Blending of Cepheids in M33

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.

v2: Fixed incorrect description of Figure 2 in text. Accepted for publication in AJ. Full data tables can be found in ASCII format as part of the source distribution. A version of the paper with higher-resolution figures can be found at http://faculty.physics.tamu.edu/lmacri/papers/chavez12.pdf

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• Properties of M31. II: A Cepheid disk sample derived from the first year of PS1 PAndromeda data
• The Cepheid distance to the maser-host galaxy NGC 4258: Studying systematics with the Large Binocular Telescope
• Panchromatic Hubble Andromeda Treasury XIII: The Cepheid period-luminosity relation in M31
• Anchors for the Cosmic Distance Scale: the Cepheids U Sgr, CF Cas and CEab Cas
• On the Metallicity Dependence of Classical Cepheid Light Amplitudes
• Evidence for Photometric Contamination in Key Observations of Cepheids in the Benchmark Galaxy IC 1613
• The Astrophysical Distance Scale: V. A 2% Distance to the Local Group Spiral M33 via the JAGB Method, Tip of the Red Giant Branch, and Leavitt Law