It is common to think of using the brightest stars to determine extragalactic
distances. Indeed, over 50 years ago,
Hubble (1936a)
first attempted to
resolve stars in other galaxies to quantify the expansion of the universe,
and since then, the use of blue and red supergiants for extragalactic distance
determinations has been explored several times (e.g.,
Sandage and Tammann
1974,
Humphreys 1983).
However, only recently has it been appreciated that young
planetary nebulae also fall into the ``brightest stars'' category and are
therefore potentially useful as standard candles. As can be seen in the
H-R diagram of Figure 14, the central stars of
these objects are almost as luminous
as the brightest red supergiants - the fact that their continuum emission
emerges in the far ultraviolet, instead of the optical or near infrared, does
not affect their detectability. On the contrary, since their surrounding
nebulae reprocess the EUV radiation into discrete emission lines, planetaries
can be viewed through interference filters which suppress the starlight from
the host galaxy. As a result, observations made through a narrow band
5007 filter can detect ~ 15%
of the energy emitted from these
extremely luminous objects, with little contamination and confusion from
continuum sources.
Figure 14. A schematic H-R diagram showing the evolutionary tracks of planetary nebula central stars. The tracks illustrate the strong dependence of magnitude and lifetime with core mass: the luminosity of a helium burning central star goes approximately as L ~ M3.5, but the lifetime of the object goes as  ~
M-9.5. It is these two relationships, coupled with a
sharply peaked core-mass distribution, which cause the abrupt truncation
in planetary-nebula luminosity function.
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Planetary nebulae have several advantages over other extragalactic distance indicators. Because PN are not associated with any one stellar population, they can be found in galaxies of all Hubble types, and hence are particularly valuable for probing the E and S0 galaxies which define the cores of large groups and clusters. Likewise, internal extinction is usually not a problem in extragalactic PN observations: unlike blue supergiants or Cepheids, PN can be found far away from star forming regions, in areas of the galaxy which are relatively dust free. Since PN are observed through narrow band filters which suppress the continuum, the identification and measurement of these objects does not require complex, crowded field photometric procedures, and, unlike variable star standard candles, PN observations are required only once. Perhaps most importantly, in a large galaxy there may be several hundred planetaries populating the brightest two magnitudes of the planetary nebula luminosity function (PNLF). With the luminosity function so well defined, distance derivations are straightforward, and the internal errors can be as small as 3% (cf. Jacoby et al. 1989).
Despite these facts, the use of PN to determine extragalactic distances is a
relatively new phenomenon. The first suggestion that PN might be a useful
standard candle can be found in the book Galaxies and Cosmology by
Hodge (1966),
where it is listed in Table 12.1 along with such well-known
distance indicators as Cepheids, RR Lyrae, and novae. Actual PN distance
measurements, however, were not not made until the 1970s, when
Ford and Jenner
(1978)
used a 50 Å wide
5007 filter and the SIT Video Camera on
the Kitt Peak 4-m telescope to find and measure the brightest PN in the bulge
of M81.
Ford (1978)
had noticed that the absolute [O
III] 5007 flux
of the brightest PN in each of seven Local Group galaxies varied by less than
25%. Thus, by comparing the [O III] 5007 fluxes of M81's brightest
PN with those of the brightest PN found in M31, Ford and Jenner (1978)
estimated the distance ratio between these two galaxies to be ~ 4.
A few years later,
Jacoby and Lesser
(1981)
used a similar argument to place
limits on the distances to five Local Group dwarfs, and
Lawrie and Graham
(1983)
estimated the distance modulus of NGC 300.
None of the above results was exceptionally persuasive, however. Some of this
skepticism arose from the analysis method which excluded all but the brightest
objects from consideration. However, the main concern at the time was that
little was known about the luminosity function of planetaries; hence the
uncertainties
associated with these distances were completely unknown. It is an irony of the
subject that distances to Galactic planetary nebulae are extremely difficult to
obtain, and that a single PN is definitely not a standard candle. (For
instance, NGC 7027, one of the best studied Galactic PN has
recent distance estimates
that range from 178 pc
[Daub 1982]
to 1500 pc
[Pottasch et
al. 1982].)
However,
while a single PN may not be a standard candle, an ensemble of these objects
can yield a well determined distance. The reason for this is the invariance of
the [O III] 5007 planetary
nebula luminosity function.