Application of Cepheids to Distance Scale: Extending to Ultra-Long Period Cepheids

Abstract

Classical Cepheids (hereafter Cepheids) belong to a class of important variable stars that can be used to determine distances to nearby galaxies via the famous period-luminosity (PL) relations, i.e. the Leavitt Law. In turn, these distances can then be used to calibrate a host of secondary distance indicators located well within the Hubble flow, and ultimately determine the Hubble constant in a manner independent of the Cosmic Microwave Background (CMB) measurements. Some recent progress in determining the Hubble constant to within ∼ 3% level via the Cepheid-based distance scale ladder (the SH0ES and the Carnegie Hubble Program) were first summarized in this Proceeding, followed by a brief discussion on the prospect of using ultra-long period Cepheids (ULPC) in future distance scale work. ULPC are those Cepheids with periods longer than 80 days, which seem to follow a different PL relation than their shorter period Cepheids. It has been suggested that ULPC can be used to determine the Hubble constant in "one-step". However, based on the two ULPCs found in M31, it was found that the large dispersion in derived distance moduli leads to a less accurate distance modulus to M31 compared to the classical Cepheids. This finding might raise an alert regarding the use of ULPCs in future distance scale work.