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Anchoring the Universal Distance Scale Via a Wesenheit Template


Daniel J. Majaess
David G. Turner
David J. Lane
Saint Mary’s University, Halifax, Nova Scotia, B3H 3C3, Canada
The Abbey Ridge Observatory, Stillwater Lake, Nova Scotia, Canada

Arne A. Henden
AAVSO, 49 Bay State Road, Cambridge, MA 02138

Tom Krajci
Astrokolkhoz Telescope Facility, Cloudcroft, NM 88317

Sonoita Research Observatory, Sonoita, Arizona

Received May 4, 2010; revised July 26, 2010; accepted August 16, 2010

A VI Wesenheit diagram featuring SX Phoenicis, δ Scuti, RR Lyrae, and type II and classical Cepheid variables is calibrated by means of geometric-based distances inferred from HST, Hipparcos, and VLBA observations (n = 30). The distance to a target population follows from the offset between the observed Wesenheit magnitudes and the calibrated template. The method is evaluated by ascertaining the distance moduli for the LMC (μ0 = 18.43 ± 0.03 σx) and the globular clusters ω Cen, M54, M13, M3, and M15. The results agree with estimates cited in the literature, although a nearer distance to M13 is favored (pending confirmation of the data’s photometric zero-point) and observations of variables near the core of M15 suffer from photometric contamination. The calibrated LMC data are subsequently added to the Wesenheit template since that galaxy exhibits precise OGLE photometry for innumerable variables of differing classes, that includes recent observations for δ Scuti variables indicating the stars follow a steeper VI Wesenheit function than classical Cepheids pulsating in the fundamental mode. VI photometry for the calibrators is tabulated to facilitate further research, and includes new observations acquired via the AAVSO’s robotic telescope network (e.g., VY Pyx: <V> = 7.25 and <V> – <I> = 0.67). The approach outlined here supercedes the lead author’s prior first-order effort to unify variables of the instability strip in order to establish reliable distances.

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