Kavli Affiliate: Wendy L. Freedman
| First 5 Authors: Barry F. Madore, Wendy L. Freedman, Kayla Owens, ,
| Summary:
This is the second of two papers exploring the effects of metallicity on the
multi-wavelength properties of Cepheids in terms of their multi-wavelength
period-luminosity (PL) relations, impacting their use as extragalactic distance
indicators, underpinning one of the most popular paths to estimating of the
expansion rate of the Universe, Ho. In Paper I (Madore & Freedman 2024) we
presented five tests for the influence of metallicity on galactic and
extragalactic Cepheid PL relations, spanning nearly 2 dex in metallicity, and
inspecting PL relations from the optical (BVI), through the near-infrared (JHK)
and into mid-infrared (at 3.4 and 4.5 microns). And,in no case were any
statistically significant results forthcoming. Here we interrogate published
spectral energy distributions constructed from theoretical (static) stellar
atmospheres, covering the surface gravity and temperature ranges attributed to
classical (supergiant, F and K spectral type) Cepheid variables, and explore
the differential effects of changing the atmospheric metallicity, down by 2 dex
from solar (a factor of 100 below the average Milky Way value) and then up from
solar by 0.5 dex (i.e., factor of 3x above the Milky Way value). The
theoretical models clearly show that metallicity systematically impacts each of
the bandpasses differentially: the level of this effect is largest in the
ultraviolet (where line blanketing is most intense), reversing sign in the
optical (due to flux redistribution from the UV), and then asymptotically
falling back to zero from the red to the far infrared. The discovered effects
of metallicity are systematic, but they are small; and as such they do not
contradict the findings of Paper I, but they do explain why the problem has
been so hard to resolve given the low level of precision of the photometry for
all but the very nearest and apparently brightest Cepheids.
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