Wesenheit Function for Galactic Cepheids: Application to the Projection Factors

TL;DR

This study uses the Wesenheit function to determine distances to over 300 Galactic Cepheids, calibrates their projection factors, and discusses discrepancies in angular diameter measurements affecting p-factor estimates.

Contribution

It re-examines the Wesenheit function for Cepheid distance measurement and calibrates the p-factor relation, highlighting non-linearity and measurement inconsistencies.

Findings

Wesenheit distances effectively calibrate the p-factor.

The p-factor relation may be non-linear with scatter.

Discrepancies in angular diameter measurements impact p-factor calibration.

Abstract

Galactic Cepheids are necessary tools for calibrating the period-luminosity relation, but distances to individual Galactic Cepheids are difficult to precisely measure and are limited to a small number of techniques such as direct parallax, main-sequence fitting to open clusters that host Cepheids and Baade-Wesselink (BW) methods. In this work, we re-examine the application of Wesenheit function in determining distances to more than 300 Galactic Cepheids, by taking advantage of the fact that the Wesenheit function is extinction free by definition. The Wesenheit distances were used to calibrate the projection factor (p-factor) for Galactic Cepheids that also possess BW distances. Based on ~70 Cepheids, we found that a period-p factor relation may exhibit a non-linear trend with a considerable scatter. During our investigation, discrepant p factors for delta Cephei were found in the literature. This may be due to inconsistent measurements of the angular diameters using different empirical techniques. We discuss the reason for the inconsistency of angular diameter measurements and offer a possible remedy for this problem.