Planetary nebulae: definition, observational characteristics, and diagnostics

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Planetary nebulae definition,observational
characteristics, and diagnostics

• David J. Frew
• Perth Observatory Macquarie University

Q.A. Parker the MASH Consortium
MASH Workshop, Sydney 16 February 2009
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Image credits B. Balick, R. Sahai, A. Hajian, M.
Meixner, P. Harrington, K. Borkowski et al.
(STScI/AURA/NASA/ESA/NOAO), G Mellema et al.
(HST, ESA, NASA), A. Block (NOAO/AURA/NSF), G.
Jacoby (WIYN/NOAO/NSF), D. Malin (AAO) and A.
Zijlstra (IPHAS).
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PN physical characteristicscf. Kohoutek (1983,
2001)

• A round or axisymmetric shape, sometimes with
  multiple shells or outer haloes. Many evolved
  PNe are perturbed through interaction with the
  ISM (Wareing et al.).
• A photoionized emission-line spectrum
  characterized by recombination lines of H and He
  as well as various collisionally-excited
  forbidden lines of heavier elements such as O, C,
  N, S, Ar and Ne. The O III lines are usually,
  but not always, the strongest lines in the
  optical.
• Thermal free-free emission in the radio spectrum.
• A nebular radius, r lt 2.5 pc (vast majority have
  r lt 1.5 pc).
• A ionized nebular mass between the empirical
  limits of 0.005 and 3 M?

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Physical characteristics

• A shell expansion velocity typically between 10
  60 kms-1 though some strongly bipolar PNe can
  have much higher expansion velocities along the
  major axis
• A hot, low-mass central star with a temperature
  of at least 20,000 K (up to 250,000 K), and a
  mass between the empirical limits of 0.55 and
  1M?. It is possible that lower mass stars might
  be formed via a common-envelope process
• Corresponding surface gravities are generally in
  the range, log g 2.5 7.5 cms-2

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How many PNe here?
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Mimics For early reviews, see Kohoutek (1983),
Stenholm Acker (1987) and Acker Stenholm
(1990)

• Emission-line stars (primarily found through
  objective prism surveys)
• Symbiotic stars
• Be and Be stars
• Wolf-Rayet stars
• Luminous blue variables
• Pre-main sequence stars
• Cataclysmic variables, including novae
• Late-type stars (with or without Balmer emission)

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Mimics

• Extended nebulae (found via imagery)
• HII regions (compact, ultra-compact, or diffuse)
• Reflection nebulae
• Galaxies (emission-line, LSB, ring)
• Symbiotic outflows
• Wolf-Rayet shell and ejecta nebulae
• LBV nebulae
• Supernova remnants
• Herbig-Haro objects / YSOs
• Miscellaneous bow-shock nebulae
• Plate defects and flaws

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The advent of VO-compliant multi-wavelength data
sets

• Ideal to combine multi-wavelength NIR, MIR,
  optical (including narrow band) and radio imaging
  with large spectral wavelength coverage in
  UV/Optical/NIR/MIR
• Previously difficult to achieve in most cases
  without detailed individual follow-up
• Advent of major wide-area surveys in the last few
  years has improved the situation considerably
  (SHS, SHASSA, IPHAS, VPHAS, SDSS, UKIDSS,
  Skymapper, MOST, NVSS, MSX, GLIMPSE)!
• These surveys offer unprecedented opportunities
  for discovery (e.g. Parker et al., Miszalski et
  al., Corradi et al.)
• New opportunities to eliminate contaminants that
  have bedevilled previous catalogues. The future
  is bright

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PN or Strömgren sphere?

• A number of large, nearby (currently accepted)
  PNe are likely to be Strömgren spheres in the
  ISM ionized by a hot pre-white dwarf or subdwarf
  (the original PN having long since dissipated).
  That is, they are HII regions!
• These nebulae have emission line ratios which
  mimic true PNe e.g. Hewett 1, DeHt 5, HDW 5, Sh
  2-68, Sh 2-174, and RE J1738665 (Hewett et
  al. 2003 Chu et al. 2004, Frew et al. 2006, Frew
  2008)
• Their true nature is currently being investigated
  where high resolution WHAM velocity maps for
  large angular size nebulae can resolve ambiguity
  (see poster by G. Madsen et al.).

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Fr 2-11, a new nebulosity discovered from SHASSA
(just outside MASH survey at b -12.5)
(Frew 2008, Frew et al. 2009, in prep.)
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Beware a PN-like spectrum!
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Abell 35 a new interpretation (Frew 2008)

• Visual binary, G8 IV DAO
• Spectrum and morphology very like Fr 2-11, a
  bow-shock associated with a CV
• No forward shock !
• Line width of gas is narrow, consistent with no
  expansion
• Systemic velocity of gas is close to LSR, Vhel
  -5 kms-1
• Radial velocity of G8 IV star is different, Vhel
  -40 kms-1
• Wind from active G-star is sufficient to drive
  shock in stationary (photoionized) ISM.

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Sh 2-174 a PN abandoned by its central star
or a nebulosity abandoned as a PN?

