Tests Of The Zero Point Of The Cepheid Distance Scale And
The Metallicity Sensitivity Of The Cepheid PL Relation
Barry F. Madore
NASA/IPAC Extragalactic Database
California Institute of Technology
Jet Propulsion Laboratory
Pasadena, California
USA
and
Wendy L. Freedman
The Observatories
Carnegie Institution of Washington
Pasadena, California
USA
Abstract
With the wide-spread application of the Cepheid Period-Luminosity
relation to extragalactic distance determinations locally, and the
subsequent calibration of secondary distance indicators based on those
distances, it is of continued importance to have independent checks
on the present accuracy and assumed universality of the adopted PL
relation. Several independent paths to checking the presently adopted
calibration are outlined.
I. The Galactic Cepheid Zero Point
Hipparcos parallaxes have recently become available for a sample of
Galactic Cepheids, and we have used these new distances to check the
calibration of the Cepheid period-luminosity (PL) relation at six
wavelengths (BVIJHK). Comparing these calibrations with previously
published multiwavelength PL relations we find agreement to within
0.07 ±0.14 mag, or 4 ±7% in distance.
Unfortunately, the
current parallax errors for the fundamental pulsators (which range in
signal-to-noise = {pi}/{sigma}{pi} from 0.3 to 5.3, at best)
preclude an unambiguous interpretation of the observed differences,
which may arise from a combination of true distance modulus, reddening
and/or metallicity effects. We have explored these effects in a
forthcoming paper (Madore, B.F. & Freedman, W.L., 1997 Astropys.J.,
in press) where we discuss their implications for the distance to the
Large Magellanic Cloud (LMC) and the Cepheid-based extragalactic distance
scale. Those results suggest a range of LMC moduli between 18.44 ±0.35
and 18.57 ±0.11 mag; however, other effects on the Cepheid PL
relation (e.g., extinction, metallicity, statistical errors)
are still as significant as any such reassessment of its zero point.
II. The Metallicity Sensitivity
Several lines of argument, of both a direct and indirect nature, suggest
that for the wavelengths (V and I) and subsequent techniques used by the
Key Project (and most others) to correct for reddening result in true
distance moduli that are insensitive to metallicity at the 10% level.
The direct evidence comes from differential tests of the metallicity
sensitivity of the final true moduli when estimating distances and
reddenings from groups of Cepheids in the same galaxy (where the
Cepheids are at the same distance) but found in regions of widely
differing metallicity (as indicated by the HII regions). This test was
first run in M31 using BVRI observations of known Cepheids.It is being
rerun by the Key Project Team using only VI observations of newly
discovered Cepheids in the giant ScI galaxy M101 using two fields at
separate galactocentric radii differing widely in the gas-phase-estimated
abundances. Both tests fail to reveal any significant effect on the
Cepheid PL relation zero point in excess of 0.2 mag/dex, where more
metal-rich Cepheids would have their distances over-estimated with
respect to metal-poor Cepheids.
At present the Key Project Cepheid distance scale is zeroed to the Large
Magellanic Cloud sample of Cepheids assuming a true distance modulus of
18.50 mag. While the young population of the LMC is known to be under-
abundant with respect to the locally observable Milky Way Cepheids,
it is also the case that, on average, the LMC metallicity is reasonably
representative of the mean metallicity of t he outer regions of those
galaxies being surveyed by HST for Cepheids. While individual moduli
may eventually require small (5-10%) corrections for metallicity,
the conclusions based on the average moduli are not expected to be
greatly affected.
An indirect indication of the low relative contribution of metallicity
to any uncertainty in the derived Cepheid distances comes from the
completely independent comparison of individual distances to galaxies
in which other methods of high precision have also been the applied.
Many Local Group galaxies now have both Cepheid (Population I) distances
as well as independently determined tip of the red giant branch (Helium
core ignition) distances using purely Population II stars. Similarly,
the planetary nebula luminosity function method has provided distances
to a large sample of galaxies slightly beyond the Local Group for which
Cepheid-based distances are also available for direct comparison. All
three methods agree remarkably well on a case-by-case basis with such
low dispersion that one is led to conclude that secondary effects (on
any one of the three independent distance indicators, not just the
Cepheids) must individually be constrained at the <10% level (e.g., Lee,
Freedman & Madore, Ap.J., 427, 553, 1993).
Conclusions
A variety of direct tests and indirect consistency arguments now suggest
that the zero point of Cepheid PL relation is well defined and that
secondary effects (specifically corrections for metallicity) will be
found affecting the present Population I distance scale at a level not
likely to exceed 10% on average.