Anchors for the Cosmic Distance Scale: the Cepheid QZ Normae in the Open Cluster NGC 6067

TL;DR

This study confirms QZ Nor as a member of open cluster NGC 6067, validating its use as a calibrator for the cosmic distance scale through multiple lines of evidence including radial velocities, photometry, and distance measurements.

Contribution

The paper provides comprehensive evidence establishing QZ Nor's membership in NGC 6067, supporting its role as a key calibrator for Leavitt's law in cosmic distance measurements.

Findings

QZ Nor and V340 Nor are confirmed cluster members.

NGC 6067 is located at approximately 1.75 kpc.

QZ Nor's properties support its use as a distance calibrator.

Abstract

Cepheids are key to establishing the cosmic distance scale. Therefore it's important to assess the viability of QZ Nor, V340 Nor, and GU Nor as calibrators for Leavitt's law via their purported membership in the open cluster NGC 6067. The following suite of evidence confirms that QZ Nor and V340 Nor are members of NGC 6067, whereas GU Nor likely lies in the foreground: (i) existing radial velocities for QZ Nor and V340 Nor agree with that established for the cluster (-39.4+-1.2 km/s) to within 1 km/s, whereas GU Nor exhibits a markedly smaller value; (ii) a steep velocity-distance gradient characterizes the sight-line toward NGC 6067, thus implying that objects sharing common velocities are nearly equidistant; (iii) a radial profile constructed for NGC 6067 indicates that QZ Nor is within the cluster bounds, despite being 20' from the cluster center; (iv) new BVJH photometry for NGC 6067 confirms the cluster lies d=1.75+-0.10 kpc distant, a result that matches Wesenheit distances computed for QZ Nor/V340 Nor using the Benedict et al. (2007, HST parallaxes) calibration. QZ Nor is a cluster Cepheid that should be employed as a calibrator for the cosmic distance scale.