Strengths of ISWM

• Predicts fast winds which are observed from
  centers of PNe
• Predicts faint halo outside main shell

22
Weaknesses of ISWM

• Cannot account for more complex structure without
  invoking additional mechanisms

23
Refinements to ISWM

• Later groups suggest double shell structure could
  be due to additional mass loss and further
  increasing wind speeds

24
Additional Mechanisms

• Magnetic fields have been observed
  observationally and have been proposed as
  collimating mechanisms for bi-polar outflows.
• Soker Magnetic fields insufficient
• Too weak
• Should decelerate winds, possibly to point of
  reversal

25
Now for the spectra
26
Spectra

• Weak continuum
• Prominent emission lines
• Many from forbidden transitions

27
Spectra

• 1791 Herschel first proposes PNe derive their
  energy from central stars
• 1922 Hubble finds correspondence between
  magnitude of central star and size of nebula

28
Rosseland Theorem

• In nebular conditions short wavelength radiation
  will be broken down into longer wavelengths

29
Rosseland Theorem

• In nebular conditions short wavelength radiation
  will be broken down into longer wavelengths
• This implies that since the central star is
  intense in the UV, the resulting nebula will be
  bright in the visual region.

30
But how does this energy conversion occur?
31
The Electron Pool

• Energy of free electrons dependant on the flux
  and the energy output by the star
• Hotter central stars ? more free electrons with
  higher energies
• Leads to stratification in which inner regions
  are more heavily ionized

32
The Electron Pool

• Electrons with less kinetic energy are quickly
  reabsorbed
• Tend to thermalize quickly

33
The Continuum

• Two main contributors
• Free-Bound Emission
• Free-Free Emission

34
Continuum Free-Bound

• Free electron from the pool is captured
• If electron falls directly to ground state, Lyman
  photon of sufficient energy to ionize another
  atom is emitted
• Since PNe are opaque to this wavelength, there
  will be no net effect

35
Continuum Free-Bound

• Electron can also fall into an upper level and
  then cascade down
• Cascade transitions are quantized, but initial is
  not and contributes to continuum

36
Continuum Free-Free

• Electron is not captured, but loses some kinetic
  energy, emitting a photon
• Does not contribute heavily to visual, but
  stronger in IR and dominates in radio

37
Emission Lines

• Three main processes
• Free-bound
• Bound-bound
• Spontaneous de-excitation

38
Emission Lines Free-Bound

• If a free electron is captured and cascades,
  total energy is broken down into several photons
  of longer wavelengths which can escape the nebula

39
Emission Lines Bound-Bound

• Atom is excited
• De-excitation of electron causes emission
• Should be dependant on relative abundances

40
Emission Lines Bound-Bound

• In some cases, the emitted wavelength for one
  common element may be equal to that of a less
  common element, exciting the less common one and
  giving rise to extra transitions from the less
  common one

41
Emission Lines Bound-Bound

• In some cases, the emitted wavelength for one
  common element may be equal to that of a less
  common element, exciting the less common one and
  giving rise to extra transitions from the less
  common one
• Ex Ionization energies of OIII and HeII similar

42
Emission Lines Spontaneous deexcitaton

• Atom is excited with electron placed into
  metastable state
• Under normal (terrestrial) conditions collisions
  will knock electrons from these states before
  they can fall out naturally
• In nebulae, density is sufficiently low that
  these transitions can occur
• This results in forbidden lines

43
Summary

• Morphology PNe are formed by interacting stellar
  winds. Additional mechanisms which are not well
  understood still necessary to explain all
  features.

44
Summary

• Morphology PNe are formed by interacting stellar
  winds. Additional mechanisms which are not well
  understood still necessary to explain all
  features.
• Spectra Comprised of continuum and emission
  lines, primarily from standard emission
  mechanisms but with the addition of forbidden
  transitions.

45
Conclusions

• PNe understanding has improved greatly in the
  past century, but much work remains to be done to
  accurately explain many features

46
Questions?