2013ApJ...772..130M

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., A_3.6/E_ B - V_= 0.18±0.06) deduced from GLIMPSE (Spitzer) data.