Planetary Nebula Luminosity Functions: Pieces of the Puzzle

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Planetary Nebula Luminosity Functions Pieces of
the Puzzle

• Robin Ciardullo

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Extragalactic PN Surveys

• Surveys for PN in other galaxies began as a way
  to
• Identify test particles for kinematic studies
• Identify test particles for chemical abundance
  studies
• Probe the stellar populations of other galaxies
• Better understand stellar evolution and Galactic
  PN
• Today, extragalactic PN observations are best
  known for their contribution to the extragalactic
  distance scale. But they are still have uses for
  all of the above!

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What Do PN Emission Line Strengths Depend On?

• To first order, estimating the brightness of a PN
  is easy. In decreasing importance, a PNs
  emission-line luminosity depends on
• The luminosity of its central star
• The density of the nebula (via two body
  interactions)
• Lots of other physics (optical thickness,
  ionization structures, shocks, radiative
  transfer, )
• For a zero-th order PNLF calculation, we can
  ignore (3) and decouple the effects of (1) and
  (2).

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The First PN Luminosity Function
Henize Westerlund (1963) argued that for faint
PN with non-evolving central stars and freely
expanding nebulae F(H?) ? Ne NH V ? NH ? R-3 ?
t-3 M(H?) ?2.5 log F(H?) C ?7.5 log t
C t ? 10-M/7.5 ? e0.307 M N(M) ?
dt/dM ? e0.307 M
The first real application of this was in the
Magellanic Clouds (Jacoby 1980). The observations
were at 5007Å, rather than H? (since the
most luminous PN have O III ?5007 as their
brightest line). The fit was good.
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The O III PNLF as a Distance Indicator

• In the late 1980s, the bright-end cutoff of the
  O III ?5007 was found to be independent of
  stellar population.

The shape of the O III PNLF was so simple, it
could be modeled by supplementing the Henize
Wester-lund law with an exponential cutoff. The
bright-end cutoff magnitude was called M.
M31s Bulge
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Observations in 50 elliptical, spiral, and
irregular galaxies show that M is (almost) a
universal constant.
In M31, the O III PNLFs of the bulge, disk, and
halo all have the same cutoff. (?M lt 0.05 mag)
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Observations in 50 elliptical, spiral, and
irregular galaxies show that M is (almost) a
universal constant.
E0
NGC 3379
E6
NGC 3377
SB0
In galaxy groups and clusters, the O III PNLFs
of systems with different colors and Hubble types
all have the same cutoff.
NGC 3384
Sab
NGC 3368
SBb
NGC 3351
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The PNLF versus Metallicity
The only (weak) shift in the O III PNLF cutoff
is with metallicity. Metal-poor galaxies with a
well-determined Cepheid distance have a slightly
fainter cutoff.
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M 4.47
The observed fading of the O III PNLF cutoff at
low metallicity is consistent with simple
arguments and more detailed predictions from
nebular and PAGB physics (Dopita et al. 1992)
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The bright end of the O III PNLF does not carry
any information about the underlying stellar
populations. But what about the faint end?
The Faint End of the PNLF
If most PN have low-mass cores, the PNLF should
follow that for non-evolving central stars, N(M)
? e0.307 M. This relation works well for the
bulge of M31.
M31s Bulge
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The bright end of the O III PNLF does not carry
any information about the underlying stellar
populations. But what about the faint end?
The Faint End of the PNLF
If most PN have low-mass cores, the PNLF should
follow that for non-evolving central stars, N(M)
? e0.307 M. But if high-mass cores are important,
the PNLF should reflect core evolution as well.
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The bright end of the O III PNLF does not carry
any information about the underlying stellar
populations. But what about the faint end?
The Faint End of the PNLF
If most PN have low-mass cores, the PNLF should
follow that for non-evolving central stars, N(M)
? e0.307 M. But if high-mass cores are important,
the PNLF should reflect core evolution as well.
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The Behavior of the Dip
The PNLFs of Pop II systems follow the Heinze
Westerlund exponential (expected from low-mass,
slowly-evolving cores). The PNLFs of star-forming
populations are non-monotonic, due to the rapid
evolutionary timescales of their higher-mass
cores. But what about intermediate populations,
whose last star-formation occurred in the recent
past?
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The Behavior of the Dip
The PNLF of the intermediate-age population of
NGC 5128 looks just like that of M31. The dip
in the PNLF must be associated with very young
(lt 2 Gyr) populations.
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The PNLF Beyond O III ?5007
O III ?5007 is a bright line, but it is not the
only line. At its best, the PNLF is a
multi-dimensional construct.

