On the Form of the Spitzer Leavitt Law and its Dependence on Metallicity
May 31, 2013Citations per year
Abstract: (IOP)
The form and metallicity dependence of Spitzer mid-infrared Cepheid relations are a source of debate. Consequently, Spitzer 3.6 and 4.5 μm period-magnitude and period-color diagrams were re-examined via robust routines, thus providing an alternative interpretation to consider. The relations (nearly mean-magnitude) appear non-linear over an extensive baseline (0.45 < log P0 < 2.0), particularly the period-color trend, which to first order follows constant (3.6-4.5) color for shorter-period Cepheids and may transition into a bluer convex trough at longer periods. The period-magnitude functions can be described by polynomials (e.g., [3.6 μm] = K0 – (3.071 ± 0.059)log P0 – (0.120 ± 0.032)log P02), and Cepheid distances computed using 3.6 and 4.5 μm relations agree with each other and the latter provides a first-order consistency check (CO sampled at 4.5 μm does not seriously compromise those distances). The period-magnitude relations appear relatively insensitive to metallicity variations ([Fe/H] ~ 0 to –0.75 |γ| < 0.1 mag dex–1), a conclusion inferred partly from comparing galaxy distances established from those relations and NED-D (n > 700), yet a solid conclusion awaits comprehensive mid-infrared observations for metal-poor Cepheids in IC 1613 ([Fe/H] ~ -1). The Cepheid-based distances were corrected for dust obscuration using a new ratio (i.e., A3.6/EB – V= 0.18 ± 0.06) deduced from GLIMPSE (Spitzer) data.Note:
- To appear in ApJ
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