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Anchors for the cosmic distance scale: the Cepheid QZ Normae in the open cluster NGC 6067 |
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| Authors: | D Majaess L Sturch C Moni Bidin M Soto W Gieren R Cohen F Mauro D Geisler C Bonatto J Borissova D Minniti D Turner D Lane B Madore G Carraro L Berdnikov |
| Institution: | 1. Halifax, Nova Scotia, Canada 2. Dept. of Astronomy, Institute for Astronomical Research, Boston University, 725 Commonwealth Ave, Boston, MA, 02215, USA 3. Instituto de Astronomía, Universidad Católica del Norte, Av. Angamos, 0610, Antofagasta, Chile 4. Departamento de Física, Universidad de La Serena, Benavente 980, La Serena, Chile 5. Departamento de Astronomía, Universidad de Concepción, Casilla 160-C, Concepción, Chile 6. Departamento de Astronomia, Universidade Federal do Rio Grande do Sul, Av. Bento Gonalves 9500, Porto Alegre, 91501-970, RS, Brazil 7. Departamento de Física y Astronomía, Facultad de Ciencias, Universidad de Valparaíso, Av. Gran Breta?a 1111, Valparaíso, Chile 8. Departamento de Astronomía y Astrofísica, Pontificia Universidad Católica de Chile, Casilla 306, Santiago, Chile 9. Department of Astronomy and Physics, Saint Mary’s University, Halifax, NS B3H 3C3, Canada 10. Carnegie Observatories, 813 Santa Barbara Street, Pasadena, CA, 91101, USA 11. ESO - European Southern Observatory, Avda Alonso de Cordova, 3107, Casilla, 19001, Santiago, Chile 12. Moscow M V Lomonosov State University, Sternberg Astronomical Institute, Moscow, 119992, Russia |
| 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 ( $-39.4\pm0.2(\sigma_{\bar {x}}) \pm1.2 (\sigma)~\mbox{km/s}$ ) 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. (Astron. J. 133:1810, 2007, HST parallaxes) calibration. QZ Nor is a cluster Cepheid that should be employed as a calibrator for the cosmic distance scale. |
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