• PN from all stellar populations are located in a
  specific region of 5007-H? emission-line space.

with L 2.4 ? 1036 ergs/sec in O III ?5007
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The PNLF Beyond O III ?5007
O III ?5007 is a bright line, but it is not the
only line. At its best, the PNLF is a
multi-dimensional construct.

• This means that measurements of 5007/H? can yield
  an upper limit on a PNs distance. Low
  excitation objects have a hard upper limit to
  their O III luminosity.

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Warning Galactic vs. Circumstellar Extinction

• The line ratios of PN will be effected by two
  different components of extinction.
• Foreground Galactic extinction is an external
  effect that must be removed.
• Circumstellar extinction is intrinsic to the
  PNLF, and (in part) defines the bright-end
  cutoff. This should not be touched.

Without circumstellar extinction, there would be
no hard cutoff to the PNLF.
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The H? PNLF
The H? PNLF is also a well-defined function. Its
cutoff, though slower (and a factor of 2 fainter
than that of O III) is also insensitive to
stellar population. At fainter magnitudes, the
luminosity function appears to flatten, and/or
turn over. But this is probably a selection
effect.
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Warning Sample Selection

• Extragalactic PN are usually identified via their
  high luminosity in O III ?5007. Some objects
  with very low (or extremely high) excitations
  will not appear in the sample.
• This introduces a bias, which can affect
  conclusions drawn from samples with different
  selection criteria.

Missing objects
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The Normalization of the PNLF
The PNLF is normalized by ?, the ratio of PN to
the (bolometric) luminosity of the underlying
population.
Local Group
Buzzoni et al. (2006)
Surveys of extragalactic PN are generally
restricted to objects in the top 1 mag of the
PNLF. Thus, measurements only yield ?0.5 or
a2.5. To get total PN populations, we need to
apply a large ( 8 mag) extrapolation.
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The Normalization of the PNLF
E/S0 galaxies and spiral bulges have 1 O
III-bright PN (within 0.5 mag of M) for every
5 ? 108 L? (bolometric).
The disks of spiral galaxies have the same ratio
of PN to light as elliptical galaxies. Despite
all the star formation, ?0.5 has neither
increased nor decreased.
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The Normalization of the PNLF
Since the bolometric corrections for spiral and
elliptical galaxies are not too different, the
V-band ?-values for the two galaxy types are
also similar (except in V, ? is a factor of 2
larger).
Schmitt 2000
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Extrapolating the Luminosity Function

• To estimate the total populations of PN, you need
  to know/assume
• a limiting magnitude for the extrapolation
• a shape for the unobserved PNLF

If the PNLF has a dip, then a extrapolation
based on the Henize Westerlund law will
overpredict the total PN popu-lation by a factor
of 1.6
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How Far Down Do You Count?
From the (single-star) theory of stel-lar energy
generation, the bolo-metric luminosity specific
stellar evolutionary flux for all old stellar
populations is B 2 x 10-11 stars yr-1 L?-1 so
the production rate of PN should be relatively
constant.
Renzini Buzzoni (1986)
To connect PN counts to this number, you need a
timescale. It really doesnt matter how far down
you count, as long as you know the timescale
associated with you limiting magnitude.
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The Normalization of the PNLF

• For example
• Begin with ?0.5 4 ? 10-9 PN/L?
• Extrapolate the Henize Weseterlune exponential
  8 mag ? ?tot 5 ? 10-7 PN/L?
• Assume a mean PN lifetime of 25,000 yr. The PN
  formation rate is then b 2 ? 10-11 PN/yr/L?

