Blending of Cepheids in M33

TL;DR

This study quantifies the blending effects on Cepheid variables in M33 using Hubble and WIYN data, revealing that a significant fraction of Cepheids are affected by nearby companions, which can bias distance measurements.

Contribution

It provides the first detailed analysis of Cepheid blending in M33 using multi-resolution data, offering empirical blending correction data for future distance estimates.

Findings

55+-4% of Cepheids have no detectable companions affecting WIYN V-band photometry.

73+-4% of Cepheids are affected below 10% in flux bias.

No significant variation in blending with period or surface brightness.

Abstract

A precise and accurate determination of the Hubble constant based on Cepheid variables requires proper characterization of many sources of systematic error. One of these is stellar blending, which biases the measured fluxes of Cepheids and the resulting distance estimates. We study the blending of 149 Cepheid variables in M33 by matching archival Hubble Space Telescope data with images obtained at the WIYN 3.5-m telescope, which differ by a factor of 10 in angular resolution. We find that 55+-4% of the Cepheids have no detectable nearby companions that could bias the WIYN V-band photometry, while the fraction of Cepheids affected below the 10% level is 73+-4%. The corresponding values for the I band are 60+-4% and 72+-4%, respectively. We find no statistically significant difference in blending statistics as a function of period or surface brightness. Additionally, we report all the detected companions within 2 arcseconds of the Cepheids (equivalent to 9 pc at the distance of M33) which may be used to derive empirical blending corrections for Cepheids at larger distances.