ON THE FORM OF THE SPITZER LEAVITT LAW AND ITS DEPENDENCE ON METALLICITY

The form and metallicity dependence of Spitzer mid-infrared Cepheid relations are a source of debate. Consequently, Spitzer 3.6 and 4.5 mu m 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 P sub(0) < 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 mu m] = K sub(0) - (3.071 + or - 0.059) log P sub(0) - (0.120 + or - 0.032) log P sub(0) super(2)), and Cepheid distances computed using 3.6 and 4.5 mu m relations agree with each other and the latter provides a first-order consistency check (CO sampled at 4.5 mu m does not seriously compromise those distances). The period-magnitude relations appear relatively insensitive to metallicity variations ([Fe/H] ~ 0 to -0.75 | gamma | < 0.1 mag dex super(-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 sub(3.6)/E sub(B- V) - 0.18 + or - 0.06) deduced from GLIMPSE (Spitzer) data.