?
BUT
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Does the Constancy of ???? Make Sense?
In late-type (constant SFR) spiral galaxies,
50 of the light comes from stars born lt 0.1 Gyr
ago. These stars provide luminosity without
making planetary nebulae.
In early-type systems, most stars cannot make
stars within 0.5 mag of M. Their core masses
are too low!
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The ? Values of Spiral Disks

• A very young (lt 0.1 Gyr) stellar population
  provides light, but no PN. Depending on the
  systems history of star-formation, this can
  depress ? by a factor of 2.
• A slightly older (lt 2 Gyr) produces a dip in the
  PNLF. Extrapolated PNLFs will overpredict ?tot
  by a factor of 1.5.

CONCLUSION Measurements of ? in spiral galaxies
are consistent with the predictions of simple,
single-star stellar evolution theory, but 50
uncertainties still exist.
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The Brightest PN of Old Populations

• The brightest 1 of PN (within 0.7 mag of M)
  present in elliptical galaxies cannot evolve from
  single stars
• M(O III) ?4.47 ? L(O III) 600 L?
• At best, the conversion of continuum luminosity
  to O III emission is 10 efficient. So
• L(O III) 600 L?????L(PNCS) gt 6,000 L?

• L(PNCS) gt 6,000 L? implies a central star mass of
  Mf gt 0.6 M?.

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The Brightest PN of Old Populations

• The brightest 1 of PN (within 0.7 mag of M)
  present in elliptical galaxies cannot evolve from
  single stars
• M(O III) ?4.47 ? L(O III) 600 L?
• At best, the conversion of continuum luminosity
  to O III emission is 10 efficient. So
• L(O III) 600 L?????L(PNCS) gt 6,000 L?

• L(PNCS) gt 6,000 L? implies a central star mass of
  Mf gt 0.6 M?.
• Mf gt 0.6 M? cores imply progenitors with Mi gt 2
  M? (? lt 1 Gyr). These dont exist in elliptical
  galaxies!

(Weidemann 2000)
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The Brightest PN of Old Populations
Traditional single star evolution can not explain
the bright PN seen in E/S0 galaxies. Neither can
common-envelope scenarios which produce
close-binaries. To reproduce the observed PN
population, one needs to build massive cores.
Blue straggler evolution can do this. Blue
stragglers have the correct evolutionary
timescales.
Ursa Minor Dwarf Galaxy
Carrera et al. (2002)
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The Brightest PN of Old Populations

• Field Blue Straggler Stars can be formed by
• Mass transfer/merger of primordial close binaries
• Dynamical evolution of triple systems via the
  Kozai mechanism and tidal friction)
• If the latter mechanism is important, then many
  (most?) O III-bright PN will have wide-binary
  companions.

Ursa Minor Dwarf Galaxy
Carrera et al. (2002)
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The Bulk of the PN of Old Populations
PN more than 0.7 mag down the O III ?5007
PNLF can be produced by the stellar populations
of normal ellipticals. These objects will have
low-mass, slowly-evolving cores, and create the
Henize Westerlund PNLF.
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Conclusions

• To a large extent, we now understand the physics
  of the PNLF. The number of PN in both spiral and
  elliptical galaxies is consistent is simple
  single-star stellar evolution. But there is
  still wiggle room at the 50 level.
• To make further progress, we need to include
  other lines in the analysis, including H?, H?, N
  II, S II, etc. But be aware that there will
  be selection effects.
• Much more work is needed to exploit the
  information contained by the multidimensional
  PNLF.