The PL calibration for Milky Way Cepheids and its implications for the distance scale

TL;DR

This paper revises the Cepheid period-luminosity relation using Galactic Cepheids and confirms its applicability for distance measurements, highlighting the importance of dust extinction properties for accurate extragalactic distance calibration.

Contribution

It provides a new calibration of the Cepheid PL relation based on Galactic Cepheids and validates it with independent distance measures, addressing previous slope change controversies.

Findings

Recalibrated PL relation with moderate scatter

Confirmed relation with HST and Hipparcos parallaxes

Validated Wesenheit formulation using NGC 4258 Cepheids

Abstract

The rationale behind recent calibrations of the Cepheid PL relation using the Wesenheit formulation is reviewed and reanalyzed, and it is shown that recent conclusions regarding a possible change in slope of the PL relation for short-period and long-period Cepheids are tied to a pathological distribution of HST calibrators within the instability strip. A recalibration of the period-luminosity relation is obtained using Galactic Cepheids in open clusters and groups, the resulting relationship, described by log L/L_sun = 2.415(+-0.035) + 1.148(+-0.044)log P, exhibiting only the moderate scatter expected from color spread within the instability strip. The relationship is confirmed by Cepheids with HST parallaxes, although without the need for Lutz-Kelker corrections, and in general by Cepheids with revised Hipparcos parallaxes, albeit with concerns about the cited precisions of the latter. A Wesenheit formulation of Wv = -2.259(+-0.083) - 4.185(+-0.103)log P for Galactic Cepheids is tested successfully using Cepheids in the inner regions of the galaxy NGC 4258, confirming the independent geometrical distance established for the galaxy from OH masers. Differences between the extinction properties of interstellar and extragalactic dust may yet play an important role in the further calibration of the Cepheid PL relation and its application to the extragalactic distance scale.