The influential effect of blending, bump, changing period and eclipsing Cepheids on the Leavitt law

TL;DR

This study uses robust statistical methods to confirm the linearity of the Leavitt law in the Large Magellanic Cloud, showing that certain Cepheid variables do not affect its validity, unlike in the Small Magellanic Cloud.

Contribution

It demonstrates that robust regression techniques reveal the Leavitt law's linearity in the LMC without excluding data, challenging previous notions of non-linearity caused by specific Cepheid types.

Findings

Leavitt law is linear in the LMC when using robust regression.

Blending, bumps, eclipses, or period changes do not affect the law in the LMC.

In the SMC, the law's linearity cannot be established due to galaxy geometry.

Abstract

The investigation of the non-linearity of the Leavitt law is a topic that began more than seven decades ago, when some of the studies in this field found that the Leavitt law has a break at about ten days. The goal of this work is to investigate a possible statistical cause of this non-linearity. By applying linear regressions to OGLE-II and OGLE-IV data, we find that, in order to obtain the Leavitt law by using linear regression, robust techniques to deal with influential points and/or outliers are needed instead of the ordinary least-squares regression traditionally used. In particular, by using $M$- and $MM$-regressions we establish firmly and without doubts the linearity of the Leavitt law in the Large Magellanic Cloud, without rejecting or excluding Cepheid data from the analysis. This implies that light curves of Cepheids suggesting blending, bumps, eclipses or period changes, do not affect the Leavitt law for this galaxy. For the SMC, including this kind of Cepheids, it is not possible to find an adequate model, probably due to the geometry of the galaxy. In that case, a possible influence of these stars could exist.