Period-Luminosity relations derived from the OGLE-III First-overtone mode Cepheids in the Magellanic Clouds

TL;DR

This study derives multi-band Period-Luminosity relations for first-overtone Cepheids in the Magellanic Clouds, identifying a period break at 2.5 days and comparing relations between the SMC and LMC to estimate their relative distance.

Contribution

It provides new multi-band PL relations for first-overtone Cepheids in both Magellanic Clouds and analyzes non-linearities and differences between the two galaxies.

Findings

Significant break in optical PL relations at 2.5 days.

No statistical evidence for non-linearity at 1 day.

Estimated relative distance modulus of 0.49 mag between SMC and LMC.

Abstract

We present multi-band Period-Luminosity (PL) relations for first-overtone mode Cepheids in the Small Magellanic Cloud (SMC). We derive optical band PL relations and the Wesenheit function using $VI$ mean magnitudes from the Optical Gravitational Lensing Experiment (OGLE-III) survey. We cross-match OGLE-III first-overtone mode Cepheids to the 2MASS and SAGE-SMC catalogs to derive PL relations at near-infrared ($JHK_s$) and mid-infrared ($3.6~\&~4.5\mu\mathrm{m}$) wavelengths. We test for possible non-linearities in these PL relations using robust statistical tests and find a significant break only in the optical-band PL relations at 2.5 days for first-overtone mode Cepheids. We do not find statistical evidence for a non-linearity in these PL relations at 1 day. The multi-band PL relations for fundamental-mode Cepheids in the SMC also exhibit a break at 2.5 days. We suggest that the period break around 2.5 days is related to sharp changes in the light curve parameters for SMC Cepheids. We also derive new optical and mid-infrared band PL relations for first-overtone mode Cepheids in the Large Magellanic Cloud (LMC). We compare multi-band PL relations for first-overtone mode Cepheids in the Magellanic Clouds and find a significant difference in the slope of the $V$-band PL relations but not for $I$-band PL relations. The slope of PL relations are found to be consistent in most of the infrared bands. A relative distance modulus of $\Delta\mu=0.49\pm0.02$~mag between the two clouds is estimated using multi-band PL relations for the first-overtone mode Cepheids in the SMC and LMC.