On Cepheid Distance Scale Bias due to Stellar Companions and Cluster Populations

Understand

State-of-the art photometric measurements of extragalactic Cepheids account for the mean additional light due to chance superposition of Cepheids on crowded backgrounds through the use of artificial star measurements.

• However, light from stars physically associated with Cepheids may bias relative distance measurements if the changing spatial resolution along the distance ladder significantly alters the amount of associated blending.
• We have identified two regimes where this phenomenon may occur: Cepheids in wide binaries and open clusters.
• We estimate stellar association bias using the photometric passbands and reddening-free Wesenheit magnitudes used to set up the distance scale.