• Pathological morphology with off-centre ionizing
  
• Ionization structure not consistent with PM
  vector of star (no rim / shock)
• Emission nebula has VLSR 0 kms-1, and narrow
  line-width from WHAM data (Madsen et al. 2006)
• Diffuse Ha emission/ within 2 at VLSR 0 kms-1
• PAGB evolutionary age is 106 years
• Is a HII region

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Sh 2-174 and Abell 21 WHAM Fabry-Perot spectra
(see Madsen et al. 2006, Madsen Frew, 2009, in
preparation, and our poster at this workshop)
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Weeding out the impostorsSeveral criteria are
used for classification

• Nebular morphology (is it irregular or otherwise
  pathological?)
• Ionization structure (including the consistency
  of any ISM interaction with proper-motion vector
  of central star)
• Nebular emission-line ratios (use diagnostic
  plots)
• MIR properties (2MASS, MSX, IRAS, Spitzer data
  use diagnostic plots)
• Systemic velocity of nebula (does it differ from
  star?)
• Line width of nebular gas
• Shklovsky (ionized) mass, but need an estimate of
  the distance
• Other bulk properties
• Local Environment
• Properties of central star, including
  evolutionary age
• In some cases, look at Galactic latitude
• No one criterion is generally enough to
  define the status of an object so we use the
  "overall body of evidence".

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Use of diagnostic emission line ratios

• Best to use lines of similar wavelengths,
  reddening errors are then minimized in
  calculation of ratios
• In blue, use OIII/Hb ratio (traditional
  diagnostic, but has caveats)
• Use HeII 4686/Hb ratio with caution (beware of
  symbiotic stars, WR stars/nebulae etc)
• In red, use both NII and SII line strengths
  (e.g. Sabbadin, Minello Bianchini 1977, Cantó
  1981, Riesgo-Tirado López 2002)
• NII/Ha ratio alone must be used with caution
• Unfortunately, very high excitation PNe
  (including many in MASH and MASH-II), have no
  measurable NII or SII emission.
• If only red spectra are available for such an
  object, then other criteria (morphology, central
  star characteristics, galactic latitude,
  environment) are needed to classify it
  unambiguously.

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Log I(Ha/NII) versus log I(Ha/SII)
diagram Sabbadin et al. (1977), Cantó (1981),
Riesgo-Tirado López (2002)
Caveat Not applicable to all PNe
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• Revised log F(Ha/NII) versus log F(Ha/SII)
  diagram (Frew et al., in preparation).

Considerable overlap between fields, especially
evolved PNe and SNRs, and HII regions and
PNe Note the empirical domain of Type I PNe
(permits preliminary classification based on red
spectral data only) Fields are consistent with
photoionization modelling (J. Köppen)
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BPT diagram(s) (Baldwin et al. 1981, Veilleux
Osterbrock 1987, Kennicutt et al. 2000, Kewley et
al. 2001, Kniazev et al. 2008)
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Kniazev, Pustilnik Zucker (2008)
Kewley et al. (2001)
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OIII/Ha vs M5007 diagram Ciardullo (2006),
Herrmann et al. (2008)
Bright PNe plot in well-defined region Excellent
utility for extragalactic PNe, but not yet used
for local samples
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Diagnostic plot from Gutierrez-Moreno, Moreno
Cortes (1995), where R1 O III4363/Hg and R2
O III5007/Hb. Three regions, A, B and C,
have been defined, containing PNe, young dense
PNe, and bona fide symbiotic stars, respectively.
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IR diagnostic plots (also see talk by M. Cohen)
Corradi et al. (2008) IPHAS and/or 2MASS data
Schmeja Kimeswenger (2001) DENIS NIR data
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Extreme bipolar nebulae Symbiotic outflows,
symbiotic PNe or proto-PNe? (Lutz, Shaw,
Corradi, Balick, Phillips, Kwok)

• Morphology is very strongly bipolar
• Extreme collimated outflows, very high velocities
• Homologous expansion, cf. interacting winds model
• Many (not all) have very red NIR colours
• Many have extreme central densities
• Binary central engine, e.g. Mira star
• Many statistically unlikely to be true PNe
  (Corradi)

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RP 916
KjPn 8
Star has MK -4.7
What is the nature of these bipolar outflows?
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Hb 5
Size does matter!
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A necessary questionWhat is a planetary nebula?

• A bit of history
• As to the question of how to define a
  planetary nebula, there is no better way than to
  accept any object in a catalogue of planetary
  nebulae if nobody has serious objections.

R. Minkowski, responding to a query from D.S.
Evans at the 1967 IAU symposium (Osterbrock
O'Dell 1968, p. 290).
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What is a PN ?

• More recent definitions
• a PN is a star that ejects some material while
  evolving from the red giant to the white dwarf
  phase. (Lutz 1993)
• an ionized shell ejected at the end of the
  AGB phase, either by a single star, or as part of
  a common-envelope ejection. (Frew 2008)

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Do PN form a heterogeneous class?

• Is there more than one evolutionary sequence
  that can produce nebulae that are catalogued as
  PNe? (Jacoby 2006, Frew 2008, de Marco 2009)

• Conventional post-AGB evolution of a single star
• Common-envelope evolution on AGB (or RGB?)
  binarity is a prerequisite
• O(He) stars and the R CrB connection
• Post-EHB evolution, producing an ejecta nebula ?
• Super-AGB star ejection event (no examples known)
  ?
• Symbiotic novae !

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Do nebulae around post-EHB or AGB-manqué stars
exist the problem of PHL 932

• PHL 932 is a subdwarf OB (sdOB) star which is
  generally regarded as being in the extended
  horizontal branch (EHB) phase of evolution.
• It has a low ionization, amorphous, elliptical
  nebula associated with it (Arp Scargle 1967),
  traditionally considered to be an evolved PN
  (Acker et al. 1992, Kohoutek 2001).
• See talk by S. OToole at this workshop.

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Topics for discussion

• Definition of a PN observational or
  phenomenological?
• Taxonomy / nomenclature are PNe a mixed bag?
• Do you have a favorite nebula?
• Be familiar with mimics
• Use multi-wavelength approach
• Consistency in the literature
• Recommended diagnostic plots know their
  limitations