OGLE-IV Period-Luminosity relation of the LMC: an analysis using mean and median magnitudes

TL;DR

This study evaluates how using median versus mean magnitudes affects the Period-Luminosity relation of Cepheids in the LMC, revealing biases and the importance of considering period breaks for precise distance measurements.

Contribution

It introduces a robust regression approach to analyze the PL relation and highlights the impact of magnitude choice and period breaks on the relation's accuracy.

Findings

Significant change in PL relation at 10 days period

Bias in zero point and slope when using median magnitudes

Corrected estimators improve distance measurement accuracy

Abstract

TThe Period-Luminosity (PL) relation for Cepheid variable stars in the Large Magellanic Cloud (LMC) is crucial for distance measurements in astronomy. This study analyzes the impact of using the median rather than the mean on the PL relation's slope and zero point. It also examines the persistence of the break at approximately 10 days and addresses specification issues in the PL relation model. Using $VI$-band median and mean magnitudes from the OGLE-IV survey, corrected for extinction, we fit the PL relation employing robust $MM$-regression, which features a high breakdown point and robust standard errors. Statistical tests and residual analysis are conducted to identify and correct model deficiencies. Our findings indicate a significant change in the PL relation for Cepheids with periods of 10 days or longer, regardless of whether median or mean magnitudes are used. A bias in the zero point and slope estimators is observed when using median magnitudes instead of mean magnitudes, especially in the $V$-band. By identifying and correcting regression issues and considering the period break, our estimators for slope and zero point are more accurate for distance calculations. Comparative analysis of the models for each band quantifies the bias introduced by using median magnitudes, highlighting the importance of considering the Cepheids' period for accurate location measure results, similar to those obtained using mean magnitudes.