Chicago–Carnegie Hubble Program: A Multiwavelength Search for the Effects of Metallicity on the Cepheid Distance Scale. II. Theoretical Models and Synthetic Spectra
This is the second of two papers exploring the effects of metallicity on the multiwavelength properties of Cepheids in terms of their multiwavelength period–luminosity (PL) relations, impacting their use as extragalactic distance indicators and underpinning one of the most popular paths to estimatin...
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| Main Authors: | , , |
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| Format: | Article |
| Language: | English |
| Published: |
IOP Publishing
2025-01-01
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| Series: | The Astrophysical Journal |
| Subjects: | |
| Online Access: | https://doi.org/10.3847/1538-4357/ada3d5 |
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| Summary: | This is the second of two papers exploring the effects of metallicity on the multiwavelength properties of Cepheids in terms of their multiwavelength period–luminosity (PL) relations, impacting their use as extragalactic distance indicators and underpinning one of the most popular paths to estimating the expansion rate of the Universe, H _0 . In Paper I, 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 the mid-infrared (at 3.4 and 4.5 μ m). 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., a factor of 3× 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. The most interesting (and useful) result of this investigation is that from a close examination of the models, we have discovered infrared regions in Cepheid spectral energy distributions that allow for the simultaneous correction for extinction and metallicity differences by the judicious combination of only two filters: one in the near-infrared and one in the mid-infrared, the former centered at 1.2 μ m and the other centered at 3.6 μ m. Even more exciting and promising is the fact that both of these bands can be simultaneously observed in single exposures using the dual-channel imager, NIRCam on JWST. |
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| ISSN: | 1538-4357 |