On the Paradoxical Impact of Blending by Red Clump Giants

The impact of blending by RCGs (red clump giants, or relatively metal-rich red horizontal branch stars) is discussed as it relates to RRab and classical Cepheids, and invariably establishing an improved distance scale. An analysis of OGLE Magellanic Cloud variables reaffirms that blending with RCGs may advantageously thrust remote extragalactic stars into the range of detectability. Specifically, simulations of Magellanic Cloud RRab and RCG blends partly reproduce bright non-canonical trends readily observed in amplitude-magnitude space ($I_c$ vs.~$A_{I_c}$). Conversely, the larger magnitude offset between classical Cepheids and RCGs causes the latter's influence to be challenging to address. The relative invariance of a Wesenheit function's slope to metallicity (e.g., $W_{VI_c}$) implies that a deviation from the trend could reveal blending and photometric inaccuracies (e.g., standardization), as blending by RCGs (a proxy of an evolved red stellar demographic) can flatten period-Wesenehit relations owing to the increased impact on less-luminous shorter-period Cepheids. That could partly explain both a shallower inferred Wesenheit function and over-estimated $H_0$ values. A consensus framework to identify and exploit blending is desirable, as presently $H_0$ estimates from diverse teams are unwittingly leveraged without homogenizing the disparate approaches (e.g., no blending correction to a sizable $\simeq 0.^{\rm m}3$).