Using Ultra Long Period Cepheids to Extend the Cosmic Distance Ladder to 100 Mpc and Beyond

TL;DR

This study investigates Ultra Long Period Cepheids, finding they are promising standard candles with a shallower Period-Luminosity relation, enabling distance measurements up to 100 Mpc and beyond.

Contribution

It demonstrates that ULP Cepheids are more luminous and have a less steep PL relation, making them useful for extending the cosmic distance ladder.

Findings

ULP Cepheids have a shallow PL relation similar to classical Cepheids.

The scatter around the PL relation is comparable to other standard candles.

ULP Cepheids can potentially measure distances up to 100 Mpc.

Abstract

We examine the properties of 17 long period (80-180 days) and very luminous (median absolute magnitude of M_I= -7.93 and M_V= -7.03) Cepheids to see if they can serve as an useful distance indicator. We find that these Ultra Long Period (ULP) Cepheids have a relatively shallow Period-Luminosity (PL) relation, so in fact they are more "standard candle"-like than classical Cepheids. In the reddening-free Wesenheit index, the slope of the ULP PL relation is ~10 times less steep than the standard PL relation for the SMC Cepheids. The scatter of our sample about the W_I PL relation is 0.22 mag, approaching that of classical Cepheids and Type Ia Supernovae. We expect this scatter to decrease as bigger and more uniform samples of ULP Cepheids are obtained. We also measure a non-zero period derivative for one ULP Cepheid (SMC HV829) and use the result to probe evolutionary models and mass loss of massive stars. ULP Cepheids main advantage over classical Cepheids is that they are more luminous, and as such show great potential as stellar distance indicators to galaxies up to 100 Mpc and beyond.